Caching is a promise about freshness. Before adding a cache, define how stale the result is allowed to be and who can invalidate it.
Three questions
- Is the response public or user-specific?
- How expensive is it to compute?
- What happens if the result is old?
An article index can tolerate a short delay. A permission decision usually cannot.
| Content | Typical policy | Invalidation |
|---|---|---|
| Static assets | Long-lived | Filename change |
| Public article page | Minutes to hours | Publish event |
| User profile | Short-lived | Profile update |
| Authorization | Minimal or none | Immediate |
Make freshness visible
When content can be stale, a timestamp helps:
Showing cached results from 2 minutes ago
Updated automatically when the source respondsThis turns a hidden trade-off into an understandable product decision.
A cache should make a known compromise explicit, not make correctness accidental.