Database Trends and Directions: Current Challenges and

Database Trends and Directions: Current Challenges and
Database
Trends
and Directions:
Database
Trends and
Directions:
Current Challenges
and Opportunities
Current Challenges
and Opportunities
George
GeorgeFeuerlicht
Feuerlicht1,21,2
Department of Information Technology,
Department
Information
Technology,
University
of Economics,
Prague
University ofofEconomics,
Prague,
W. Churchill
Sq. 4, Prague,
Czech Republic
2
W.
Churchill
Sq. 4, Prague,
Czech Republic
Faculty
of Engineering
and Information
Technology,
University of Technology, Sydney,
P.O. Box
Broadway,Technology
Sydney, NSW
2007, Australia
Faculty of Engineering
and123
Information
University
of Technology, Sydney
P.O. Box 123 Broadway, Sydney, NSW 2007, Australia
[email protected]
1
1
2
Abstract. Database management has undergone more than four decades of
evolution producing vast range of research and extensive array of technology
solutions. The database research community and software industry has
responded to numerous challenges resulting from changes in user requirements
and opportunities presented by hardware advances. The relational database
approach as represented by SQL databases has been particularly successful and
one of the most durable paradigms in computing. Most recent database
challenges include internet-scale databases – databases that manage hundreds of
millions of users and cloud databases that use novel techniques for managing
massive amounts of data. In this paper we review the evolution of database
management systems over the last four decades and then focus on the most
recent database developments discussing research and implementation
challenges presented by modern database applications.
Keywords: Relational Databases, Object-Relational Databases, NoSQL
Databases
1 Introduction
Databases, in particular relational databases, are a ubiquitous part of today’s
computing environment. Database management systems support a wide variety of
applications, from business to scientific and more recently various types of internet
and electronic commerce applications. Database management systems (DBMS) are a
core technology in most organizations today and run mission-critical applications that
banks, hospitals, airlines, and most other types of organizations rely on for their day
to day operation. Over the last three decades relational DBMS technology has proven
to be highly adaptable and has evolved to accommodate new application requirements
and the ever-increasing size and complexity of data. But, there are indications that
some of the recently emerging data-intensive applications (e.g. internet searches)
cannot be satisfactorily addressed using existing DBMS technology, and some experts
argue that significant innovation is needed (a new database paradigm) to overcome
the limitations of the current generation of database technology.
The combination of inexpensive and high capacity storage and the prevalence of
digital devices (digital cameras, sound recorders, video recorders, mobile phones,
J. Pokorný, V. Snášel, K. Richta (Eds.): Dateso 2010, pp. 163–174, ISBN 978-80-7378-116-3.
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RFID readers, and various types of sensors) is creating a deluge of digital
information. According to a recent article in the Economist [1] the amount of data
collected by various sensors, computers, and devices is growing at a compound
annual rate of 60%. A 2008 study by International Data Corporation (IDC) predicted
that over a thousand exabytes of digital data will be generated in 2010 [2]. Scientific
applications in astronomy, earth sciences, etc. (e-science) tend to produce massive
amounts of data; well-documented examples include the Large Hadron Collider at
CERN [3] that generates 40 terabytes of data every second. Storing and analyzing
such volumes of data represents an insurmountable challenge for the current
generation of database technology. Another relatively recent development that may
require a revision of current database paradigms are internet-scale applications (e.g.
search engines, social networking applications, cloud computing services, etc.) that
typically process petabytes of data, use thousands of servers, and serve millions of
users that demand sub-second access to information. Companies like Google,
Facebook, Amazon, and eBay manipulate petabytes of data every day. For example,
Facebook handles 20 petabytes of data, managing 20 billion photographs in 4
different resolutions, growing by 2 billion photographs per month. The Facebook
database is serving 600,000 photographs per second for a user base of 300 million
active users [4]. Google manages vast amounts of semi-structured data: billions of
URLs with associated internet content, crawl metadata, geographic objects (roads,
satellite images, etc.), and hundreds of terabytes of satellite image data, with hundreds
of millions of users and thousands of queries per second [5]. The scale and level of
functionality required for such “big data” applications has not been anticipated by
commercially available DBMSs, and almost invariably internet companies were
forced to develop their own database solutions. But, even more traditional database
applications manage increasingly large volumes of data; for example the retail chain
WalMart handles more than one million transactions per hour, and manages databases
with more than 2.5 petabytes of data.
