NGINX Cache

2017-08-02 ·

A proxy cache is located between a client and an origin server. It stores the content that passes between. If a client requests content that the cache has stored, it is returned directly without contacting the origin server. This improves performance. The application servers don't need to generate pages each time. Their system resource usage (CPU, RAM and I/O) is reduced.

NGINX provides a full set of caching features. You can enable with proxy_cache_path and proxy_cache directives. The first one sets the cache location along with its configuration. The second one activates the cache.

proxy_cache_path /path/to/the/cache levels=1:2 keys_zone=just_cache:10m max_size=10g inactive=69m use_temp_path=off;

server {
  
  location / {
    proxy_cache just_cache;
    proxy_pass http://some_upstream;
  }
}
  • levels defines a directory hierarchy under under the given path. 1:2 sets it as two-level directory. This is recommended if there is a lot of files on single level. By default, it's just one level i.e. a single directory.
  • keys_zone sets up a shared memory zone just for the cache keys. Having those keys in memory improves lookup time, thus request/response time. 10m defines a 10 megabytes zone which can hold about 80,000 keys.
  • max_size defines the maximum size of the cache. In the example above it is set as 10 gigabytes. This parameter is optional, not specifying a value makes the cache grow to use available disk space.
  • inactive defines how long an item should be kept in the cache without being accessed. 69m specifies a 69 minutes widow: a file that has not been requested within that time is deleted.
  • use_temp_path forces NGINX not to use the temporary cache area and to use directly the cache directory specified.
  • proxy_cache activates the caching for requests that match URL of the parent location block. You can specify proxy_cache within server block to apply it to all location blocks.

NGINX can be also configured to deliver stale content from its cache when it's not possible to get fresh content from the origin servers. This provides an extra level of fault tolerance for the servers being proxed by NGINX. It also ensures the content availability in the case of server failures or traffic spikes. This functionality is enabled with proxy_cache_use_stale directive.

location / {
  proxy_cache_use_stale error timout http_500 http_502 http_503 http_504;
}

The configuration above states that when NGINX receives and error, timeout or a 50x response and it has a stale version of the requested file in the case, that file will be delivered to the client instead of the actual error.

There is a set of optional directives for fine-tuning the cache performance.

  • proxy_cache_revalidate tells NGINX to use conditional GET requests to refresh the content from origin servers. Once the cache item is expired, NGINX uses If-Modified-Since header for the request send to the origin server. This saves bandwidth as the server sends the full response only if the actual content has been modified. It uses the Last-Modified header for that.
  • proxy_cache_min_uses defines the number of times an URL must be requested before it's being cached by NGINX. The default value is 1.
  • proxy_cache_use_stale has an additional updating parameter to deliver stale content if it's in the process of being updated from the origin server. By setting proxy_cache_background_update directive to on those updates will be done in the background.
  • proxy_cache_lock set to on is useful when there are several requests for the same content. That directive ensures that only one of those identical requests is sent to the origin server. The remaining requests wait for it to finish and then they are served directly from the cache.

NGINX's cache configuration allows to split the cache distribution between multiple hard drives. $request_uri is used to determine which cache to use for each request. This way request for the same URL are cached in the same location.

proxy_cache_path /path/to/hd1 levels=1:2 keys_zone=cache_hd1:10m max_size=10g inactive=69m use_temp_path=off;
proxy_cache_path /path/to/hdd2 levels=1:2 keys_zone=cache_hd2:10m max_size=10g inactive=69m use_temp_path=off;

split_clients $request_uri $just_cache {
  50%  "cache_hd1";
  50%  "cache_hd2";
}


server {
    location / {
        proxy_cache $just_cache;
        proxy_pass http://upstream;
    }
}

NGINX follows Cache-Control header in responses from the origin server to determine if something should be cached or not. It does not cache responses with Cache-Control set to Private, No-Cache, No-Store or with Set-Cookie in the response header. NGINX doesn't cache responses if proxy_buffering is set to off as well. Cache-Control headers can be ignored with proxy_ignore_headers set to Cache-Control.

By default, only GET and HEAD requests are cached. This can be changed with proxy_cache_methods. Its default value is GET HEAD and it can be extended with POST if needed.

Pragma header with no-cache value is used by clients to bypass cache layer and go straight to the origin server. This header is not activated by default. This feature can be configured using proxy_cache_bypass directive set to $http_pragma.

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