1. Cache Guide
  2. Defining Cache Groups
  3. Asynchronous WriteThrough (AWT) Cache Group
  4. What an AWT Cache Group Does and Does Not Guarantee

What an AWT Cache Group Does and Does Not Guarantee

An AWT cache group comes with some guarantees.

An AWT cache group can guarantee that:

  • No transactions are lost because of communication failures between the TimesTen and Oracle databases.

  • If the replication agent is not running or loses its connection to the Oracle database, automatic propagation of committed changes on the TimesTen cache tables to the cached Oracle Database tables resumes after the agent restarts or reconnects to the Oracle database.

  • Transactions are committed in the Oracle database in the same order they were committed in the TimesTen database.

An AWT cache group cannot guarantee that:

  • All transactions committed successfully in the TimesTen database are successfully propagated to and committed in the Oracle database. Execution errors on the Oracle database cause the transaction in the Oracle database to be rolled back. For example, an update on the Oracle database may fail because of a unique constraint violation. Transactions that contain execution errors are not retried.

    Execution errors are considered permanent errors and are reported to the TimesTenDatabaseFileName.awterrs file that resides in the same directory as the TimesTen database's checkpoint files. See Reporting Oracle Database Permanent Errors for AWT Cache Groups.

  • The absolute order of Oracle Database updates is preserved because TimesTen does not resolve update conflicts. The following are some examples:

    • In two separate TimesTen databases (DB1 and DB2), different AWT cache groups cache the same Oracle Database table. An update is committed on the cache table in DB1. An update is then committed on the cache table in DB2. The two cache tables reside in different TimesTen databases and cache the same Oracle Database table. Because the writethrough operations are asynchronous, the update from DB2 may get propagated to the Oracle database before the update from DB1, resulting in the update from DB1 overwriting the update from DB2.

    • An update is committed on a cache table in an AWT cache group. The same update is committed on the cached Oracle Database table using a passthrough operation. The cache table update, which is automatically and asynchronously propagated to the Oracle database, may overwrite the passed through update that was processed directly on the cached Oracle Database table depending on when the propagated update and the passed through update is processed on the Oracle database. For this and other potential error conditions, TimesTen recommends that you do not run DML statements directly against Oracle Database tables cached in an AWT cache group. For more information, see Restrictions with AWT Cache Groups.