Scale Up And Then Scale Out: How do I keep a Cache and DB in sync?

Scale Up And Then Scale Out: How do I keep a Cache and DB in sync?

While speaking with java architects, I often get asked the question - When I cache information, what happens if the data in the underlying database changes? How do I keep the cache and DB in sync. This blog addresses some of the common ways to address this issue.

Broadly speaking, there are 2 ways to approach this. Firstly you can put the onus on the underlying memory store to fetch the information periodically or when the store determines that the information is stale. Secondly you can put the onus on the database to "push" updates either periodically or whenever data changes. Lets discuss further.

The immediate and most straightforward approach that comes to mind is to set the TTI or TTL on the cache so that the data will be expired periodically. The next request after expiration will result in a cache miss and can be configured to pull in the current value from the underlying system.
Couple of things to note here. Obviously there is the potential for a window when the data is in the cache and not consistent with the underlying system. Plus, the "miss" is taken by the user thread that could be interpreted as a performance penalty.

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