Host Failure and Switchover of the DB Service

Use

In the event of DB host failure, the network connection of SAP work processes to the DBMS is lost. Such an error might, for example, be due to the failure of the DB host, a database shutdown, or a network failure. If a work process encounters an error in the database connection, the built-in DB Reconnect mechanism starts, and tries to re-establish the database connection.

Features

The DB Reconnect feature makes sure that all work processes of all SAP NetWeaver AS instances are automatically reconnected to the DB as soon as the DB service is restarted and becomes available again. This means the work processes can transparently recover after temporary DB failure.

All local memory buffers of the host machines of the SAP NetWeaver AS instances are preserved. System performance on the machines after switchover is as optimal as before because the local memory buffers remain well tuned and buffer hit rates are correspondingly high.

To the end user, the temporary DBMS failure is almost fully transparent, apart from the time taken for the DB service to be switched over and become operational again. Note that the functionality offered depends on the type of access service involved – that is, dialog, batch, or update.

DB Reconnect avoids a shutdown of all work processes and therefore of all SAP instances.

If pre-defined errors are returned from the database to a work process, this process is set to “reconnect” status. The transaction run by the process is terminated. However, the process itself continues to run and informs all other work processes (irrelevant of type) on the host about the DB restart. If the DB is not available, all work processes switch to this status within a short period of time.

Whenever a user request is received, a work process in this status tries to reconnect to the database system before it starts the requested transaction. If the database is accessible again, a work process in the reconnect state lets the transaction start without terminating. This is transparent to the user. If the database connection cannot be re-established, the transaction does not start and the user is informed by a popup message about the lost database connection.

For more information, see SAP Note 98051.

Activities

In principle, you can use DB Reconnect in all situations where the DB fails, such as host failure, planned shutdown, temporary interruption of the connection to the DB host. Both the CI and AS instances are configured to take advantage of the reconnect. However, if an instance is started or restarted without being able to access the database, the instance stops. There is no reconnect at startup time. The same applies to the restarted work process: if the initial connect fails, the work process is stopped and is not restarted.