Wanted: suggestions on how to invert proxy_pass return codes

Server NGINX
1

Hi all -

I have an odd requirement which I’m not quite able to solve - is there
anyone out there with a solution?

For reasons I won’t go into, but which are unfortunately immovable and
quite real, I need to come up with a location stanza that exhibits the
following behaviour:

location /healthcheck/ {
if (!-f /tmp/flag) {
return 503;
}
proxy_intercept_errors on;
proxy_pass http://backend.server.fqdn.local/status;
error_page 200 = @backend_up;
error_page 404 500 501 502 503 504 = @backend_down;
}
location @backend_up {
return 503;
}
location backend_down {
return 200;
}

Yes - you read that correctly :frowning:

Before making the proxy_pass call, check a marker on disk and serve a
(real, not translated to =200) 503 if it exists.
If I get a 200 from the backend status page, actually serve a 503.
If I get a 404/5xx from the backend status page, serve a 200.

I’m pretty sure the last time I tried something horrible like this,
error_page complained that it could only override response codes >400

  • hence this email. Can anyone see a way of achieving the behaviour I
    need?

The behaviour described is all that’s important - no other part of
the config, above, is fixed. I can’t introduce any more external
services unless there’s absolutely no choice, however.

Thanks for having a think about this. I’ll tell you what it’s for if
you’re able to help - I’m pretty sure you’ll be both surprised and a
little bit proud that your idea got into this project, if you do …
:slight_smile:

TIA all,
Jonathan

Jonathan M. // Oxford, London, UK
http://www.jpluscplusm.com/contact.html

2

A slight thinko crept in to my original mail; there’s a small
difference (which does remove a minor complexity) as I’ve marked below
…

On 4 March 2013 23:58, Jonathan M. [email protected]
wrote:
[snip]

location /healthcheck/ {
if (!-f /tmp/flag) {
return 503;
}

SHOULD BE:

"
if (-f /tmp/flag) {
return 200;
}
"

[snip]

Before making the proxy_pass call, check a marker on disk and serve a
(real, not translated to =200) 503 if it exists.

SHOULD BE: “… serve a 200 if it exists.”

I don’t think this changes the meat of the problem - it just removes
one minor niggle.

Any thoughts?

Jonathan M. // Oxford, London, UK
http://www.jpluscplusm.com/contact.html

3

On Tue, Mar 05, 2013 at 12:17:03AM +0000, Jonathan M. wrote:

}

Before making the proxy_pass call, check a marker on disk and serve a
(real, not translated to =200) 503 if it exists.

SHOULD BE: “… serve a 200 if it exists.”

I don’t think this changes the meat of the problem - it just removes
one minor niggle.

Any thoughts?

Well, if you can add headers on a backend …

: http {
: map $connection $fail {
: ~[02468]$ 1;
: }
:
: server {
: server_name backend;
: listen 8001;
: add_header X-Accel-Redirect /backend_up;
:
: if ($fail) { return 503 “real 503\n”; }
: return 200 “real 200\n”;
: }
:
: server {
: listen 8000;
:
: location = /test {
: proxy_pass http://127.0.0.1:8001;
: proxy_intercept_errors on;
: error_page 503 = @backend_down;
: }
:
: location @backend_down {
: return 200 “wrapped 200\n”;
: }
:
: location = /backend_up {
: internal;
: return 503 “wrapped 503\n”;
: }
: }
: }

$ repeat 10 curl http://127.0.0.1:8000/test
wrapped 503
wrapped 200
wrapped 503
wrapped 200
wrapped 503
wrapped 200
wrapped 503
wrapped 200
wrapped 503
wrapped 200

(See Module ngx_http_proxy_module for the meaning of
X-Accel-Redirect.)