It is estimated that structured data constitutes only about 5% of the total volume of
generated data, with the rest of this “digital universe” in semi-structured or
unstructured form, making it more difficult to manage and to extract meaningful
information from it. This massive increase in the volume and complexity of data is
challenging available database management techniques and technologies, forcing a reevaluation of the direction of database research. Some fundamental questions arise,
including what constitutes a database application. Can applications that search
petabytes of unstructured data (e.g. Web pages) using thousands of servers working in
parallel be classified as database applications?
In this paper we firstly review the past achievements of database research and
technology solutions (section 2), and then discuss the research challenges and
opportunities created by new types of database applications (section 3). The final
sections (section 4) are our conclusions.
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2 Evolution of Database Technology
While the origin of commercial database management systems can be traced to
hierarchical and CODASYL (Conference on Data Systems Languages) databases of
1960s and 1970s it was the emergence of relational DBMS during the 1980s that
started a revolution in data management. The simplicity and elegance of the relational
model proposed by E.F. Codd in 1970 [6] resulted in unprecedented volume of
research activity and the emergence of highly successful relational DBMS (RDBMS)
implementations. Relational databases are a rare example of a theoretical model
preceding and guiding the implementation of technologies. Codd is often credited
with turning the previously black art of data management into an engineering
discipline providing a blueprint for the design and implementation of databases and
the foundation of modern database technology. The basic idea of the relational model
is to represent data as two-dimensional tables with well-defined properties and to use
of a high-level query language for data access. This remarkably simple set of ideas
based on the underlying relational theory had a major impact on the development of
database technology over the following two decades. Relational databases solved two
major interrelated problems of the earlier database approaches. The first achievement
was to de-couple the database from application programs by providing effective
support for data independence. Second, and equally important achievement of the
relational approach was to free database application developers from the burden of
programming navigational access to database records by introducing a non-procedural
query language.
A number of different relational languages were proposed following Codd’s
original description of the relational model, notably a language called QUEL (Ingres
DBMS) developed at University of California at Berkeley, and IBM’s Structured
Query Language (SQL) developed at the IBM San Jose Research Laboratory. The
next major milestone in the evolution of relational databases was the acceptance by
ANSI (American National Standards Institute) of a subset of IBM’s SQL as the first
version of the standard relational database language - SQL86. Although SQL86
lacked many important features of the relational model as originally proposed by
Codd, including key aspects of the model such as referential integrity and domains, it
quickly became universally accepted as the database language for relational DBMS
systems. The shortcomings in SQL86 were largely rectified in the subsequent releases
of the SQL standard (SQL89, SQL92) and SQL has evolved from a relatively simple
language into a comprehensive database language implemented in all significant
RDBMS products today. Many of the enhancements incorporated into SQL over the
last two decades were integral features of the relational model omitted from the earlier
standard specifications, other features, such as triggers, role-based security, and stored
procedures were retrofitted into the standard as a result of their widespread use in
commercial products.
Given the computing environment of the 70s and early 80s, relational databases
were initially used for relatively simple business applications running on large
mainframe computers; data used in such traditional business applications (e.g.
financial and banking) can be structured into tables and stored in a relational database
with relative ease. The main concern of early RDBMS implementations was to ensure
adequate performance, in particular for online transaction processing (OLTP)
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applications. Initially, relational DBMSs had inferior performance when compared
with earlier DBMS approaches as SQL uses expensive join operations and relies on a
query optimizer to determine how to access data records instead of using faster
pointer-based navigational access implemented in hierarchical and CODASYL
databases. For that reason the main use of relational DBMSs was initially confined to
decision support applications that did not involve users waiting for query results
online. However, as computer hardware became more powerful and optimization
techniques improved, relational systems became the technology of choice in most
application environments, including those with stringent response time requirements.
