Different Types of Distribution Transparency

Distribution Transparency

Distribution transparency allows a physically dispersed database to be managed as though it were a centralized database. The level of transparency supported by the DDBMS varies from system to system. Three levels of distribution transparency are recognized:

• Fragmentation transparency is the highest level of transparency. The end user or programmer does not need to know that a database is partitioned. Therefore, neither fragment names nor fragment locations are specified prior to data access.

• Location transparency exists when the end user or programmer must specify the database fragment names but does not need to specify where those fragments are located.

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• Local mapping transparency exists when the end user or programmer must specify both the fragment names and their locations.

To illustrate the use of various transparency levels, suppose you have an EMPLOYEE table containing the attributes EMP_NAME, EMP_DOB, EMP_ADDRESS, EMP_DEPARTMENT, and EMP_SALARY. The EMPLOYEE data are distributed over three different locations: New York, Atlanta, and Miami. The table is divided by location; that is, New York employee data are stored in fragment E1, Atlanta employee data are stored in fragment E2, and Miami employee data are stored in fragment E3. Consider the following figure.

Now suppose that the end user wants to list all employees with a date of birth prior to January 1, 1960. To focus on the transparency issues, also suppose that the EMPLOYEE table is fragmented and each fragment is unique. The unique fragment condition indicates that each row is unique, regardless of the fragment in which it is located. Finally, assume that no portion of the database is replicated at any other site on the network.
Depending on the level of distribution transparency support, you may examine three query cases.

Case 1: The Database Supports Fragmentation Transparency

The query conforms to a non-distributed database query format; that is, it does not specify fragment names or locations.

The query reads:

SELECT *
FROM EMPLOYEE
WHERE EMP_DOB < '01-JAN-196';

Case 2: The Database Supports Location Transparency

Fragment names must be specified in the query, but the fragment’s location is not specified. The query reads:
SELECT *
FROM E1
WHERE EMP_DOB < '01-JAN-1960';
UNION
SELECT *
FROM E2
WHERE EMP_DOB < '01-JAN-1960';
UNION
SELECT *
FROM E 3
WHERE EMP_DOB < '01-JAN-1960';

Case 3: The Database Supports Local Mapping Transparency

Both the fragment name and its location must be specified in the query. Using pseudo-SQL:

SELECT *
FROM El NODE NY
WHERE EMP_DOB < '01-JAN-1960';
UNION
SELECT *
FROM E2 NODE ATL
WHERE EMP_DOB < '01-JAN-1960';
UNION
SELECT * FROM E3 NODE MIA
WHERE EMP_DOB < '01-JAN-1960';

Distribution transparency is supported by a distributed data dictionary (DDD), or a distributed data catalog (DDC). The DDC contains the description of the entire database as seen by the database administrator. The database description, known as the distributed global schema, is the common database schema used by local TPs to translate user requests into sub queries (remote requests) that will be processed by different DPs. The DDC is itself distributed, and it is replicated at the network nodes. Therefore, the DDC must maintain consistency through updating at all sites.

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