Distributed database

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A distributed database is a database that is under the control of a central database management system (DBMS) in which storage devices are not all attached to a common CPU. It may be stored in multiple computers located in the same physical location, or may be dispersed over a network of interconnected computers.

Collections of data (eg. in a database) can be distributed across multiple physical locations. A distributed database is distributed into separate partitions/fragments. Each partition/fragment of a distributed database may be replicated (ie. redundant fail-overs, RAID like).

Besides distributed database replication and fragmentation, there are many other distributed database design technologies. For example, local autonomy, synchronous and asynchronous distributed database technologies. These technologies' implementation can and does definitely depend on the needs of the business and the sensitivity/confidentiality of the data to be stored in the database. And hence the price the business is willing to spend on ensuring data security, consistency and integrity.

Contents

1 Basic architecture
2 Important considerations
3 Advantages of distributed databases
4 Disadvantages of distributed databases
5 See also
6 References

Basic architecture

A database server is the software managing a database, and a client is an application that requests information from a server. Each computer in a system is a node. A node in a distributed database system act as a client, a server, or both, depending on the situation.

Horizontal fragments: subsets of tuples (rows) from a relation (table).
Vertical fragments: subsets of attributes (columns) from a relation (table).
Mixed fragment: a fragment which is both horizontally and vertically fragmented.
Homogeneous distributed database: uses one DBMS (eg: Oracle).
Heterogeneous distributed database: uses multiple DBMS's (eg: Oracle and MS-SQL and PostgreSQL).

Users access the distributed database through:

Local applications: applications which do not require data from other sites.
Global applications: applications which do require data from other sites.

Important considerations

Care with a distributed database must be taken to ensure that:

The distribution is transparent — users must be able to interact with the system as if it was one logical system. This applies to the systems performance, and methods of access amongst other things.
Transactions are transparent — each transaction must maintain database integrity across multiple databases. Transactions must also be divided into subtransactions, each subtransaction affecting one database system.

Advantages of distributed databases

Reflects organizational structure — database fragments are located in the departments they relate to.
Local autonomy — a department can control the data about them (as they are the ones familiar with it.)
Improved availability — a fault in one database system will only affect one fragment, instead of the entire database.
Improved performance — data is located near the site of greatest demand, and the database systems themselves are parallelized, allowing load on the databases to be balanced among servers. (A high load on one module of the database won't affect other modules of the database in a distributed database.)
Economics — it costs less to create a network of smaller computers with the power of a single large computer.
Modularity — systems can be modified, added and removed from the distributed database without affecting other modules (systems).

Disadvantages of distributed databases

Complexity — extra work must be done by the DBAs to ensure that the distributed nature of the system is transparent. Extra work must also be done to maintain multiple disparate systems, instead of one big one. Extra database design work must also be done to account for the disconnected nature of the database — for example, joins become prohibitively expensive when performed across multiple systems.
Economics — increased complexity and a more extensive infrastructure means extra labour costs.
Security — remote database fragments must be secured, and they are not centralized so the remote sites must be secured as well. The infrastructure must also be secured (eg: by encrypting the network links between remote sites).
Difficult to maintain integrity — in a distributed database enforcing integrity over a network may require too much networking resources to be feasible.
Inexperience — distributed databases are difficult to work with, and as a young field there is not much readily available experience on proper practice.

References

M. T. Ozsu and P. Valduriez, Principles of Distributed Databases (2nd edition), Prentice-Hall, ISBN 0-136-59707-6

Federal Standard 1037C
Elmasri and Navathe, Fundamentals of database systems (3rd edition), Addison-Wesley Longman, ISBN 0-201-54263-3