Middleware-based Database Replication: The Gaps

Middleware-based Database Replication: The Gaps
Middleware-based Database Replication:
The Gaps Between Theory and Practice
Emmanuel Cecchet
George Candea
Anastasia Ailamaki
EPFL
Lausanne, Switzerland
EPFL & Aster Data Systems
Lausanne, Switzerland
EPFL & Carnegie Mellon University
Lausanne, Switzerland
[email protected]
[email protected]
[email protected]
ABSTRACT
1. INTRODUCTION
The need for high availability and performance in data
management systems has been fueling a long running interest in
database replication from both academia and industry. However,
academic groups often attack replication problems in isolation,
overlooking the need for completeness in their solutions, while
commercial teams take a holistic approach that often misses
opportunities for fundamental innovation. This has created over
time a gap between academic research and industrial practice.
This paper aims to characterize the gap along three axes:
performance, availability, and administration. We build on our
own experience developing and deploying replication systems in
commercial and academic settings, as well as on a large body of
prior related work. We sift through representative examples from
the last decade of open-source, academic, and commercial
database replication systems and combine this material with case
studies from real systems deployed at Fortune 500 customers. We
propose two agendas, one for academic research and one for
industrial R&D, which we believe can bridge the gap within 5-10
years. This way, we hope to both motivate and help researchers in
making the theory and practice of middleware-based database
replication more relevant to each other.
Categories and Subject Descriptors
C.2.4 [Distributed Systems]: Distributed
[Systems]: Distributed databases
databases; H.2.4
General Terms
Performance, Design, Reliability.
Keywords
Middleware, database replication, practice and experience.
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Despite Gray’s warning on the dangers of replication [18] over a
decade ago, industry and academia have continued building replication systems for databases. The reason is simply that replication
is the only tried-and-true mechanism for scaling performance and
availability of databases across a wide range of requirements.
There exist replication “solutions” for every major DBMS, from
Oracle RAC™, Streams™ and DataGuard™ to Slony-I for
Postgres, MySQL replication and cluster, and everything inbetween. The naïve observer may conclude that such variety of
replication systems indicates a solved problem; the reality,
however, is the exact opposite. Replication still falls short of
customer expectations, which explains the continued interest in
developing new approaches, resulting in a dazzling variety of
offerings.
Even the “simple” cases are challenging at large scale. We
deployed a replication system for a large travel ticket brokering
system at a Fortune-500 company faced with a workload where
95% of transactions were read-only. Still, the 5% write workload
resulted in thousands of update requests per second, which
implied that a system using 2-phase-commit, or any other form of
synchronous replication, would fail to meet customer performance
requirements (thus confirming Gray’s prediction [18]). This
tradeoff between availability and performance has long been a
hurdle to developing efficient replication techniques.
In practice, the performance/availability tradeoff can be highly
discontinuous. In the same ticket broker system mentioned above,
the difference between a 30-second and a one-minute outage
determines whether travel agents retry their requests or decide to
switch to another broker for the rest of the day (“the competition
is one click away”). Compounded across the hundreds of travel
agencies that connect to the broker system daily for hotel
bookings, airline tickets, car rentals, etc., the impact of one minute
of downtime comes close to that of a day-long outage. The
replication system needs to be mindful of the implied failover
requirements, and obtaining predictable behavior is no mean feat.
Our premise is that, by carefully observing real users’ needs and
transforming them into research goals, the community can bridge
the mismatch between existing replication systems and customers’
expectations within the coming decade. We sift through the last
decade of database replication in academic, industrial, and opensource projects. Combining this analysis with 45 person-years of
experience building and deploying replicated database systems,
we identify the unanswered challenges of practical replication.
We find that a few “hot topics” (e.g., reliable multicast and lazy
replication [21]) attract the lion’s share of academic interest, while
other equally important aspects (e.g., availability and
management) are often forgotten—this limits the impact research
systems can have on the real world. Motivated by these findings,
we draft possible agendas for academic and industrial research.
This paper concentrates exclusively on middleware-based1
replication for OLTP workloads. The prevalent architecture is
shared-nothing, where cluster nodes use local disks to store data.
We make two contributions. First, we identify gaps between
database research and practice on four different levels: RDBMS
engine, SQL language, middleware, and system management. We
show how overlooking seemingly small details can undermine
replication systems. Second, we distill a few research topics that
provide low-hanging fruit for closing these gaps, in the realms of
middleware design, consistency models, availability, and system
evaluation. We also describe what we believe industry ought to do
with respect to interfaces, transaction abstractions, system
management, and dynamic upgrades. As this paper is not intended
to serve as an area survey, the reference list is by no means
exhaustive. Rather, we choose representative examples that help
us characterize the academia/industry gap and its consequences.
The rest of the paper is structured as follows: Section 2 describes
the replication schemes presently favored in the field. Section 3
surveys representative academic proposals for replication. Section
4 discusses in detail the practical challenges we have encountered
while deploying numerous middleware-based replication systems
at customers ranging from small startups to Fortune 500
companies. Section 5 distills the main challenges and outlines
roadmaps for both academic research and industrial R&D that can
bridge the identified gaps. Section 6 concludes the paper.
2. REPLICATION IN PRACTICE
There are two main reasons to employ database replication: to
improve performance and to increase availability. Section 2.1
discusses commonly used architectures for performance-focused
deployments, while Section 2.2 describes availability-focused
solutions.
2.1 Improving Performance via Replication
Database replication is typically used to improve either read
performance or write performance, while improving both read and
write performance simultaneously is a more challenging task.
Figure 1 depicts master-slave replication, a popular technique
used to improve read performance. In this scenario, read-only
content is accessed on the slave nodes and updates are sent to the
master. If the application can tolerate loose consistency, any data
can be read at any time from the slaves given a freshness
guarantee. As long as the master node can handle all updates, the
system can scale linearly by merely adding more slave nodes.
Examples of commercial products providing asynchronous
master-slave replication are Microsoft SQL Server replication,
Oracle Streams, Sybase Replication Server, MySQL replication,
1
By middleware we mean the software layer that lies between an
application and the database replicas.
IBM DB2 DataPropagator, GoldenGate TDM platform, and
Veritas Volume Replicator.
A special instance of read throughput improvement relates to
legacy databases: often an old DB system is faced with increased
read performance requirements, that it can no longer satisfy, yet
replacing the DB is too costly. Recently emerged strategies, such
as satellite databases [29], offer a migration path for such cases. In
the case of an e-commerce application, the main legacy database
is preserved for all critical operations, such as orders, but less
critical interactions, such as catalog browsing, can be offloaded to
replicas. Such configurations typically use partial replication—all
orders could be solely on the main legacy database, while only the
catalog content is replicated. As an application might also be
using multiple database instances inside the same RDBMS, the
user can choose to replicate only specific database instances.
Application 1
Application 2
Application 3
Database replication Middleware
Write requests
Master
database
Read only requests
DB1
DB2
DB3
DB4
Asynchronous write propagation
Figure 1. Database scale-out scenario
Multi-master replication allows each replica owning a full copy
of the database to serve both read and write requests. The
replicated system then behaves as a centralized database, which
theoretically does not require any application modifications.
