Question

Is it possible to tell nginx an alternative location where to look when given path does not exist? I’d like to serve static assets from my Rails application, however sometimes the compiled asset might not be available and I’d like to have a fallback.

production.rb

  # Disable Rails's static asset server (Apache or nginx will already do this)
  config.serve_static_assets = false

nginx.conf:

  location ~ ^/assets/ {
               expires max;
               add_header Cache-Control public;
               add_header ETag "";
               break;
  }

UPDATE:
nginx.conf

  #cache server
  server {
        listen 80;

        # serving compressed assets
        location ~ ^/(assets)/  {
                root /var/app/current/public;
                gzip_static on; # to serve pre-gzipped version
                expires max;
                add_header Cache-Control public;
                add_header ETag "";
        }

        try_files $uri /maintenance.html @cache;

        location @cache {
            proxy_redirect off;
            proxy_pass_header Cookie;
            proxy_ignore_headers Set-Cookie;
            proxy_hide_header Set-Cookie;
            proxy_set_header Host $host;
            proxy_set_header X-Real-IP $remote_addr;
            proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
            proxy_cache one;
            proxy_cache_key app$request_uri;
            proxy_cache_valid 200 302  5s;
            proxy_cache_valid 404      1m;
            proxy_pass http://127.0.0.1:81;
        }
  }

  #real rails backend
  server {
        listen 81;
        root /var/app/current/public;
        error_log /var/app/current/log/error.log;

        rails_env production;
        passenger_enabled on;
        passenger_use_global_queue on;
  }

Answer 1

yes with the try files directive:

# note: you don't need the overhead of regexes for this location
location /assets/ {
   try_files $uri /alternative_to_try
   # ... add back in rest of your assetts config
}

this will try the requested url and if not found try the alternative uri (you can also add a 3th, 4th, … option)

Note that /alternative uri can be a named location (with for instance the directives for passing the url to the rails app)

see http://nginx.org/en/docs/http/ngx_http_core_module.html#try_files for more details and some examples regarding try_files

update:

right so change your assets’ location to

location /assets/  {
   try_files $uri @cache;
   root /var/app/current/public;
   gzip_static on; # to serve pre-gzipped version
   expires max;
   add_header Cache-Control public;
   add_header ETag "";
}

in other words for all url’s where the part starts with /assets/:

1. check if there’s an actual file corresponding to the path (that’s the $uri part of the try_files directive)
2. if not, pass the request on to the named location @cache (that’s the @cache part of the try_files directive)
3. if we get to the @cache location, it will first check the proxy cache zone one for a match
4. if no cache match is found it will reverse proxy the request to the rails app at http://127.0.0.1:81