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What is Cache Callback in Cache?

The cache object has dependencies e.g. relationships to the file it stores. Cache items remove the object when these dependencies change. As a work around we would need to simply execute a callback method whenever items are removed from the cache to add the items back in cache.

e.g.: We have a string variable string var="hello" and store it in cache. This item would be removed from the cache if value is changed from "hello" to "bye". In such a case we need to simply right a callback function which would add updated var back in cache as it's been removed as a result of its dependencies changing.