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# Cache Invalidation

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By default, objects remain in the session cache until they are explicitly deleted (see "Deleting Objects") or garbage collected when using a weak identity map (see "Configuring Cache Type and Size at the Project Level").

Alternatively, you can configure any object with a `CacheInvalidationPolicy` that lets you specify, either automatically or manually, under what circumstances a cached object is invalid: when any query attempts to read an invalid object, TopLink will go to the data source for the most up-to-date version of that object, and update the cache with this information.

You can use any of the following `CacheInvalidationPolicy` instances:

- `DailyCacheInvalidationPolicy`: the object is automatically flagged as invalid at a specified time of day.

- `NoExpiryCacheInvalidationPolicy`: the object can only be flagged as invalid by explicitly calling `oracle.toplink.sessions.IdentityMapAccessor` method `invalidateObject`.

- `TimeToLiveCacheInvalidationPolicy`: the object is automatically flagged as invalid after a specified time period has elapsed since the object was read.

You can configure a cache invalidation policy in the following ways:

- At the project level that applies to all objects ("Configuring Cache Expiration at the Project Level")

- At the descriptor level to override the project level configuration on a per-object basis ("Configuring Cache Expiration at the Descriptor Level")

- At the query level that applies to the results returned by the query ("Configuring Cache Expiration at the Query Level")

If you configure a query to cache results in its own internal cache (see "Caching Query Results in the Query Cache"), the cache invalidation policy you configure at the query level applies to the query's internal cache in the same way it would apply to the session cache.

If you are using a coordinated cache (see "Cache Coordination"), you can customize how TopLink communicates the fact that an object has been declared invalid. For more information, see "Configuring Cache Coordination Change Propagation at the Descriptor Level".