Relational DBMSs proved to be extraordinarily successful in taking advantage of
new computing platforms, architectures and environments. The first significant
demonstration of the adaptability of relational databases was the extension of the
relational model to cover distributed database environments. The origin of distributed
relational database was IBM’s research project System R* (continuation of the Project
R) which addressed distributed database issues including distributed query
optimization, distributed transactions, and catalog management. Following on from
the System R* database researchers solved most of the problems that concern running
applications transparently across multiple databases. Most commercial RDBMSs
incorporate a whole range of distributed database features, including reliable (twophase commit) distributed transactions, optimized distributed queries, and advanced
replication facilities. Similarly, relational DBMSs were among the first technologies
to support applications with large number of users in distributed client/server
environments. This was largely due to the non-procedural nature of SQL, which made
it possible for database queries to be packaged and send over a computer network as
messages from a client application to a database server. This type of client/server
interaction later supplemented with remotely executed database stored procedures
using RPC calls (Remote Procedure Calls) enabled the implementation of scalable
client/server database applications.
Relational DBMSs were quick to take advantage of the new multiprocessor
architectures and provide support for parallel execution of SQL queries. Query
decomposition, necessary for parallel execution is made possible by the declarative
nature of the SQL language enabling queries to be decomposed into well-defined subqueries that run in parallel across multiple processors. Parallel SQL was implemented
for shared memory, shared-disk, and shared-nothing parallel architectures with
excellent performance and scalability. Both distributed and parallel databases benefit
from the theoretical underpinning provided by the relational model. As a result of
such developments relational databases became the fastest and most scalable
commercially available DMBS systems.
2.1 Objects and Databases
RDBMs have shown remarkable ability to take advantage of new computing
platforms and continuously improve functionality, performance and scalability to a
point where relational databases became the dominant database technology in 1990s,
supporting mission-critical environments with tens of thousands of users. However,
by mid 90s it became quite clear that the simple data structures and a limited set of
Database Trends and Directions: Current Challenges and Opportunities
167
data types that characterize SQL92 relational DBMSs constitute a significant
drawback when implementing new types of applications that use complex data.
Modern database applications are characterized by four categories of requirements:
(1) need to store and manage large multimedia data objects – images, sound
clips, videos, maps, etc.
(2) requirement for database data types to mirror application-level data types,
including the ability for users to define their own data types as needed by
specific applications
(3) representation of complex relationships, including composition and
aggregation, e.g. multi-level component assemblies used in CAD (ComputerAided Design) and similar applications
(4) need for seamless integration with object-oriented programming languages;
with Java in particular
Such requirements are particularly evident in applications that use multimedia data,
GIS (Geographical Information Systems), e-science and web applications. Web
applications typically contain a whole range of multimedia data types such as textual
information, images, video and audio clips, and fragments of program code. Many
modern applications require specialized data types, for example GIS applications
involve spatial data types (e.g. points, lines, polygons, etc.) and spatial operations
(e.g. distance, area, etc.). The initial solution adopted in relational databases to
accommodate non-traditional data (e.g. multimedia, GIS, etc.) was to allow the
storage of large objects (LOBs) as columns in database tables. However, using this
approach multimedia data is treated as unstructured large granularity objects – the
data type of the object is not explicitly recognized by the database type system and
only very limited processing of the object data is supported.
In addition to the need to store large and complex objects in the database, there is
another important requirement that motivated the introduction of object support at the
database level. Most modern applications are developed using object-oriented
programming languages (i.e. Java, C++, C#) and close integration of the database
language SQL with object-oriented programming languages reduces impedance
mismatch (i.e. differences between the type systems, error handling, etc.) with
corresponding improvements in programmer productivity. This requirement, while
not new gained urgency with the emergence of Java as a de facto standard
programming language for internet applications, making it imperative to ensure that
Java objects can be easily mapped into database objects.
