Connections

A connection is an association between an application process and a local or remote database server. Connections are managed by applications.

An application process must be connected to the application server facility of a database manager before SQL statements that reference tables or views can be executed. An application can use the CONNECT statement to establish a connection to a database server and make that database server the current server of the application process.

Commit processing: When DB2® for z/OS® acts as a requester, it negotiates with the database server during the connection process to determine how to perform commits. If the remote server does not support two-phase commit protocol, DB2 downgrades to perform one-phase commits. Otherwise, DB2 always performs two-phase commits, which allow applications to update one or more databases in a single unit of work and are more reliable than one-phase commits. Two-phase commit is a two-step process:

1. First, all database managers involved in the same unit of work are pooled to determine whether they are ready to commit.
2. Then, if all database managers respond positively, they are directed to execute commit processing. If all database managers do not respond positively, they are directed to execute backout processing.

DB2 can also provide coordination for transactions that include both two-phase commit resources and one-phase commit resources. If an application has multiple connections to several different database servers, and if any of the connections are one-phase commit connections, then only one database that is involved in the transaction can be updated. The connections to all the other databases that are involved in the transaction are read-only.

Supported SQL statements and clauses: For the most part, an application can use the statements and clauses that are supported by the database manager of the current server, even though that application might be running via the application requester of a database manager that does not support some of those statements and clauses. Restrictions to this general rule for DB2 for z/OS are documented in IBM DB2 SQL Reference for Cross-Platform Development.

To execute a static SQL statement that references tables or views, the bound form of the statement is taken from a package that the database manager previously created through a bind operation or when a version of a native SQL procedure was defined.