Replicas, however, need to synchronize in order to agree on a
serializable execution order of transactions, so that each replica
executes the update transactions in the same order. Also,
concurrent transactions might conflict, leading to aborts and
limiting the system’s scalability [18]. Even though real
applications generally avoid conflicting transactions, there are still
significant research efforts trying to solve this problem in the
replication middleware layer. The volume of update transactions,
however, remains the limiting performance factor for such
systems. As every replica has to perform all updates, there is a
point beyond which adding more replicas does not increase
throughput, because every replica is saturated applying updates.
Examples of commercial multi-master architectures include
Continuent uni/Cluster and Xkoto Gridscale for middleware
replication, and MySQL Cluster and DB2 Integrated Cluster for
database in-core implementations. Shared-disk architectures, such
as Oracle RAC, are out of the scope of this paper.
Finally, data partitioning techniques can be used to address write
scalability. Figure 2 shows an example where data is logically
split into 3 different partitions, each one being replicated.
Common partitioning criteria are based on a table primary key and
include techniques such as range partitioning, list partitioning and
hash partitioning. The benefits of this approach are similar to
RAID-0 for disks: updates can be done in parallel to partitioned
data segments. Read latency can also be improved by exploiting
intra-query parallelism and executing the sub-queries in parallel
on each partition.
Application 1
Application 2
Application 3
directs the requests to the master and, when the master’s failure is
detected, requests are rerouted to the slave.
The hot standby or slave node, either local or at a remote site, has
computing capabilities similar to the master node. It applies
updates as they arrive from the master. As soon as the master fails
(detected by a simple heartbeat mechanism), the load is shifted to
the slave node. Various techniques can be used for this, such as
virtual IP [10] or reconfiguration of the driver or application.
Application
Database replication Middleware
Heartbeat
DB1
DB2
Partition 1
DB3
DB4
Partition 2
DB5
DB6
Partition 3
Figure 2. Database partitioning for increased write
performance
2.2 Increasing Availability via Replication
High availability requires low downtime. Planned downtime is
incurred during all software and hardware maintenance
operations, while unplanned downtime can strike at any time and
is due to predictable or unpredictable failures (hardware failures,
software bugs, human error, etc.). Downtime is usually the
primary metric being optimized for, with performance relegated to
a secondary role (although, in practice, both are desired).
A system’s availability is the ratio of its uptime to total time. In
practice, it is computed as the ratio between the expected time of
continuous operation between failures to total time, or
where MTTF is mean-time-to-failure and MTTR is mean-time-torepair. Since MTTF >> MTTR, the unavailability (ratio of
downtime to total time) is approximately MTTR/MTTF.
The goal of replication, together with failover and failback, is to
reduce MTTR and thus reduce unavailability. Failover is the
capability of switching users of a database node over to another
database node containing a replica of the data, whenever the node
they were connected to fails. Failback is the reverse and occurs
when the original replica recovered from its failure and users are
re-allocated to it.
Hot standby is the most commonly deployed configuration using
database replication in both open-source and commercial
databases. Whether it uses single-master or multi-master
techniques, the end goal remains the same: to provide fast
recovery from node failures. A node serves all queries and, upon
failure, the workload is transferred to the hot standby node.
Figure 3 shows a hot standby setup using Slony-I for PostgreSQL
[31]. There are two replicas, one acting as a master and the other
as a slave. The application connects to a simple load balancer that
IP alias
Heartbeat
Slony-I
Slony-I
Master
Slave
Figure 3. Hot standby configuration
Determining which transactions are lost when the master fails
remains a manual procedure that requires careful inspection of the
master’s transaction log (when it is available). The best guarantee
that is usually offered is 1-safe (i.e., transactions commit at the
master without consulting the slave) with an upper-bound time
window (e.g., at most all transactions committed in the past 5
minutes have been lost). These guarantees are usually considered
weak, but good enough for maintaining uptime in the face of
single faults. 2-safe database replication forces the master to
commit only when the backup has also confirmed receipt of the
update (even though the backup may not have written to disk
immediately). This avoids transaction loss, but increases latency.
In order to not completely waste the slave’s computing resources
under normal operation, the slave is typically used for read-only
reporting tasks that run in batch mode. In a heavily loaded
production system, however, the lag between the master and slave
node can become significant. Our customers report several hours
of downtime when commercial databases failover clients from a
master to a hot-standby slave node. The reason is typically that the
trailing updates are applied serially at the slave, whereas the
master processes them in parallel. The “solution” is usually to
slow down the master (during normal operation) so as to keep the
slave synchronized to within a small time window.
WAN replication is the gold standard for sustaining availability in
the face of disasters that would affect an entire cluster
(earthquakes, floods, etc.). In the case of disaster recovery, unlike
regular failover, clients’ requirements for synchronization
windows are less stringent. Replicating data asynchronously
between sites, possibly by interconnecting middleware replication
solutions, usually involves both data partitioning and multi-way
master/slave replication (i.e., each site is master for its local
geographical data)—see Figure 4.
Major challenges in WAN replication are partitions resulting from
network failure or datacenter infrastructure failures. Although
some companies can afford their own data centers interconnected
by dedicated leased lines, most use professional hosting centers,
to reduce costs. Hosting centers emphasize efficient multiplexing
and management of resources so, even if individual applications
are not demanding in terms of resources, there are often hundreds
or thousands of them running on various combinations of software
stacks in one datacenter. In such dense environments, one
problem can have domino effects. Our experience with an
academic GRID (600 CPUs) and a number of smaller clusters (10100 nodes) indicates that, on average, one fatal failure (software
or hardware) occurs per day per 200 processors, not including air
conditioning or power outages. Thus, keeping replicas in sync can
be challenging when failures are frequent.
Asian
users
happens behind the scenes, by coordinating the different database
engines.
The major advantage of this approach is that it does not require
any change on the client side; it does require, however, integration
with the database engine. This restricts the ability of different
database engines (or even different versions of the same engine)
to interact with each other. For this approach to be viable in
practice, the replication APIs must be adopted and integrated in
the main development line of the database engine itself. In the
case of Postgres-R, the failure to transfer the complex piece of
replication code to the PostgreSQL core team led to a gradual
divergence, eventually rendering Postgres-R obsolete. In the case
of closed source databases, this is an even greater challenge.
M
Asian site
WAN replication Middleware
Application
Application
Native database
driver
Native database
driver
Database native protocol
M
DB1
M
DB2
DB3
Database replication Middleware
US
users
American site
European site
EU
users
Figure 4. Worldwide multi-way master/slave replication
3. ACADEMIC APPROACHES TO
MIDDLEWARE-BASED REPLICATION
In this section, we describe some of the leading architectures
adopted by modern academic prototypes, as well as the major
points of research interest. Many academic proposals have not
(yet) made their way into practice because of practical challenges
we describe later on (Section 4).
3.1 System Design
Most of middleware-based replication research focuses on multimaster replication. Master-slave implementations are either incore implementations or third-party tools that extract database
binlogs and ship them to another database. Slony-I [31] and
Ganymed [28] for PostgreSQL are among the few middleware
solutions that provide direct master-slave replication capabilities.
With suitable load balancers, multi-master systems like C-JDBC
[8] or Tashkent [14] can also be used in master-slave mode.
Postgres-R [20] was the first significant academic research
prototype to provide a fully functional database replication
mechanism for an open source database. Even though not
implemented 100% in middleware, the replication mechanism
interacts with mostly unmodified database core components
(subsequently, Middle-R [27] implemented Postgres-R concepts
in middleware). As illustrated in Figure 5, the database
client/server communication is untouched and the replication only
Figure 5. Query interception at the database engine level
Figure 6 shows another approach, that intercepts queries directly
at the database native protocol level; a prototype intercepting the
PHP/MySQL protocol to route queries is described in [3].