While there was a wide agreement within the database research community about
the need to support objects at the database level, there was a considerable divergence
of opinion about how this should be achieved. Two competing approaches emerged:
the revolutionary approach, seeking to develop a completely new fully object-oriented
database solution [7], and the evolutionary approach which took the path of adding
object features to SQL. In early 90s a number of database management systems were
developed ground-up as pure object DBMS (ODBMS) systems with the goal to
address the limitations of relational databases by adopting a completely new database
model with support for objects with unique identifiers, methods, inheritance,
encapsulation, polymorphism and other features commonly associated with object
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systems. The basic idea was to build on top of object-oriented programming
languages and provide persistence for application objects achieving homogeneous
programming environment with close correspondence between application objects
and objects stored in the database. This (revolutionary) approach popularized by the
Object Database Management Group (ODMG) resulted in the proposal for a new
database model and Object Query Language (OQL). As the commercial ODBMS
products appeared on the market and attempted to capture market share from the
established relational DBMSs, many regarded object-oriented databases as the next
generation of database technology destined to supersede relational databases in much
the same way as relational technology superseded earlier databases approaches.
However, this radical attempt to break with the past has been largely unsuccessful as
ODBMSs have not been able to match RDBMS technology in a number of important
aspects, including reliability, scalability and level of standardization. Even more
importantly, while popular in some niche application areas (e.g. CAD/CAM), object
databases have not been able to address the wider requirements of mainstream
corporate applications.
As a response to ODBMS enthusiasts a number of influential database researches
formed a Committee for Advanced DBMS Function with the objective to define the
requirements for the next generation database systems, and published the ThirdGeneration Database Systems Manifesto [8] as a blueprint for future database
development. While recognizing the limitations of relational databases, this important
effort argued that the next generation database systems should subsume the existing
(second generation) DBMSs and preserve the benefits of relational databases, in
particular non-procedural access and data independence. The essential point of
difference from the advocates of object-oriented databases was the insistence on
natural evolution from the existing relational DBMSs technology, and the
implementation of object identity, abstract data types, inheritance, and other object
features as relational database extensions.
The evolutionary approach resulted in a new breed of hybrid Object-Relational
database technology. In retrospect, Object-Relational databases to a very large extent
achieved the original objective of the Third-Generation Database Systems Manifesto,
to preserve the benefits of relational database and at the same time to take advantage
of object features. However, bringing object features into SQL did not turn out to be
an easy task, and the evolutionary approach has struck numerous challenges and
produced a number of changes in direction. At a superficial level there seems to be a
good match between relations and objects, more specifically the concepts of relational
rows and object instances. But, at closer inspection there are deep conflicts between
the two models. For example, encapsulation, a key feature of object systems is
difficult to reconcile with a database query language, as encapsulated data cannot be
queried directly and requires access via methods, imposing unacceptable performance
overheads. Various attempts at the unification of relations and objects using concepts
such as ADTs (Abstract Data Types) have been proposed and discussed extensively
by the ISO WG3 (Working Group 3), the working group responsible for database
language standardization, but failed to gain the necessary wide support. After more
than five years of intensive work by the WG3 Working Group on Database
Languages some of the early ambitious attempts to incorporate object-orientation into
the SQL standard were significantly scaled down. The resulting SQL:1999 standard is
Database Trends and Directions: Current Challenges and Opportunities
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a very pragmatic solution, which addresses the main limitation of relational databases
by enabling data type extensibility, providing the basis for a rich database type system
[9]. The mainstream database vendors (Oracle, IBM, and others) have strongly
endorsed the object-relational approach and actively participated in the development
of SQL:1999, and most leading DBMS products support object-relational features of
SQL:1999.
2.2 XML and Databases
Another challenge to the dominance of relational databases that echoed the efforts to
incorporate object support into databases in the 90s arose approximately a decade
later with the emergence of XML. XML became the de facto standard formatting
language for semi-structured data and has been widely adopted in e-business
applications as a standard data interchange language and a core standard for Web
Services and related technologies. The availability of a standard XML query language
XQuery [10], and a standard schema definition language XML Schema [11], and
numerous other XML tools and languages (XPath, SAX, DOM, XQL, etc.) provided a
basis for the implementation of XML DBMSs. This lead to the development of a
number of research prototypes, e.g. Lore [12], XTABLES [13], SilkRoute [14], and
some commercial products, e.g. Tamino [15].
