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Leonard Ehrenfried
Asked: 2020-05-20 06:55:08 +0800 CST

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

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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?

nginx

1 Answers

  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.

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