Proxying queries at the DBMS native protocol level is elegant,
because the middleware is not coupled to the database system and
can evolve independently. This approach, however, does not work
if the protocol is protected by copyright, licensing or patent
restrictions. It also does not support more than one DB engine at
the low level.
Application
Application
Native DBMS
driver
Native DBMS
driver
DBMS native protocol
Database replication Middleware
DBMS native protocol
DB1
DB2
DB3
Figure 6. Query interception at the DBMS protocol level
It is also possible to intercept the native protocol on the client
side, to reuse existing native drivers and remap the calls to a
standard API, such as JDBC or ODBC. Myosotis [26], for
instance, intercepts MySQL and PostgreSQL protocols and
remaps them to the Sequoia/JDBC protocol [30]. This enables the
use of Sequoia-specific drivers on supported platforms and the use
of native libraries or drivers for other platforms or for accessing
non-clustered databases.
Most contemporary academic prototypes nowadays are based on
the JDBC proxying concept introduced by C-JDBC [8], depicted
in Figure 7. This approach typically requires the database driver to
be replaced in the client application. This should normally not
require any application code changes, since the new driver
implements the same interface as the old one (JDBC, ODBC,
etc.). In addition, this approach allows the replication system to
span heterogeneous database systems. Some examples of systems
designed in this fashion are Tashkent [14], Ganymed [28], and
Middle-R [27].
Application
Application
JDBC driver
JDBC driver
Middleware protocol
Database replication Middleware
DB1 driver
DB2 driver
DB3 driver
DBMS native protocol
DB1
DB2
DB3
Figure 7. Query interception in JDBC-based replication
3.2 Load Balancing
A replicated database built for high availability must eliminate all
single points of failure (SPOF). Often, projects focusing on
performance overlook the replication needs of core components,
such as load balancers or certifiers. Even though such redundancy
is technically feasible, achieving it is more than mere engineering,
because it affects the replication middleware substantially.
To our knowledge, there is no off-the-shelf load balancer for
databases, mostly because load balancing is intrinsically tied to
the replication middleware. Load balancing can be implemented
at the connection level, transaction level or query level.
Connection-level load balancing allocates new client connections
to replicas according to a specified policy; all transactions and
requests on that connection go to the same replica until the
connection is closed. This approach is simple, but offers poor
balancing when clients use connection pools or persistent
connections. Transaction-level or query-level load balancing
perform finer grain load balancing by directing queries on a
transaction or query basis, respectively.
As an example, Tashkent+ [13] provides transaction-level load
balancing and exploits knowledge of the working sets of
transactions to allow in-main-memory execution at every replica.
The result is an improved throughput of more than 50% over
previous techniques; however, the approach uses a centralized
load balancer that is not replicated. A failure of this component
brings down the entire system. The recovery procedure requires
retrieving state from every replica to rebuild the load balancer’s
soft state. Similar issues would be observed for a certifier failure.
It is possible to achieve near-optimal load balancing with a
stateful, centralized load balancer. A failure of the load balancer,
however, not only causes all in-flight transactions to be lost, but
also causes a complete system outage. Replicating a stateful load
balancer or certifier requires extra communication and
synchronization that significantly impacts performance.
Unfortunately, recovery procedures are rarely described and
almost never evaluated in terms of the overhead they introduce in
system performance and recovery time.
3.3 Data Consistency
Much of today’s research chooses snapshot isolation (SI) for
enforcing consistency in the database. SI, introduced by [6], is a
weaker transactional guarantee than one-copy serializability
(1SR), the original standard correctness criterion for replicated
data. SI does not offer serializability, but decouples reads from
updates to increase concurrency. Each transaction operates on its
own copy of data (a snapshot), allowing read-only transactions to
complete without blocking.
Postgres-R [20] originally proposed an eager replication protocol
equivalent to “strong” SI. Various proposed protocols, such as
DISCOR and NODO, aim at optimizing performance in this
context [19] and are also implemented in Middle-R [27]. [22]
extends that work and provides 1-copy SI, also called global
strong SI. Ganymed [28] also provides a form of global strong SI
called “replicated snapshot isolation with primary copy”
(RSI-PC), but it focuses on master/slave architectures and satellite
databases. Tashkent [14] relies on generalized snapshot isolation
(GSI) and implements prefix-consistent SI (PCSI). [11] proposes
global weak SI and evaluates it using simulation. C-JDBC [8]
provides pluggable consistency protocols and uses 1SR by
default.
3.4 Prototype Evaluation
Evaluation of research prototypes mostly relies on benchmarks
from the Transaction Processing Council [33]. TPC-W or
RUBiS [2] are used for web-related workloads (see Tashkent [14],
Ganymed [28], C-JDBC [8]). Micro-benchmarks are also widely
used to measure replicated system performance (see
Postgres-R [20], Middle-R [27]). System performance is
evaluated in terms of throughput (transactions per second, web
interactions per seconds, etc.) and latency. If the system under test
forms a closed-loop system with the load generator(s), then
latency can be directly inferred from throughput.
Scalability measurements almost always use a scaled load to find
the best achievable performance (e.g., 5 times more requests for a
system with 5 replicas). This usually hides the system overhead at
low or constant load. As most production systems operate at less
than 50% load, it would be interesting to know how the proposed
prototypes perform when under-loaded. To the best of our
knowledge, management operations such as backup/restore or
adding a node to the system are practically never measured either.
Availability aspects of replication are usually not evaluated in
academic prototypes; even recent papers on adaptive
middleware [25] focus on performance adaptation in case of
workload variations, but do not address adaptation in the presence
of failures. In fact, important parameters for evaluating database
replication systems (such as mean-time-between-failure, meantime-between-system-abort,
or
mean-time-between-critical-
failure) are not used, despite them being well explained in the
literature. MTTR and MTTF would also seem to be natural
metrics for the evaluation of recovery effectiveness. We propose
further options in Section 5.1
according to a single total serializable order. Only cross-database
accesses must be synchronized, but separating these from the
overall workload requires the middleware to have a complete
view of all database instance accesses.
4.1.2 Isolation Level
4. PRACTICAL CHALLENGES
Why, given so many well-explained, thoroughly evaluated
academic proposals, is database replication still such a challenge
in practice? In this section, we use a bottom-up approach to
describe the various challenges we have encountered in the field
that we believe constitute the primary hurdles in bringing
sophisticated replication to real systems. Figure 8 shows the main
domains we have identified.
System Management
Replication Middleware
SQL
SQL
RDBMS
RDBMS
Figure 8. Layering of practical challenges
We begin with RDBMS-related issues (§4.1), then look at SQLspecific issues (§4.2), middleware-level challenges (§4.3), and
finally analyze the management/administration of replicated
databases (§4.4).
Database replication research has been largely addressing
snapshot isolation (SI) and its variations (GSI, strong SI, weak SI,
strong session SI, etc. [22]) in order to provide client applications
with a behavior as close to 1-copy serializability as possible. SI is
provided by Oracle (strongest isolation), DB2, PostgreSQL and,
more recently, Microsoft SQL Server 2005. Database systems,
such as Sybase or MySQL, do not provide SI. Nevertheless, the
default setting in all DBMS is the weaker read-committed form,
which most production applications use for performance reasons.