Native XML Databases (NXD) that store XML documents in their native format
and use XML query languages for retrieval were regarded by some as a new
generation of DBMS technology destined to supersede relational DBMS. However,
similar to ODBMS, NXD did not replace relational DBMS and remain a solution in
niche application domains, mainly in document and content management applications.
A detailed analysis of the benefits and limitations of NXD databases is available in
[16].
As an alternative to the Native XML Database approach, ORDBMSs (e.g. Oracle)
provide a repository functionality called XML native type to store XML documents in
the database without any conversion, and support updates, queries, indexing, and
views on this data type. Another option available in ORDBMS is to convert XML
data into a relational or object-relational form (so called shredding) and store the
resulting rows of data in corresponding typed tables. The SQL/XML specification
[17] that is a part of the SQL:2003 standard defines the XML data type and provides
mapping rules between XML schemas and SQL structures, as well as functions that
support manipulation of XML data within SQL queries.
3 Database Research Directions
As per our discussion in the previous section (section 2), database research and
associated standardization activities have successfully guided the development of
database technology over the last four decades and SQL relational databases remain
the dominant database technology today. This effort to innovate relational databases
to address the needs of new applications is continuing today. Recent examples of
database innovation include the development of streaming SQL technology that is
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used to process rapidly flowing data (“data in flight”) minimizing latency in Web 2.0
applications [18], and database appliances that simplify DBMS deployment on cloud
computing platforms [19]. It is also evident from the above discussion that the
relational database approach has proven to be extraordinarily durable, and has adapted
to new hardware architectures as well as new application requirements, successfully
subsuming both object-oriented and XML paradigms. Database research has played
an important role in solving key research problems and facilitating rapid technology
transfer making DBMS technology one of the most successful efforts in computer
science [20]. However, it is equally evident that database research is facing major new
challenges due to explosion of data, novel usage scenarios, and a major shift in
computing architectures. A recent meeting of leading database experts characterized
the present situation as a “turning point in database research” and identified a number
of trends that necessitate re-evaluation of research directions, and at the same time
present new research opportunities. “The Claremont Report on Database Research”
[21] identified the following trends:
(1)
(2)
(3)
(4)
(5)
Big Data (applications that process very large volumes of data, e.g. Web
search, e-science, etc)
Data analysis as a profit center (increasing number of companies where the
main business is data)
Ubiquity of structured and unstructured data (mainly originating from
various Web sources)
Developer demands (as adoption of open source relational DBMS
accelerates, developers demand more intuitive programming models)
Architectural shifts in computing (emergence of cloud computing services
brings about a fundamental change in software architecture towards parallel
clusters of computers; shift away from increasing CPU clock speed to
increasing the number of processor cores)
The report goes on to identify the following research opportunities:
(1) Re-design of architecture of database engines, to overcome the limitations of
current relational databases (RDBMS provide poor price/performance for
many popular applications, including text indexing, serving web pages, and
media delivery)
(2) Declarative Programming for Emerging Platforms (support for data
independence, declarative programming and cost-based optimization for new
programming models, e.g. MapReduce)
(3) The Interplay of Structured and Unstructured Data (managing a rich collection
of structured, semistructured and unstructured data, spread over many
repositories in the enterprise and on the Web)
(4) Cloud Data Services (improving manageability of cloud databases, Federated
cloud architectures, etc.)
(5) Mobile Applications and Virtual Worlds (manage massive amounts of diverse
user created data, and provide real-time services)
Database Trends and Directions: Current Challenges and Opportunities
171
The report notes that a number of additional research areas were not included as
these are the subject of ongoing investigation, and includes a summary of past reports
in the appendix. Setting agenda for database research is a challenging task, and forty
years of database research and development illustrates both the successes and failures
of such efforts. Database management is a very pragmatic field and many promising
research ideas were discarded as they did not provide any practical benefits, or turned
out not to be a database problem (e.g. deductive databases [22], expert databases [23],
etc.).
