bryancall commented on issue #9796:
URL: https://github.com/apache/trafficserver/issues/9796#issuecomment-4008618639
There's no built-in "cache-only" mode in ATS. The challenge is that the
remap destination is part of the cache key, so you can't point the remap at a
dummy origin without breaking cache key alignment with your PUSH requests.
A few approaches depending on your version:
1. **Plugin approach (cleanest):** A plugin on
`TS_HTTP_CACHE_LOOKUP_COMPLETE_HOOK` can detect a cache miss and return a 404
immediately, without attempting an origin connection. On ATS 10.x+, this is
straightforward with a Cript.
2. **Lightweight origin:** Run a minimal HTTP server on the origin host that
returns 404 for all requests. On a cache miss, ATS connects to the origin, gets
a 404, and returns it to the client. On a cache hit (from PUSH), the cached
content is served normally. This works on any ATS version with no plugins
needed.
3. **header_rewrite (partial):** You can use `cond %{CACHE} =miss` with
`set-status 404` and `set-body "Not Found"` on `SEND_RESPONSE_HDR_HOOK` to
rewrite the response, but this only fires *after* the origin connection
attempt, so it doesn't prevent the round-trip. There's no cache lookup complete
hook available in header_rewrite to short-circuit earlier.
Note that 7.1.x has been end-of-life for several years. If you're still
working on this use case, upgrading to a current release is recommended.
Closing this out given the age. Feel free to reopen or file a new issue if
you have further questions.
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