Applications try to avoid transaction aborts with SI or deadlocks
when using multi-version concurrency control (MVCC) at all
costs. Enhancing current replication frameworks to support
multiple isolation levels efficiently under a weaker isolation
assumption is still an open area of research.
Related to isolation, the handling of request failures is different in
the various implementations. The reasons why a request might fail
range from malformed SQL to integrity constraint violations.
PostgreSQL, for instance, aborts a transaction as soon as an error
occurs, whereas MySQL continues the transaction until the client
explicitly rolls back or aborts. To the best of our knowledge, no
study has investigated error handling in replicated databases. This
is a real problem in practice, especially when errors are combined
with database updates that cannot be rolled back (DDL, autoincremented keys, sequences, etc.).
4.1.3 Heterogeneous Clustering
4.1 RDBMS-level Challenges
There are numerous challenges faced at the level of the database
engine itself. The ones we have encountered most frequently are
queries spanning multiple database instances, lack of flexibility in
isolation levels, problems introduced by cluster heterogeneity, the
handling of temporary tables, and suitable access control.
4.1.1 Multi-Database Queries
An RDBMS can manage multiple database instances (as created
by CREATE DATABASE) and queries can span instances.
Triggers, for example, are often used to perform reporting tasks
and may update a different reporting database instance. Research
efforts, however, focus primarily on replicating independent
database instances [17] and have led to the concept of virtual
databases. Virtualization of an entire RDBMS has not been
addressed, so queries spanning multiple databases are usually not
handled correctly by replication that works on a per-databaseinstance basis. Furthermore, RDBMSes generally lack
mechanisms for taking a consistent snapshot of multiple
databases.
Databases are sometimes also used as substitutes for schemas (as
in CREATE SCHEMA). Some systems do not support the notion
of schema at all (like MySQL), while for others, isolating data in
different database instances is preferable over schemas for
performance or security reasons.
Managing databases and schemas in replicated systems can
sometimes be solved with engineering tweaks, but novel
algorithms are still required when synchronizing multiple
databases, in order to prevent synchronization of all databases
Hardware heterogeneity is a fact of life in clusters of replicated
databases. Heterogeneity, along with the ensuing unpredictability
and performance disparity, inevitably occurs over time, because
hardware components have limited lifetimes and replacing a piece
of hardware by the same model 6 months later is difficult and not
cost-effective. Heterogeneity issues can sometimes be addressed
using dynamic load balancing techniques, such as LPRF [8].
Even when the cluster’s hardware is homogeneous, the larger it is
and the longer it has been in operation, the higher the variance in
hardware and software performance, the higher the skew in data
layout affecting disk throughput, workload asymmetries, etc. A
few examples: a RAID controller with battery-backed write-back
caches suddenly becomes 2x slower when the battery fails, and
the OS rarely finds out; when a disk is replaced in a RAID-5,
reconstruction severely impacts array performance; if a single
strand in an Ethernet cable is crimped, throughput can drop from 1
Gbps to 100 Mbps. These anomalies are not addressed or
evaluated by research focusing on load balancing strategies [4].
Software heterogeneity can be of 2 kinds: (1) It might be
necessary to run multiple versions of the same database engine
(more on this under software upgrades—see Section 4.4.3), or
(2) aggregate data may be stored in two different database
engines.
The first scenario usually happens during a migration phase, or
when a legacy application that cannot be updated requires an older
version of the database engine. As different versions of the
database might require different drivers, either the application or
the replication middleware have to ensure the proper driver is
used in accordance with the accessed replica. Moreover, new
functionality must be properly identified, so that queries using
new features are only forwarded to capable replicas. Replication
systems based on log shipping using binary formats have to
handle all versions of storage formats and operate appropriate
conversions.
The second scenario (aggregating data from multiple
heterogeneous sources) is often seen when consolidating data
from different departments of a company. Another common case
occurs when companies are merged and databases have to be
accessed from applications as a single data source. Replication
adds complexity to these use cases, that already bring their own
challenges. One option is for the application to use the smallest
common denominator for all databases involved. Even pure ANSI
SQL 92 compliant queries might not be able to execute similarly
on all databases. Furthermore, some applications use middleware
or object relational mappers that generate the queries on their
behalf for a given database. In that case, the replicated database
has to present itself as a data source of a specific kind (e.g.,
database engine A) and the replication middleware adapts
automatically queries for database engines of other types. This
adaptation can take the form of on-the-fly query rewriting, like in
C-JDBC [8]. Also, Ganymed [29] scales a master database with
different satellite databases that can possibly have a different
schema and run different queries than the master.
4.1.4 Temporary Tables
Temporary tables are often used to simplify queries and store
processed data between requests or transactions. The visibility of
a temporary table varies between databases. In some cases, it is
global, requiring a unique name, whereas in other
implementations it is only visible to the current transaction or
connection. Sybase, for instance, does not authorize the use of
temporary tables within transactions. This forces the replication
middleware to keep track of temporary tables so that connections
stick to the same replica while using a given temporary table.
Temporary tables are not always persistent and are rarely backed
up, even though they can persist across transaction boundaries.
Management operations dealing with database backup/restore
operations to bring new replicas online must make sure that no
temporary tables are in use when a snapshot is taken, because this
information cannot be made part of the snapshot.
The lack of conventions on temporary tables makes it difficult for
the replication middleware to detect the true lifespan of a
temporary table. Most applications do not explicitly delete
temporary tables, but rather drop the connection, allowing the
database to automatically free the corresponding resources on its
own. Other implementations free temporary tables at commit
time. Such drastic differences make it nearly impossible to
implement a generally applicable middleware replication solution
for temporary tables.
4.1.5 Access Control
Every connection starts with an authentication phase. Over time,
databases
have
accommodated
popular
authentication
mechanisms supporting a wide variety of access control methods.
Middleware-based replication systems that intercept connections
necessarily tamper with the database authentication mechanisms
by hiding the original location of the client. However, it is
necessary to capture client information, so that requests are
replayed on behalf of the right user; as each user may have their
own set of triggers, the same SQL statement might have a
different impact, depending on which user is executing it.
The lack of user information standardization in the DBMS results
in ad-hoc configurations and settings for each implementation.
Despite the recent trend to store user data in the database
information schema, access control information is often
considered orthogonal to database content. This is a major
problem when databases need to be cloned (even more so when it
is a complete RDBMS with multiple database instances). Backup
tools typically capture only data, without user-related information,
raising issues when trying to clone a replica. Note that triggers
and stored procedures are also rarely backed up (e.g., in ETL—
Extraction, Transformation and Loading—tools, that focus mainly
on data transformation without addressing user-related
information).
4.2 SQL-level Challenges
In this section we discuss two main challenges induced by SQL
semantics: stored procedures and large objects.
4.2.1 Stored Procedures
Stored procedures were initially introduced by Sybase and have
been heavily used since then, with many legacy applications
relying on stored procedures. The integration of Microsoft SQL
Server 2005 with the .NET CLR has expanded the use of stored
procedures [32] by allowing them to access thousands of pre-built
classes and routines of the .NET Framework Base Class Library
(BCL). Replication of stored procedures, however, raises several
issues.
Statement replication can only broadcast calls to stored
procedures, so stored procedure execution must be deterministic,
to prevent cluster divergence. As there is no schema describing
the behavior of a stored procedure, it is usually impossible to
know which tables it accesses, thereby limiting concurrency
control options for the middleware. Moreover, by replicating a
stored procedure call, all the read queries will be executed by all
nodes, resulting in no speedup and thus a waste of resources.
