エッジ関数のトリガーとリクエストライフサイクル

When a request reaches a CDN edge, it passes through distinct phases. Edge functions can attach to specific phases to inspect or transform the request and response.

• Viewer request: as the request arrives
• Origin request: before fetching from origin
• Origin response: after origin replies
• Viewer response: before returning to the user

The viewer request trigger runs on every incoming request, even cache hits. Use it for lightweight logic like redirects, header normalization, or A/B routing.

The origin request trigger runs only on cache misses, just before the edge contacts the origin. It is ideal for rewriting the origin path or choosing a different backend.

The origin response trigger fires after the origin replies but before the response is cached. Use it to adjust cache headers or inject security headers.

The viewer response trigger runs before the response goes to the user, on both hits and misses. Use it for final header tweaks like adding CORS or HSTS.

Knowing which triggers run on cache hits matters for performance and cost. Viewer triggers run every time; origin triggers run only on misses. Put heavy logic on origin triggers to avoid paying for it on every hit.

An edge function can return a response directly instead of forwarding it, short-circuiting the lifecycle. This is great for maintenance pages, auth gates, or geo-blocks.

Request triggers can change the URI, headers, and query string before routing. Response triggers can change status codes and headers. Choose the trigger that matches what you need to mutate.

Edge functions run inside tight limits on CPU time and memory because they execute on every request at scale. Keep them small, avoid network calls in viewer triggers, and prefer synchronous logic.

• Tiny scripts equal predictable latency
• Heavy logic belongs on origin triggers or full edge runtimes

A quick decision guide:

• Redirect or auth gate: viewer request
• Rewrite origin path: origin request
• Adjust cache TTL: origin response
• Add response headers for everyone: viewer response

Test each trigger in isolation. Log the phase and inspect headers added by the CDN (like the viewer country) to confirm your function runs where you expect.

Confirm your understanding of edge trigger points.

You learned the four edge function trigger points across the request lifecycle: viewer request, origin request, origin response, and viewer response. You saw which run on cache hits versus misses, how to short-circuit a request, and how to pick the right trigger while respecting edge runtime limits.