The Silent API Killer: How Caching Broke My Integration and How I Fixed It

Source: Dev.to

Problem

I recently spent two days chasing a ghost in a machine‑to‑machine API integration. The integration worked flawlessly in staging, but in production our webhook endpoint sporadically failed to acknowledge receipts from a critical third‑party service, causing duplicate processing and missed events. The third‑party logs showed a 200 OK response from our endpoint, while our logs showed that the corresponding background job never ran. The issue was intermittent and impossible to reproduce locally—a classic sign of a state or environmental problem.

Root Cause

After exhaustive logging, I discovered that our production API gateway (a layer we didn’t control) was aggressively caching POST requests. Because our idempotency‑key header was static for a period (a business‑logic decision to group events), the gateway served a cached 200 response for subsequent identical POSTs before they even reached our application server. Our app never saw the request, so the job never enqueued. The “healthy” 200 response from the gateway masked a total failure of our business logic.

Fix

1. Add caching directives – We added a Cache-Control: no-store header on this specific webhook endpoint to instruct all intermediate caches to bypass storage.
2. Revise idempotency strategy – We re‑architected our idempotency‑key approach to be truly unique per event, ensuring every request is distinct and thus uncachable.

Takeaways

• Always verify caching behavior at every proxy, CDN, or gateway layer, especially for non‑GET requests.
• A successful HTTP status code from a downstream cache can be a catastrophic lie.
• Treat caching headers as a first‑class concern in API design; what you think is a “GET” operation might be interpreted as a “cacheable” operation by the network, and that assumption can silently break your system.