As stored procedures are often used to manipulate large amounts
of data without transferring it to the client, performing writeset2
extraction in such a context would be expensive in terms of
resources, thus making it impractical in many cases.
Stored procedure replication is a domain that has been mostly
overlooked by the research community. Even in a master/slave
context, most databases have significant limitations on stored
procedure replication and require the user to expand the stored
procedure definition with ad-hoc extensions. Similar issues can be
observed with user-defined functions.
4.2.2 Large Objects
Large objects, whether text (CLOB) or binary (BLOB), are
implemented differently in different database engines (like many
SQL data types). Object-relational DBMSes provide object
identifiers (OIDs) and an API to retrieve an object’s content.
If the replication middleware relays request and results, it must
track resources properly to prevent the stream from remaining
open indefinitely upon user program errors or failures. Moreover,
certain drivers provide fake streaming APIs and require the
application to have enough memory to hold the entire object in
2
Writeset: the set of data W updated by a transaction T, such that
applying W to a replica is equivalent to executing T on it.
memory. Hence, multiple large objects, when streamed
simultaneously, may quickly overwhelm the replication
middleware.
4.2.3 Sequences
Database sequences, used to generate unique or auto-incremented
keys, have only been standardized in SQL-2003 [12]. Even if, in
most implementations, sequences can be retrieved as part of the
database schemas, these objects are not persisted in the
transactional log. This results in the need for workarounds to
backup and restore sequences consistently with the other data.
Additionally, sequences are non-transactional database objects, so
they cannot be rolled back. Sequence numbers generated for a
failed query or transaction are lost and generate “holes” in the
sequence of numbers. Moreover, sequence semantics vary
significantly among implementations; in most implementations,
they bypass isolation mechanisms such as MVCC and are subject
to subtle ordering problems.
4.3 Middleware-level Challenges
Middleware-based replication uses a middleware layer between
the application and the database engines to implement replication,
and there are multiple design choices for how to intercept client
queries and implement replication across multiple nodes. We
describe the most common alternatives with their pros and cons.
4.3.1 Intercepting Queries
Query interception needs may force driver changes on the
application side as database protocols evolve over time. A new
driver on the application side might offer new functionality, such
as support for transparent failover or load balancing. Moreover,
protocol version implementation may vary from one platform and
language to another. For example, each MySQL JDBC, ODBC
and Perl driver has its own bugs and ways to interpret the
protocols. The Microsoft SQL Server JDBC driver and the
FreeTDS open source implementation also exhibit different
behavior, even though they are based on the exact same TDS
protocol, originally designed by Sybase. Hence, it is difficult for
the middleware to infer the application’s intentions from the
various implementations of a protocol in different drivers. In
addition, some drivers exploit loopholes in the protocols to carry
information for database extensions, such as geographic
information services (GIS). This makes it even more difficult for
the middleware to distinguish illegitimate input from
undocumented extensions.
Updating drivers on the client side can be a real showstopper for
sites with large clusters of application servers. If a customer has,
e.g., 500 client machines accessing a cluster of 4 database server
nodes, updating the driver is orders of magnitude more complex
than upgrading the four nodes.
While JDBC and ODBC cover a large portion of database access
methods for most recent applications, native APIs are still widely
used by PHP and legacy applications. Supporting all APIs on all
platforms quickly becomes unrealistic; for example, MySQL
provides 14 main programming APIs for a database engine that is
used on 16 different platforms (14 x 16 = 224 combinations).
4.3.2 Statement vs. Transaction Replication
Multi-master replication can be implemented either by
multicasting every update statement (i.e., statement replication) or
by capturing transaction writesets and propagating them after
certification (i.e., transaction replication). Both approaches face
significant challenges when put in production with real
applications.
Non-deterministic queries are an important challenge: statementbased replication requires that the execution of an update
statement produce the same result on each replica. However, SQL
statements may legitimately produce different results on different
replicas if they are not pre-processed before being issued.
Time-related macros such as ‘now’ or ‘current_timestamp’ are
likely to produce a different result, even if the replicas are
synchronized in time. Simple query rewriting techniques can
circumvent the problem by replacing the macro with a hard-coded
value that is common to all replicas. Of course, all replicas must
still be time-synchronized and set in the same timezone, so that
read queries provide consistent results.
Other macros, such as ‘random’ or ‘rand’, cannot always be
replaced by a statically computed random number. Consider a
statement like ‘UPDATE t SET x=rand()’—a database engine
would assign a different random value to each row of table t.
Rewriting the query to hardcode a value like ‘UPDATE t SET
x=5’ assigns the same value to each row, which was evidently not
the programmer’s intention. In this case, transaction replication
would do the right thing, while statement replication would not.
Other queries may have non-deterministic results. For example,
SELECT … LIMIT can create non-deterministic results in
UPDATE statements. In ‘UPDATE FOO SET KEYVALUE=‘x’
WHERE ID IN (SELECT ID FROM FOO WHERE KEYVALUE
IS NULL LIMIT 10)’, the SELECT does not have an ORDER BY
with a unique index. Therefore, broadcasting such a statement can
cause each replica to update a different set of rows leading to
divergence in the cluster.
Writeset extraction is usually implemented using triggers, to
prevent database code modifications. This requires declaring
additional triggers on every database table, as well as changing
triggers every time the database schema is altered. This can be
problematic both from an administrative as well as a performance
standpoint when applications use temporary tables. If the
application already uses triggers, writeset extraction through
triggers might require an application rewrite. Materialized views
also need special handling, to avoid duplicate writeset extraction
by the triggers on the view and those on the underlying tables.
Writeset extraction does not capture changes like autoincremented keys, sequence values, or environment variable
updates. Queries altering such database structures change the
replica they execute on and can contribute to cluster divergence.
Moreover, most of these data structures cannot be rolled back (for
instance, an auto-incremented key or sequence number
incremented in a transaction is not decremented at rollback time).
Statement-based replication, at least, ensures that all these data
structures are updated in the same order at all replicas. With
transaction replication, if no coordination is done explicitly from
the application, the cluster can end up in an endless effort to
converge conflicting key values from different replicas.
Locking and performance are harder issues in statement-based
replication. In particular, locking granularity is usually at the table
level, as table information can be obtained through simple query
parsing; however, this limits performance. Finer granularity (i.e.,
row level) would require re-implementing a large fraction of the
database logic inside the middleware. Moreover, the middleware
locking regime might not be compatible with the underlying
database locking, leading to distributed deadlocks between the
databases and the middleware.
reliable communication channels. Reliable failure detectors are
critical for failover and failback.
4.3.3 Failover
A large body of research has been devoted to group
communication protocols (a survey appears in [17]). Database
replication requires reliable multicast with total order to ensure
that each replica applies updates in the same order. Even though
various optimizations have been developed, the group
communication layer is an intrinsic scalability limit for such
systems.
Failover requires one or more failure detection mechanisms, so
that everyone in the system can identify the same set of faulty
components. Most problems are related to network timeouts
(explained in Section 4.3.4). The state-of-the-art in failover has
surprisingly not evolved much during the past decade. Even if
failover does not require a system reconfiguration with automated
reconnection mechanisms, in-flight sessions are still lost.
