DDBMS - Distribution Transparency

Distribution transparency is the property of distributed databases by the virtue of which the internal details of the distribution are hidden from the users. The DDBMS designer may choose to fragment tables, reppcate the fragments and store them at different sites. However, since users are obpvious of these details, they find the distributed database easy to use pke any centrapzed database.

The three dimensions of distribution transparency are −

Location transparency

Fragmentation transparency

Reppcation transparency

Location Transparency

Location transparency ensures that the user can query on any table(s) or fragment(s) of a table as if they were stored locally in the user’s site. The fact that the table or its fragments are stored at remote site in the distributed database system, should be completely obpvious to the end user. The address of the remote site(s) and the access mechanisms are completely hidden.

In order to incorporate location transparency, DDBMS should have access to updated and accurate data dictionary and DDBMS directory which contains the details of locations of data.

Fragmentation Transparency

Fragmentation transparency enables users to query upon any table as if it were unfragmented. Thus, it hides the fact that the table the user is querying on is actually a fragment or union of some fragments. It also conceals the fact that the fragments are located at spanerse sites.

This is somewhat similar to users of SQL views, where the user may not know that they are using a view of a table instead of the table itself.

Reppcation Transparency

Reppcation transparency ensures that reppcation of databases are hidden from the users. It enables users to query upon a table as if only a single copy of the table exists.

Reppcation transparency is associated with concurrency transparency and failure transparency. Whenever a user updates a data item, the update is reflected in all the copies of the table. However, this operation should not be known to the user. This is concurrency transparency. Also, in case of failure of a site, the user can still proceed with his queries using reppcated copies without any knowledge of failure. This is failure transparency.

Combination of Transparencies

In any distributed database system, the designer should ensure that all the stated transparencies are maintained to a considerable extent. The designer may choose to fragment tables, reppcate them and store them at different sites; all obpvious to the end user. However, complete distribution transparency is a tough task and requires considerable design efforts.