Background Fetch Plugin

This is a plugin for Apache Traffic Server that allows you to proactively fetch content from Origin in a way that it will fill the object into cache. This is particularly useful when all (or most) of your client requests are of the byte-Range type. The underlying problem being that Traffic Server is not able to cache request / responses with byte ranges.

Using the plugin

This plugin functions as either a global or per remap plugin, and it takes an optional argument for specifying a config file with inclusion or exclusion criteria. The config file can be specified both via an absolute path or via a relative path to the install dir

To activate the plugin in global mode, in plugin.config, simply add:

background_fetch.so --config <config-file>

To activate the plugin in per remap mode, in remap.config, simply append the below to the specific remap line:

@plugin=background_fetch.so @pparam=<config-file>

Functionality

Examining the responses from origin, we decide to trigger a background fetch of the original (Client) request under these conditions:

  • The request is a GET request (we only support these right now)
  • The response is a 206 response
  • The original client request, and the Origin server response, is clearly indicating that the response is cacheable. This uses the new API c:func:TSHttpTxnIsCacheable(), which also implies honoring current Traffic Server configurations.

Once deemed a good candidate to performance a background fetch, we’ll replay the original client request through the Traffic Server proxy again, except this time eliminating the Range header. This is transparent to the original client request, which continues as normal.

Only one background fetch per URL is ever performed, making sure we do not accidentally put pressure on the origin servers.

The plugin now supports a config file that can specify exclusion or inclusion of background fetch based on any arbitrary header or client-ip:

background_fetch.so --config <config-file>

The contents of the config-file could be as below:

include User-Agent ABCDEF
exclude User-Agent *
exclude Content-Type text
exclude X-Foo-Bar text
exclude Content-Length <1000

The plugin also now supports per remap activation. To activate the plugin for a given remap, add the below on the remap line:

@plugin=background_fetch.so @pparam=<config-file>

Future additions

  • Limiting the background fetches to content of certain sizes