MySQL provides automatic reconnection inside its drivers, and
application servers, like WebLogic [5], use multiple connection
pools (multipools) for failover purposes. These techniques, or the
one proposed in [22], offer session failover, but not failover of the
transactional context. To the best of our knowledge, Sequoia [30]
(the continuation of the C-JDBC project) is the only middleware
that provides transparent failover without losing transactional
context. Failover code is available in the middleware to handle a
database failure, and additional code is available in the client
driver to handle a middleware failure. Fully transparent failover
requires consistently replicated state kept at all components, and is
more easily achieved using statement-based rather than
transaction-based replication. In the latter case, the transaction is
only executed at a single replica; if the replica fails, the entire
transaction has to be replayed at another replica, which cannot
succeed without the cooperation of the application.
Even though a replicated database could handle internal
component failures transparently to the clients, there is currently
no API to pause, transfer and resume transaction contexts.
Phoenix/COM+ [24] enhances the .NET runtime to serialize
ODBC connection state in the database and allows COM-based
applications to recover and failover transparently. Application
server clusters typically operate on top of a replicated database; in
the case when an application server replica fails, there is no way
for the other replicas to retrieve the database connections of the
failed replica and continue its transactions—even though the
underlying database is capable of transparent failover. This is a
manifestation of the more fundamental problem of failures being
treated in isolation by each tier in a multi-tier architecture.
Systems need to define user sessions and transaction contexts that
cross tier boundaries, in order to treat failover issues globally and
to ensure transparent failover throughout the system.
Connection pools are usually a major issue for failback. At failure
time, all connections to a bad replica will be reassigned to another
replica, or just removed from the pool. When the replica recovers
from its failure, it requires the application to reconnect explicitly
to that replica; this can only happen if the client connection pool
recycles aggressively its connections, but this defeats the
advantages of a connection pool. Most database APIs do not
provide information on the endpoint of a database connection.
Therefore, it is not possible for the connection pool to distinguish
between connections to different replicas. In order to implement
new load balancing and failover/failback strategies in connection
pools, more contextual information on database connections is
needed through standard database APIs.
4.3.4 Networking
Replicated databases are a distributed system, so they have to deal
with network communication and related problems. As data loss is
not acceptable during normal operation, it is necessary to have
4.3.4.1 Group Communication
Group communication performance varies according to a large
number of parameters [1], making configuration and tuning a real
headache in practice. Even the developers of Spread, the most
widely used group communication toolkit, admit that tuning UDPbased group communication is challenging even to a specialist.
There is a subtle multi-dimensional tradeoff between ease of
configuration (static vs. dynamic group membership),
performance (UDP multicast vs. TCP performance in network
switches, UDP multicast parallelism vs. TCP packet duplication,
etc.), flow control (TCP flow control vs. credit-based flow control
on UDP) and reliability (TCP in-kernel implementation with
KeepAlive timeouts vs. UDP user-space error management
implementations with tunable timeouts). Even though some issues
can and must be addressed at the group communication level,
cooperation with the replication middleware is key. For example,
it is inefficient to perform state transfers when a new replica joins
a cluster using group communication, because of the large amount
of state to transfer.
Recent efforts have tried to extend multi-master replication to
WAN environments [23]. The network latency and unreliability of
long distance links are still making it impractical to have any
reasonable production implementation of fast reliable multicast.
Even though bandwidth availability is greatly improving, latency
is unlikely to evolve dramatically on worldwide distances due to
physical limitations. In practice, asynchronous replication is
preferred over long distance links when replicating data between
remote sites. Applications are usually partitioned and written
using ad-hoc techniques to work around current technology
limitations.
It is unlikely that group communication alone will be able to solve
the database replication problem over WAN. 1-copyserializability is unlikely to be successful in the WAN by
extending existing LAN techniques. New data access models will
have to be proposed to address the fundamental differences that
one has to face when replicating in a WAN environment.
4.3.4.2 TCP/IP Communication
Database drivers currently communicate with DBMSes through
TCP connections, because TCP offers reliable communication
with flow control that is efficiently implemented in the operating
system kernel. However, TCP relies on timeouts to detect
connection failures. Even though it is technically feasible to set up
TCP timeouts on a per-connection basis, all drivers we know of
rely on the default system-wide settings.
Upon a network failure, the TCP communication is blocked until
the keep-alive timeout expires. This results in unacceptably long
failure detection (ranging from 30 seconds to 2 hours, depending
on the system defaults). Even though external heartbeat
mechanisms are used to detect node failures, connections remain
blocked on the client or server or both sides until the TCP
timeouts expire. It is impractical to devise failover solutions for
in-flight transactions over TCP, unless the underlying OS can be
configured for each installation.
Altering operating system settings for the TCP KeepAlive value
affects all applications running on that machine, and that is
usually undesirable. A shorter TCP KeepAlive value generates
false positives under heavy load by classifying slow connections
as failed. Database drivers must either not rely on TCP for
database communication, or have a built-in heartbeat mechanism
for reliable and timely detection of connection failures.
4.3.4.3 Network Partitions
The issue of network partitions or “split brain” has been addressed
by the research community mostly at a theoretical level; typically
quorum-based solutions [19] are used. In practice, however, nodes
often fail simultaneously (e.g., due to a rack-level power outage,
or a network switch failure). If the remaining quorum does not
constitute a majority, the system must shut down and make the
customer unhappy.
The “CAP Principle” [15] states that a distributed storage system
can favor any two of Consistency, high Availability, or resilience
to Partitions. Systems such as search engines usually favor A and
P over C, whereas a replicated database necessarily must favor C
and A over P. The most common approach to handling network
partitions is, therefore, to try and avoid them. If a network
partition occurs, detecting it can be challenging, especially if the
partition was due to a transient failure. When the system is
partitioned, updating each partition independently leads to replica
divergence. Some ETL and reconciliation tools do exist for fixing
this [7], but the process remains largely manual; reconciliation
policies are typically ad-hoc and application-dependent.
Partitions over WAN configurations usually require manual
intervention of the human administrators at the various sites. If the
network is indeed down, phone calls are usually used to diagnose
the failure accurately and to coordinate a plan of action. The
failover procedure usually has a wider scope than just the database
replication system and typically involves DNS failover and other
network-related reconfigurations.
4.4 System Management-level Challenges
Performing backups and adding/removing replicas are standard
management operations for replicated databases. However, many
customers desire at least 5 nines of availability for their database
replication systems (99.999% availability, or at most 5 minutes of
downtime per year), including all planned and unplanned
downtime—this places tremendous pressure on the administrators.
In this section we highlight some of the main challenges in
managing replicated database systems: backup (§4.4.1),
adding/removing replicas (§4.4.2), software upgrades (§4.4.3),
routine maintenance (§4.4.4), and performance evaluation
(§4.4.5).
4.4.1 Backup
Backup is part of normal database system operation, but is also
fundamental in a replicated system, because backups are used to
bring new replicas up-to-date. For most problematic backup and
restore operations, databases can be taken offline (cold backup).
ETL tools usually use database-specific extensions to access
information such as user access rights, stored procedure or trigger
definitions. Hot backup techniques exist, but they are still limited,
because they only provide a read-consistent copy of the database,
without handling active transactions. Database performance is
typically degraded during backup. For example, in Oracle, when a
database block is modified for the first time since the backup
started, the entire block is written into the online redo logs. Under
normal operation, only the changed bytes are written.
