Distributed request

Distributed request enables a user or application program to read or update data at one or more databases within a single SQL statement.

Within one SQL statement, an application running against a local database can direct SQL requests to one or more remote databases. For example, a program can perform updates to table A on database 1 and table B on database 2 within one SQL statement. The following figure illustrates this idea.

IBM i distributed relational database supports a subset of the distributed request functionality. It allows update access or read-only access to a single local or remote database in one SQL statement.

The target database of the SQL statement is controlled by the user or application by specifying database qualifiers on referenced objects. For more information on object qualification, see SQL Reference.

Remote database connections are controlled by the database management system. The user or application can not use CONNECT TO, SET CONNECTION, RELEASE or DISCONNECT against these remote database connections. Remote database connections are tied to the local database connection. They are started as needed and ended when the local database connection is ended.

Special registers and global variables are propagated over remote database connections. If special registers or global variables do not exist on the remote database, they are ignored. If global variables exist but do not have the same attributes, the operation fails. Updates to the local special registers or global variables are propagated as needed over existing remote database connections.

When the application is ready to commit the work, it initiates the commit; commitment coordination is performed by a synchronization-point manager or a transaction manager.

Whether an application can update a given database in a SQL statement is dependent on the level of DRDA (if DRDA is used to access the remote relational database) and the order in which the connections and updates are made.