A 503 page gets written to my proxy cache, overwriting the 200 (3 replies)

Hi,

I'm trying to use the proxy cache to store regular pages (200) from my web server so that when the web server goes into maintenance mode and starts returning 503 nginx can still serve the good page out of cache. It works great for a few minutes but then at some point (5 to 10 minutes in) nginx will overwrite the good 200 page in the cache with the bad 503 page and then start handing out the 503. Looking at my config I don't understand how a 503 could ever get written to cache but it is. And the 200 page was brand new (written 10 minutes before) so it shouldn't be the "inactive" time on the proxy_cache_path setting causing nginx to delete the good file. Can anyone tell me what I'm missing? Here are the relevant pieces of my config:

proxy_cache_path /var/www/cache levels=1:2 keys_zone=my-cache:500m max_size=3000m inactive=120h;
proxy_temp_path /var/www/cache/tmp;
proxy_cache_key "$scheme$host$request_uri";

map $http_cookie $GotSessionCookie {
default "";
"~(?P<sessionid>\bSESS[^;=]+=[^;=]+)" $sessionid;
}

server {
listen 80;
server_name _;

proxy_cache my-cache;

location / {
proxy_pass http://production;
proxy_cache_valid 200 301 302 30m;
proxy_cache_valid 404 1m;
}

# don't cache pages with php's session cookie
proxy_no_cache $cookie_$GotSessionCookie;

# bypass the cache if we get a X-NoCache header
proxy_cache_bypass $http_nocache $cookie_$GotSessionCookie;

proxy_cache_use_stale http_500 http_503 error timeout invalid_header updating;
}

I can't imagine how a 503 would ever get cached given those proxy_cache_valid lines but maybe I don't understand something. Thanks for any ideas!

-Rick