Since backup operations can take several hours, depending on
how large the database is, it is important for hot backups and
incremental backups to interplay with the replication middleware.
It is unreasonable to expect applications that use large databases
with high update rates to rely on cold backups with replication,
since the backup time is not only the time it takes for the data to
be dumped, but also the time needed to resynchronize the replica
by reapplying all updates missed while doing the backup.
Thus, it is necessary for the replication middleware to collaborate
with the replica and the backup tool, to make sure that the dumped
data is consistent with respect to the entire cluster. This means
that the middleware must be aware of exactly which transactions
are contained in the dump and which ones must be replayed (or
have their writesets applied), to properly resynchronize a backend.
Replication middleware that supports partial replication affords a
variety of optimizations for backup/restore.
4.4.2 Adding/Removing Replicas
Over time, replicas have to be removed from the system, usually
for maintenance operations. If the replica is removed from the
system due to a failure, a recovery operation is needed. Many
systems, like MySQL cluster, require the entire cluster to be shut
down and all replicas to be synchronized offline when adding new
replicas. This implies long downtimes and unhappy customers.
Other solutions, like Emic Networks m/cluster, systematically use
an active replica, bring it offline to transfer its state to the added
replica, and then apply to both replicas the updates that occurred
during transfer. This has the disadvantage of bringing the system
down when only one replica is left in the system. Also, a node has
to be taken offline when adding a new replica, which reduces
performance during the operation. If the new replica is added with
the intention of boosting performance, the operation has to be
carefully planned, since overall system performance drops for the
whole duration of the synchronization.
Sequoia [30] uses a recovery log that records all update statements
executed by the system. When a node is removed from the cluster,
a checkpoint is inserted, pointing to the last update statement
executed by the removed node. When the node is re-added to the
system, the recovery log is replayed from the checkpoint on.
Offline nodes that have been properly checkpointed by the system
can also be backed up. The resulting data dump can be used to
initialize new replicas and resynchronize them from the recovery
log, without having to use resources of active replicas.
Minimizing the cost of a cluster-wide checkpoint, while
respecting transaction boundaries, is still an unsolved problem.
Replaying the recovery log to resynchronize a replica requires the
extraction of parallelism from the log to prevent reapplying
updates serially, in which case a new replica may never catch up
if the workload is update-heavy. Once a replica has replayed the
entire recovery log, it is also necessary to enact a global barrier, to
ensure that no in-flight request is missed by the newly-added
replica. A large body of optimizations could be operated on
replica synchronization, to minimize the resources and the time
necessary to get to the online state.
Failures could often be recovered relatively easily. E.g., a replica
might stop working because its log is full or its data partition ran
out of space. However, the replication middleware has often no
information on which transactions committed successfully prior to
the failure; this information is only known to the database. As
there is no standard API to query a database about the status of
transactions, usually a full recovery has to be performed— in
large production databases, this means hours of dump/restore and
resynchronization. Fast resynchronization of failed nodes is as of
yet still a hard problem.
Autonomic provisioning of database replicas [9] depends to a
large extent on the system’s ability to add and remove replicas.
Being able to model and predict replica synchronization time and
its associated resource cost is key to efficient autonomic
middleware-based replicated databases. Determining the relevant
metrics and exporting accurate resource usage predictions are
challenges that need solutions before we can expect major
breakthroughs in the area of autonomic replicated databases.
4.4.3 Software Upgrades
Software upgrades are part of planned maintenance operations. In
a replicated database, there are three different types of
components that can be upgraded: the engine itself, the replication
middleware, and the drivers (database driver, middleware driver,
or both, depending on the design).
Database upgrade. Database upgrades are usually relatively easy
between minor releases, where a simple patch can be applied.
Upgrades between major version numbers often require migration
tools for both configuration and data files. If the replication
middleware requires database modifications or extensions, it
might not be possible to upgrade databases one by one without
bringing the entire cluster down. A database upgrade while
keeping the system online requires the replication middleware to
support (at least temporarily) a heterogeneous cluster
configuration, possibly using different driver versions for old and
new database versions.
Middleware upgrade. As any software component, the replication
middleware itself must be upgraded. If all components are
replicated, it might be possible to upgrade them one by one,
relying on standard failover techniques to handle the online
upgrade. However, the protocols between replicated components
must remain compatible, so that the old version of an upgraded
component can still communicate with the newer version during
the upgrade. This might require additional engineering, like onetime migration protocols, to allow upgrades between major
versions of the replication middleware. If the replication
middleware relies on a group communication library, upgrading
the library requires protocol compatibility between versions.
Driver upgrade. Driver upgrades are rarely viewed as part of the
database upgrade problem. However, it is quite common to have
large web sites with tens or hundreds of application servers
connecting to the same replicated database system. In such cases,
upgrading the drivers is a much more complex issue than the
database upgrade itself, which only affects a small number of
machines and configurations.
4.4.4 Routine Maintenance
When running a production system, logs have to be purged,
indexes have to be rebuilt, optimizer statistics need to be updated
(vacuum-like operations), backups have to be made, etc. These
operations have a significant impact on database performance
both during and after their execution. Significant engineering
efforts have gone into simplifying and automating database
maintenance, but most of these efforts target centralized
databases, leaving many open issues for replicated databases.
So far, there are no accepted “best practices” for performing
maintenance on a replicated database system. What operations
must be executed sequentially or in parallel? What is the impact
on load balancing policies? Is it better to execute operations
online or on an offline replica?
The vast majority of production systems have a monitoring
infrastructure. Failures can be detected and reported by different
sensors. It is not clear what interactions the database management
framework should have with this global monitoring infrastructure.
Whose responsibility is it to trigger an automatic replica repair
operation when a failure is detected? A classification of replicated
management operations is necessary in order to define the needed
sensors, actuators, and to implement good management policies.
4.4.5 Performance Prediction
Database replication is often envisaged as a solution for
performance issues. However, database replication usually only
provides scalability, that is, if one adds resources proportionally to
the load increase, the performance perceived per client will
remain constant. Furthermore, the replication middleware itself
imposes an overhead that often deteriorates query latency.
More insight is needed into the latency deterioration induced by
moving from a single database to a replicated system, when the
load could be handled without contention by the resources of a
single database. We have observed that, when faced with
workloads that have little parallelism, replicated databases usually
perform poorly when load is low, because low latency is critical to
the performance of sequential (non-parallel) queries. For example,
a sequential batch update script will usually run much slower on a
replicated database than on a single-instance database. OLTPstyle sub-millisecond queries suffer the most from latency
overheads imposed by the replication middleware, more so than
heavyweight queries that take seconds or minutes to execute.
The lack of tools for accurately predicting the performance of a
replicated database makes it difficult to properly size a system
(“capacity planning”) or to estimate the scalability limits of a
system. As a result, database clusters tend to be small, between
2-4 replicas, rarely going up to 8 replicas. When it comes to
OLTP databases, users feel safer to invest in fewer powerful
machines than several less powerful machines. This helps limit
complexity, as well as save on licensing and maintenance costs.
5. BRIDGING THE GAP
We now suggest a number of directions and areas for both
academic research and industrial R&D, where innovative
solutions could have a significant practical impact.
