Explain the Cache-Aside pattern and how you implement it.

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Accepted Answer

The Cache-Aside pattern, also called Lazy Loading, is the most common caching strategy. The application manages the interactions between the cache and the database itself.

The read flow works as follows. First, the application receives a request for data. Second, it checks if the data exists in the cache. Third, if it exists, that is a cache hit, and we return the data directly. Fourth, if it does not exist, that is a cache miss, and we query the database. Fifth, we store the result in the cache with a TTL, time to live. Sixth, we return the data.

For writes, the application updates the database, then invalidates or updates the corresponding entry in the cache.

The advantages of this pattern are numerous. The application has full control over what gets cached. Only data that is actually requested gets cached, which avoids filling the cache with unused data. The cache can fail without the application stopping, it will simply fall back to the database.

The disadvantages are as follows. Each cache miss incurs additional latency, the database query plus the cache write. There is a risk of stale data if invalidation is not handled correctly. The code must explicitly manage caching logic.

This pattern is ideal when reads are much more frequent than writes and when you can tolerate slightly stale data.