nginx forgets original request regex variables when updating stale cache entries in the background

Question

I'm running a map tile server behind an nginx proxy. The tile calculation is expensive and that's why I cache the response for 24 hours. I've configured nginx to serve a stale response even when the cache is expired and at the same time updating the cache in the background.

My config is this:

 location ~* ^/tiles/(streets|satellite-overlay)/(.*)$ { proxy_pass http://127.0.0.1:5000/styles/$1/$2; # configure server-side cache proxy_cache tiles; proxy_cache_valid 200 1d; proxy_buffering on; # whilst tileserver recalculates the new tile, serve the old tile instead proxy_cache_background_update on; # only one request per tile proxy_cache_lock on; proxy_cache_use_stale error timeout invalid_header updating http_500 http_502 http_503 http_504; add_header X-Cached $upstream_cache_status; # cache client-side for 24 hours add_header "Cache-Control" "public, max-age=86400"; } 

So a request to /tiles/streets/1/2/3.png is being correctly proxied to http://127.0.0.1:5000/styles/streets/1/2/3.png.

However, after the 24 cache period, when the cache is supposed to be updated in the background, the following URL is requested from the tile server: http://127.0.0.1:5000/styles/streets//. This leads to a 404 and the cache is never updated.

It appears that the contents of the $1 and $2 variables is being lost somewhere.

Can someone spot a configuration error? Is it possible to use these variables in proxy_pass in conjuction with proxy_cache_background_update?

Answer 1

This looks like an nginx bug to me. However, as a possible workaround, you can try using named captures:

location ~* ^/tiles/(?<type>streets|satellite-overlay)/(?<tile>.*)$ { proxy_pass http://127.0.0.1:5000/styles/$type/$tile; ... } 

I remember some occasions when I needed to use named captures because positional captures did not work properly.

However, I haven't faced a similar issue, so I'm not sure if this helps in this case.