TopicTopicDistributed DBMSDistributed DBMS

Database Management Database Management Systems Fall 2012 Systems Fall 2012

Presented by:

Osama Ben Omran

OutlineOutline What is a Distributed Database System? Homogeneous and Heterogeneous distributed

database Distributed DBMS Advantages DISADVANTAGES OF DDBMS What is not a DDBS? Distributed Data Storage Data Replication Advantages of Replication Disadvantages of Replication Data Fragmentation Advantages of Fragmentation

What is a Distributed Database System?

A distributed database (DDB) is a collection of multiple, logically interrelated databases distributed over a computer network.

A distributed database management system (DDBMS) is the software that manages the DDB and provides an access mechanism that makes this distribution transparent to the users.

What is a Distributed Database System? A distributed database system is a collection of logically related databases that co-operate in a transparent manner.

Transparent implies that each user within the system may access all of the data within all of the databases as if they were a single database.

There should be location independence: as the user is unaware of where the data is located it is possible to move the data from one physical location to another without affecting the user.

Homogeneous and Heterogeneous distributed database

Homogeneous and Heterogeneous distributed database

homogeneous distributed database

All sites have identical software. Are aware of each other and agree to

cooperate in processing user requests. Each site surrenders part of its

autonomy in terms of right to change schemas or software.

Appears to user as a single system

Homogeneous and Heterogeneous distributed database

In a heterogeneous distributed database

Different sites may use different schemas and software.

Sites may not be aware of each other and may provide only limited facilities for cooperation in transaction processing.

Distributed DBMS Advantages

Transparent management of distributed, fragmented, and replicated data.

Improved reliability, availability through distributed transactions.

Improved performance. Easier and more economical system

expansion.

Distributed DBMS Advantages

Improved Processing power. Instead of one server handling the full database we now have a collection of machines handling the same database.

Removal of reliance on a central site. If a server fails, then the only part of the system that is affected is the relevant local site. The rest of the system remains functional and available.

DISADVANTAGES OF DDBMS

Architectural complexity. Cost. Security. Integrity control more difficult. Lack of standards. Lack of experience. Database design more complex.

What is not a DDBS?

A timesharing computer system. A loosely or tightly coupled

multiprocessor system. A database system which resides at

one of the nodes of a network of computers -this is a centralized database on a network node.

Distributed Data Storage

Replication- System maintains multiple copies of data, stored in different sites, for faster retrieval and fault tolerance.

Fragmentation- Relation is partitioned into several fragments stored in distinct sites.

Replication and fragmentation can be combined- Relation is partitioned into several fragments: system maintains several identical replicas of each such fragment.

Data Replication

A relation or fragment of a relation is replicated if it is stored redundantly in two or more sites.

Full replication of a relation is the case where the relation is stored at all sites.

Fully redundant databases are those in which every site contains a copy of the entire database.

Advantages of Replication

Availability: failure of site containing relation r does not result in unavailability of r.

Parallelism: queries on r may be processed by several nodes in parallel.

Reduced data transfer: relation r is available locally at each site containing a replica of r.

Disadvantages of Replication

Increased cost of updates: each replica of relation r must be updated.

Increased complexity of concurrency control: concurrent updates to distinct replicas may lead to inconsistent data unless special concurrency control mechanisms are implemented.

Data Fragmentation

Division of relation r into fragments r1, r2, …, rn which contain sufficient information to reconstruct relation r.

Horizontal fragmentation: each tuple of r is assigned to one or more fragments.

Vertical fragmentation: the schema for relation r is split into several smaller schemas.

Advantages of Fragmentation

Horizontal: allows parallel processing on fragments of a relation. allows a relation to be split so that tuples are located

where they are most frequently accessed.

Vertical: allows tuples to be split so that each part of the tuple

is stored where it is most frequently accessed. tuple-id attribute allows efficient joining of vertical

fragments. allows parallel processing on a relation.

References

http://it.becs.ac.in/content/sukanta_das/projslides/36-40(Distributed%20Database%20System).pdf

https://cs.uwaterloo.ca/~tozsu/courses/cs856/F02/lecture-1-ho.pdf

http://pages.cs.wisc.edu/~dbbook/openAccess/thirdEdition/slides/slides3ed-english/Ch22b_DistributedDBs-95.pdf