Using Queries with Coherence Read-Through Caches
- by jpurdy
Applications that rely on partial caches of databases, and use read-through to maintain those caches, have some trade-offs if queries are required. Coherence does not support push-down queries, so queries will apply only to data that currently exists in the cache. This is technically consistent with "read committed" semantics, but the potential absence of data may make the results so unintuitive as to be useless for most use cases (depending on how much of the database is held in cache).
Alternatively, the application itself may manually "push down" queries to the database, either retrieving results equivalent to querying the cache directly, or may query the database for a key set and read the values from the cache (relying on read-through to handle any missing values).
Obviously, if the result set is too large, reading through the cache may cause significant thrashing. It's also worth pointing out that if the cache is asynchronously synchronized with the database (perhaps via database change listener), that an application may commit a transaction to the database, then generate a key set from the database via a query, then read cache entries through the cache, possibly resulting in a race condition where the application sees older data than it had previously committed. In theory this is not problematic but in practice it is very unintuitive. For this reason it often makes sense to invalidate the cache when updating the database, forcing the next read-through to update the cache.