4 Conclusions
The Claremont Report on Database Research made an interesting observation noting
that the database research community has doubled in size over the last decade (as
measured by the number of publications and number of database related conference
sessions), but at the same time there was a perception that the quality of reviews (and
consequently, the quality of publications) has been decreasing over time.
Notwithstanding this massive research effort most significant recent innovations came
out of research labs of various companies (e.g. Google, Facebook, etc.), who are
facing urgent challenges of unprecedented size and complexity of data, and millions
of users running many thousands of transactions per second. These developments
have not been fully anticipated by the database research community and interestingly
not even by the traditional database vendors whose products offerings were dwarfed
by the scale and complexity of new application domains.
Recent rise of the NoSQL movement whose proponents regard the existing
relational DBMSs as inefficient, complex and expensive, and favor open source nonrelational solutions, demonstrates this point. For example, the MapReduce
programming model developed by Google [24], and its open source clone Hadoop
[25] initially used to simplify the construction of inverted indexes, has been applied to
text processing and numerous other tasks that require parallel computation over a very
large set of data. MapReduce is designed to automatically parallelize and execute a
program on a large cluster of commodity machines (typically, tens of thousands of
machines), managing data partitioning, task scheduling, inter-machine
communication, and recovery from machine failures. The combination of Hadoop
with Hypertable [26] (an open source version of Google BigTable [26]), enables the
concurrent execution of programs on tens of thousands of machines processing
petabytes of data on a daily basis [27].
These types of applications were traditionally the domain of parallel databases, and
a number of commercial database machines (e.g. Teradata, Oracle Exadata, etc.) have
been available for some time with proven performance characteristics for processing
very large data volumes. The comparison of MapReduce and parallel databases has
been the subject of a recent publication [28], concluding that “using MapReduce to
perform tasks that are best suited for DBMSs yields less than satisfactory results”, and
that MapReduce resembles more Extract-Transform-Load (ETL) system rather than a
DBMS, and therefore is a complimentary technology rather than competing with
DBMS. But, these conclusions are being hotly disputed by MapReduce proponents
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who claim that the scalability benefits of this approach will eventually “relegate
relational DBMS to the status of legacy technology”. Database vendors are taking
different approach to adopting the open source version of MapReduce (Hadoop),
some implementing this technology in their products (e.g. Teradata, and IBM), while
others (e.g. Microsoft) adopting a more cautious attitude [29].
Other vendors, notably Netezza have developed massively parallel database
machines (Data Warehouse Appliances) that perform data filtering directly on the
disk so that only the relevant portions of the data are propagated to the SQL database,
gaining significant performance improvements over more traditional parallel database
architectures. Additional functionality such as data analytics functions can also be
implemented directly in hardware, achieving further performance gains [30]. Such
approaches are using standard SQL database technology and are betting on further
advances in computer hardware (larger and less expensive computer memory and
more powerful CPUs as predicted by Moore’s Law), and innovative, massively
parallel database architectures to overcome the challenges of big data.
A key to understanding present and likely future database developments is a firm
view of what constitutes the database paradigm, i.e. defining the scope of database
research problems. Many of the recent challenges (in particular those faced by
internet companies such as Google, Facebook, etc.), concern situations where three
key elements that normally constitute a database environment are not present. While
these applications are clearly data-intensive, there is no database (data is not loaded
into a database), there is no database schema (data is mostly semi-structured and
sparse), and there is no support for database queries (application involve mainly text
search over semi-structured data). It is therefore difficult to regard such applications
as database applications. Learning from history we can observe that a similar situation
arose in the 1990s with Object-Oriented databases and later with XML databases, and
conclude that there is little benefit in applying database solutions to problems that do
not fit the database paradigm.
ACKNOWLEDGMENT
This research has been supported by GAČR (Grant Agency, Czech Republic) grant No.
P403/10/0092 - Advanced Principles and Models for Enterprise ICT Management, University
of Economics, Prague IGA (Internal Grant Agency) grant No. IG406040 – Cloud Computing
Adoption and Governance, and the Research Centre for Human Centered Technology Design at
the University of Technology, Sydney.
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