Cache-Aside vs Write-Through

Posted Aug 15, 2026 · 0 views

Both patterns put a cache in front of the database. They differ in who fills it, and when.

Cache-aside (lazy loading): the application checks the cache first. On a miss it reads the database and writes the value back. Writes go straight to the database and invalidate the cached copy, so the next read takes an L-shaped detour to refill it. Misses are easy to reason about, but a burst of concurrent misses on a hot key hits the database at once, and a failed invalidation serves a stale value until it expires.

Write-through: the write goes through the cache, which updates the database synchronously and does not return until both are done. The cache is never stale and reads after a write always hit — paid for on every single write, including data nobody will ever read.

The animation runs the same read-after-write under both: the detour appears in one and never in the other.