Caching

Caching lets you offload work from your application servers by returning cached resources directly from the load balancer. It can be applied to any reusable content that is safe to be shared among multiple clients, such as:

• CSS, Javascript and icon files
• API responses (e.g. JSON) that do not contain client-specific data
• Small, static HTML pages

The cache runs in memory and does not store state on disk. This makes it very fast, but not durable past a reload of the load balancer. We recommend storing objects for a short duration, such as less than a minute to reduce the chance of serving stale content.

Enable the cache Jump to heading #

1. Add a cache section to your configuration.

   haproxy
   cache mycache
      total-max-size 4
      max-object-size 10000
      max-age 240

   haproxy
   cache mycache
      total-max-size 4
      max-object-size 10000
      max-age 240

   Directive	Description
   total-max-size	Sets the total memory the cache can consume in megabytes. The maximum value is 4095.
   max-object-size	Sets the max size in bytes for a single object; Larger objects will not be stored. It cannot be larger than half of total-max-size.
   max-age	Sets the seconds for an object to stay the cache; It can be overridden with a Cache-Control header.

2. Add an http-request cache-use and an http-response cache-store directive to a frontend or listen section to enable caching in that section. The filter line is required only when the section includes other filter lines, but otherwise can be omitted, since it is implicitly defined.

   haproxy
   frontend  fe_main
      bind :80
      filter cache mycache
      http-request cache-use mycache
      http-response cache-store mycache
      default_backend be_main

   haproxy
   frontend  fe_main
      bind :80
      filter cache mycache
      http-request cache-use mycache
      http-response cache-store mycache
      default_backend be_main

   Directive	Description
   http-request cache-use	Fetches the requested resource from the cache. Place an if or unless statement afterwards to include or exclude a resource from being cached.
   http-response cache-store	Saves the resource to the cache once it comes from the server.

Cache based on a condition Jump to heading #

You can select which resources to cache by using an if statement on the http-request cache-use line. You need an if statement only on the http-request cache-use line and not the http-response cache-store line too. The load balancer will not store items in the cache if there is no chance of them being used.

Below, we add the ACL path_beg /api/ to cache responses when the requested URL path begins with /api/:

haproxy
cache mycache
  total-max-size 4095
  max-object-size 10000
  max-age 30
backend servers
  http-request cache-use mycache if { path_beg /api/ }
  http-response cache-store mycache
  server s1 192.168.1.25:80 check
  server s2 192.168.1.26:80 check

haproxy
cache mycache
  total-max-size 4095
  max-object-size 10000
  max-age 30
backend servers
  http-request cache-use mycache if { path_beg /api/ }
  http-response cache-store mycache
  server s1 192.168.1.25:80 check
  server s2 192.168.1.26:80 check

Cache restrictions Jump to heading #

Objects are cached only if all of the following are true:

• The size of the resource does not exceed max-object-size
• The response from the server is 200 OK
• The response does not have a Vary header
• The response does not have a Cache-Control: no-cache header

See also Jump to heading #

• Cache reference