Sunday, August 30, 2009

gemfire GET subverted by(cache miss -> read through)

get() usually is a "const" operation (c++ jargon), but gemfire CacheLoader.java can intercept the call and write into the cache. Such a "trigger" sits between the client and the DB. Upon a cache miss, it loads the missing entry from DB.

When Region.get(Object) is called for a region entry that has a null value, the load method of the region's cache loader is invoked. The load method *creates* the value for the desired key by performing an operation such as a database query.

A region's cache loader == a kind of DAO to handle cache misses.

In this set-up, gemfire functions like memcached, i.e. as a DB cache. Just like the PWM JMS queue browser story, this is a simple point but not everyone understands it.

http://community.gemstone.com/display/gemfire60/Database+write-behind+and+read-through --

When an application requests for an entry (for example entry key1) which is not already present in the cache, if read-through is enabled Gemfire will load the required entry (key1) from DB. The read-through functionality is enabled by defining a data loader for a region. The loader is called on cache misses during the get operation, and it populates the cache with the new entry value in addition to returning the value to the calling thread.

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