5.1 An Agenda for Academic Research
Tradeoffs for portability, upgradability, and performance vary
across different designs for intercepting queries at the middleware
level. Statement and transaction-based replication offer tradeoffs
between performance, availability, and intrusiveness into the
application or the RDBMS engine. Availability aspects, such as
single points of failure cannot be overlooked. Failover and
failback are tightly coupled with the usual networking issues and
the black art of tuning timeouts. Performance is usually limited by
group communication or writesets propagation. We see ample
opportunities for optimization in systems operating under partial
load or capacity, or at low consistency levels. A practical,
deployment-worthy solution must address all these issues.
Research prototypes have mainly focused on performance, largely
ignoring other system aspects. Every additional functionality,
however, impacts the replication middleware’s design, so the
practicality of proposed concepts can only be assessed in a global
context.
Middleware design. Database replication requires new
abstractions in order to replicate more than one database instance
at once. A RDBMS may host multiple database instances that
appear as a single logical unit to the application or the customer.
RDBMS replication poses new challenges for inter-database
queries and cross-database management operations.
Partial replication is also a challenge: tables cannot be arbitrarily
replicated, since queries might span over multiple tables and
require distributed joins to perform select or update operations.
Database backup is a complex distributed operation, since it might
require multiple replicas to obtain a full consistent snapshot.
Adding or removing partial replicas while still offering
availability and service continuity is a completely open problem.
Stored procedure execution should be handled by the replication
middleware. New algorithms are needed for optimizing the
cluster-wide execution of stored procedures. If stored procedures
were compiled in the middleware instead of the DBMS, queries
and transactions could be better scheduled and balanced.
Consistency. SI and its variations attract substantial attention, as
they improve performance over 1SR. Most probably, new
optimizations or consistency models will be developed to address
different needs. These new models, such as eventual
consistency [34], could also require applications to be written
differently, to better cooperate with the database and with new
architectures, such as computing clouds. We ought to extract from
past work the necessary interfaces and abstractions needed from
both the replication middleware and the DBMS and to make these
protocols pluggable in a replicated system. This would both
encourage industry to provide standard APIs for replication and
foster new research into other consistency models (e.g., targeting
the very common read-committed transaction isolation level).
WAN environments impose both latency and availability
constraints that are different from geographically centralized
clusters. New consistency models for the WAN that are less strict
than 1SR or SI, but stronger than fully asynchronous replication,
require new protocols and probably programming paradigms.
SPOF and Availability. Production systems cannot tolerate any
single points of failure, since service continuity is key. All
management operations (e.g., backups or adding a replica) should
be doable without service interruption. This requires fully
transparent failover and failback that go beyond standard
application/database boundaries. Availability might have to be
thought of more globally, so that all failure detection mechanisms
can synchronize to take proper coordinated actions. Connections
and transactions should be addressed globally, so they can be
transferred or failed over. Recovery procedures, distributed
checkpointing, and replica state reconstruction are vast areas to
investigate. Research on autonomic replicated databases ought to
expand beyond performance, to tackle all aspects of availability.
Software upgrades are inevitable. All components including
driver, middleware and database engine, must be upgradeable
without service interruption. New solutions are required to allow
such upgrades and to minimize the duration of these operations. A
system with 5 nines of availability can be unavailable for no more
than 5.26 minutes per year—this number marks the sole
acceptable upper bound when evaluating new availability
techniques. Similarly, metrics such as MTTF and MTTR should
be considered when evaluating a design and/or prototype.
Evaluation. As database replication is about more than peak
throughput, it is necessary to assess performance in the presence
of failures, in degraded modes, as well as under low loads.
Another area of performance that is not evaluated is the impact of
management operations and faults on the system. New availability
metrics should be defined, or combined with performance metrics,
to better assess true overall performance.
To this end, researchers need new benchmarks that are not
necessarily closed-loop systems, that could integrate fault
injection or management operations. It would be interesting to
have a wider variety of workloads, or to be able to capture
workloads from existing applications. Even though it is possible
to capture in various logs the execution of a workload, we know
of no way yet to replay that exact same workload: the inherent
parallelism in the original workload implies non-determinism in
the execution order that is decided by the DBMS. Replaying a
statistically equivalent workload is possible, but replaying the
exact original workload at the same level of parallelism while still
providing the same execution order requires instruction-level
hardware simulation, which is still very expensive.
5.2 An Agenda for Industrial R&D
Database-agnostic middleware replication is challenging in
practice, due to database implementation discrepancies. We have
identified variations at the RDBMS level in transactional
behavior, SQL semantics (e.g., temporary tables, stored
procedures, large objects) and access control mechanisms. Despite
efforts such as the GORDA project [16], the lack of
standardization also affects management operations as well as
recovery procedures. Hardware and software upgrades without
service interruption lead to temporarily heterogeneous clustering
and require innovative solutions.
Integration. Innovation in middleware-based replication with
commercial databases has been limited to interactions using
publicly available APIs. Workarounds, such as triggers for
writeset extraction, have been implemented, but industry
standards should be defined to better integrate with middleware
replication. New mechanisms have to be developed to allow
replication middleware to plug its own replacement for nondeterministic functions (e.g., time or random number functions).
Transaction abstraction. The notion of transaction as currently
exposed by databases should be expanded to provide additional
meta-information, such as readset and writeset, lock set, execution
cost, etc. Having such information would allow for higher
diagnosability at recovery time, would enable more efficient
caches at the middleware level, and would improve decisions on
which transactions to abort. Transactions are currently tightly
coupled with driver connections—when the connection is lost, the
transaction is lost; this precludes failover. It is currently not
possible to pause a transaction, serialize and transfer transaction
state to another connection, and resume a transaction.
Management. Backup/restore operations have to be improved to
capture a consistent snapshot of a database without limiting
themselves to data content. User information, access rights, views,
triggers, and stored procedures must also be captured if a replica
is to be properly cloned. The lack of standardization in this area
thwarts further development of heterogeneous clustering with
different database engines.
Software upgrades. Finally, software upgrades are not only a
problem for the database engine itself, but for all applications
requiring a driver upgrade. New language runtimes, such as recent
Java virtual machines, allow on-the-fly replacement of classes’
implementations. This offers an infrastructure for dynamically
upgrading drivers at the client side. The complexity of driver and
database deployment could be considerably reduced by rethinking
the driver lifecycle. Drivers used by client applications could be
reduced to a minimum bootstrap, the database server providing
the appropriate driver code at the first connection. Similar
approaches could be used for database or middleware drivers.
6. CONCLUSION
In this paper, we reported challenges faced by middleware-based
replication systems when deployed in production settings at real
customers. We identified performance, availability and
management issues that are insufficiently (or not at all) addressed
by academic prototypes. Availability, in particular, poses several
unsolved problems, from reliable failure detection to transparent
failover/failback. Common management operations such as
backups and hardware/software upgrades require the focused
attention of the research community, to provide innovative
solutions and optimizations. Database performance is not an issue
that can be separated from availability and management issues.
We proposed four main themes to be investigated by academic
research: replication middleware design, consistency, availability
and evaluation. We also suggested new approaches to industrial
R&D: improving the integration of databases with replication
middleware, rethinking the way in which transactions are exposed
to applications, standardizing management operations, and
simplifying software upgrades.
7. ACKNOWLDEGEMENTS
We are grateful to our academic and industrial partners for their
feedback and contributions and especially to Gustavo Alonso,
Robert Hodges, Bettina Kemme and Dheeraj Pandey for their
detailed comments which helped improve the final version of this
paper. This work was partially supported by NSF grants IIS0133686, CCR-0205544, and IIS-0713409.
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