Chapter 12 Distributed Database
Management Systems
Comprehensive Exam Study Guide
Latest Updated 2024/2025
,Chapter 12 Distributed Database
Management Systems
Comprehensive Exam Study Guide
Latest Updated 2024/2025
Chapter 12
Distributed Database Management Systems
Discussion Focus
Discuss the possible data request scenarios in a distributed database environment.
1. Single request accessing a single remote database. (See Figure D12.1.)
Figure D12.1 Single Request to Single Remote DBMS
REQUEST
The most primitive and least effective of the distributed database scenarios is based on a single SQL
statement (a "request" or "unit of work") is directed to a single remote DBMS. (Such a request is known
as a remote request.). We suggest that you remind the student of the distinction between a request and a
transaction:
A request uses a single SQL statement to request data.
A transaction is a collection of two or more SQL statements.
2. Multiple requests accessing a single remote database. (See Figure D12.2.)
Figure D12.2 Multiple Requests to a Single Remote DBMS
REQUEST
REQUEST
REQUEST
,Chapter 12 Distributed Database
Management Systems
Comprehensive Exam Study Guide
Latest Updated 2024/2025
418
, Chapter 12 Distributed Database
Management Systems
Comprehensive Exam Study Guide
Latest Updated 2024/2025
Chapter 12
Distributed Database Management Systems
A unit of work now consists of multiple SQL statements directed to a single remote DBMS. The local
user defines the start/stop sequence of the units of work, using COMMIT, but the remote DBMS
manages the unit of work's processing.
3. Multiple requests accessing multiple remote databases. (See Figure D12.3.)
A unit of work now may be composed of multiple SQL statements directed to multiple remote DBMSes.
However, any one SQL statement may access only one of the remote DBMSes. As was true in the
second scenario, the local user defines the start/stop sequence of the units of work, using COMMIT, but
the remote DBMS to which the SQL statement was directed manages the unit of work's processing. In
this scenario, a two-phase COMMIT must be used to coordinate COMMIT processing for the multiple
locations.
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