Why Cache Aside Can Fail: Hidden Risks and the Double-Delete Fix

What is Cache Aside?

Cache Aside is a cache‑update strategy that is the preferred solution for scenarios where strong consistency between cache and database is not required.

Read strategy

First try to read data from the cache; if the cache hits, return the data directly. If the cache misses, load the data from the database, return it to the client, and store it in the cache.

Write strategy

Update the database first, then delete the corresponding entry from the cache.

Problems with Cache Aside

1. Thread A reads data; the cache misses, so it fetches from the database (value 20). At this moment its CPU time slice ends.

2. Thread B performs an update, changes the database value to 21, and deletes the cache entry. Its CPU time slice ends, and then Thread A gets CPU time again.

3. Thread A writes the cached value 20 back, which is stale because the database now holds 21, leading to inconsistency between cache and database.

Inconsistent cache‑database data occurs when:

The read operation finds no data in the cache, forcing a database load.

A write operation happens concurrently with the read.

The read operation takes longer than the write.

During concurrent reads and writes, the read obtains an old value.

Solution

The double‑delete (cache‑double‑delete) approach can resolve cache‑database inconsistency.

Cache Aside works well for read‑heavy, write‑light scenarios, but it is unsuitable for write‑heavy workloads because frequent cache invalidations reduce cache hit rates.