Chapter 23 - Distributed Database Concepts

distributed database (DDB)

A collection of multiple logically related database distributed over a computer network, and a distributed database management system as a software system that manages a distributed database while making the distribution transparent to the user.

Mixed (Hybrid) fragmentation

A combination of Vertical fragmentation and Horizontal fragmentation.

Fragmentation schema

A definition of a set of fragments (horizontal or vertical or horizontal and vertical) that includes all attributes and tuples in the database that satisfies the condition that the whole database can be reconstructed from the fragments by applying some sequence of UNION (or OUTER JOIN) and UNION operations.

Primary site technique

A single site is designated as a primary site which serves as a coordinator for transaction management.

Distributed commit

A transaction may be fragmented and they may be executed by a number of sites. This require a two or three-phase commit approach for transaction commit.

Primary site approach with no backup site

Aborts and restarts all active transactions at all sites. Elects a new coordinator and initiates transaction processing.

Homogeneous

All sites of the database system have identical setup, i.e., same database system software. The underlying operating system may be different. For example, all sites run Oracle or DB2, or Sybase or some other database system. The underlying operating systems can be a mixture of Linux, Window, Unix, etc.

Easier expansion (scalability)

Allows new nodes (computers) to be added anytime without chaining the entire configuration.

Fragmentation transparency

Allows to fragment a relation horizontally (create a subset of tuples of a relation) or vertically (create a subset of columns of a relation).

Transaction management

Concurrency control and commit are managed by this site. In two phase locking, this site manages locking and releasing data items. If all transactions follow two-phase policy at all sites, then serializability is guaranteed.

Replication transparency

Copies of data stored at multiple sites. This is done to minimize access time to the required data.

Failure of individual sites

Database availability must not be affected due to the failure of one or two sites and the recovery scheme must recover them before they are available for use.

Data Replication

Database is replicated to all sites. achieved through a replication schema.

federated

Each site may run different DBMS but the data access is managed through a single conceptual schema.

Recovery from a coordinator failure

In both approaches a coordinator site or copy may become unavailable. This will require the selection of a new coordinator.

Primary Copy Technique

In this approach, instead of a site, a data item partition is designated as primary copy. To lock a data item just the primary copy of the data item is locked.

Allocation schema

It describes the distribution of fragments to sites of distributed databases. It can be fully or partially replicated or can be partitioned.

Horizontal fragmentation

It is a horizontal subset of a relation which contain those of tuples which satisfy selection conditions.

Vertical fragmentation

It is a subset of a relation which is created by a subset of columns. Thus a vertical fragment of a relation will contain values of selected columns. There is no selection condition used in vertical fragmentation.

Derived horizontal fragmentation

It is the partitioning of a primary relation to other secondary relations which are related with Foreign keys.

Semijoin

Objective is to reduce the number of tuples in a relation before transferring it to another site.

Distributed deadlock

Since transactions are processed at multiple sites, two or more sites may get involved in deadlock. This must be resolved in a distributed manner.

Data Fragmentation

Split a relation into logically related and correct parts.

Primary site approach with backup site

Suspends all active transactions, designates the backup site as the primary site and identifies a new back up site. Primary site receives all transaction management information to resume processing.

Dealing with multiple copies of data items

The concurrency control must maintain global consistency. Likewise the recovery mechanism must recover all copies and maintain consistency after recovery.

multidatabase

There is no one conceptual global schema. For data access a schema is constructed dynamically as needed by the application software.

Concurrency control based on voting

There is no primary copy of coordinator. Send lock request to sites that have data item. If majority of sites grant lock then the requesting transaction gets the data item. Locking information (grant or denied) is sent to all these sites. To avoid unacceptably long wait, a time-out period is defined. If the requesting transaction does not get any vote information then the transaction is aborted.

Communication link failure

This failure may create network partition which would affect database availability even though all database sites may be running.

Data Distribution (Data Allocation)

This is relevant only in the case of partial replication or partition. The selected portion of the database is distributed to the database sites.

Primary and backup sites fail or no backup site

Use election process to select a new coordinator site.

Distribution and Network transparency

Users do not have to worry about operational details of the network.

Heterogeneous

federated and multidatabase

transparency

pertains to logical and physical data independence for application developers.

Reliability

refers to system live time, that is, system is running efficiently most of the time.

Availability

the probability that the system is continuously available (usable or accessible).