Serve stale content from cache if origin responds with 5xx

[rrva]

As requested in this forum thread, I want to be able to use caddy as a reverse caching proxy which makes an service more resilient to outages of the origin server, so that if the origin goes down or responds with errors, caddy serves old content.

Specifically:

1. I only want http 200 responses to be cached
2. If an origin server responds with http 5xx, I want to serve stale content from the cache.

Also attaching a corresponding nginx.conf which achieves what I want for reference.

nginx.conf.gz

[rrva]

I define stale content as content for which the cache-control max-age has already expired, but it is to be kept in the cache anyway and served as a fallback if the origin is unresponsive/responds with errors

[francislavoie]

@mholt I think this issue should be moved to https://github.com/caddyserver/cache-handler

[mholt]

@francislavoie Agreed -- and this feature should be pretty easy to implement. Caddy has a ResponseRecorder which can help the cache decide whether to respond or not.

[gc-ss]

@rrva are you still interested in this and/or willing to test an implementation?

[rrva]

@gc-ss Sorry, no interest has faded for the moment (not using caddy in this capacity right now).

[frederichoule]

Maybe we can take into count stale-if-error from Cache-Status - https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Cache-Control

[frederichoule]

Should this issue be moved to https://github.com/darkweak/souin ?

[darkweak]

In the Souin codebase we support the stale-if-error directive

[fliespl]

@darkweak do you think it might be possible to make it configurable and not depending on upstream response cache-control?

I would like to enable fallback to server STALE page in case my upstream goes down.

[darkweak]

@fliespl you may use the mode bypass_response to bypass the response RFC checks. Or you could use the header_down caddy file directive to remove the cache-control response header.

[fliespl]

@darkweak thanks - do you have any example of that? I have tried a few things without success...

[darkweak]

I'm not sure but it could be something like this (cc @francislavoie about header_down reverse_proxy):

route {
  cache
  reverse_proxy 127.0.0.1:81 {
    header_down -Cache-Control
  }
}

Or using the mode cache directive:

route {
  cache {
    mode bypass_response
  }
  reverse_proxy 127.0.0.1:81
}

[fliespl]

@darkweak actually, I was not able to use this. Steps tried:

Simple test.php file with:

Cache-Control:
max-age=10, stale-if-error=120

I get cached on first request, disable php upstream. Cache works for 10 seconds and after that I get bad gateway 502 error and not stale content.

Souin; fwd=uri-miss; key=GET-https-example.com-/test.php; detail=SERVE-HTTP-ERROR

STALE_GET-https-example.com-/test.php still has ttl of 3471 seconds.

Am I missing something?

[darkweak]

@fliespl what's your request? (curl)

[fliespl]

curl https://domain
or
curl -I https://domain

No matter if it's GET / HEAD same output.

[darkweak]

If your request doesn't send a stale-if-error directive the HTTP cache won't serve the stale response because the client doesn't allow it as explained in the RFC 7234. But reading the "new" 9111 RFC, they consider stale to be allowed by default. Maybe I misunderstood something in these RFCs.

[fliespl]

@darkweak isn't stale-if-error only response header? I haven't seen any browser utilizing it for "request".

I would assume it's origin response to cache servers that instructs them that in case they fail, cache server is allowed to serve this content to client.

[darkweak]

I meant use the max-stale request directive sorry

[fliespl]

@darkweak okay so my initial request still stands - allow serving stale content no matter what request sends (since max-stale is not used). Maybe it should allow it if mode is bypass request the same way it does for no-cache?

[darkweak]

Yes, I will create a PR for that! 👍

[fliespl]

@darkweak also found one other issue which is probably connected.

At default setup (without souin) if reverse proxy (php fastcgi) returns status 500 with html it gets displayed (i.e. wordpress error: https://ss.codeone.pl/ss-2024-01-21-22-20-19-1705872019-F83puPwH.png)

With souin enabled I get empty response and default chrome error: https://ss.codeone.pl/ss-2024-01-21-22-21-00-1705872060-HGzbuNTQ.png I feel like it's a bug somewhere with souin altering proxy response/headers.

[darkweak]

@fliespl there is a PR already open for that darkweak/souin#441 (thanks to @vejipe) 😄

[fliespl]

@darkweak @vejipe Didn't realize it's connected - thanks much! :) Great job by the way!