Regex Revalidate Plugin

This plugin allows for the creation of rules which match regular expressions against mapped URLs to determine if and when a cache object revalidation should be forced.

Purpose

This plugin’s intended use is the selective forcing of revalidations on cache objects which are not yet marked as stale in Traffic Server but which may have been updated at the origin - without needing to alter cache control headers, pre-emptively purge the object from the cache manually, or adjust the global cache revalidation settings (such as fuzz times) used by other plugins.

Forced cache revalidations may be as specifically or loosely targeted as a regular expression against your origin URLs permits. Thus, individual cache objects may have rules created for them, or entire path prefixes, or even any cache objects with a particular file extension.

Installation

To make this plugin available, you must enable experimental plugins when building Traffic Server:

./configure --enable-experimental-plugins

Configuration

This plugin is enabled via the plugin.config configuration file, with two required arguments: the path to a rules file, and the path to a log file:

regex_revalidate.so -c <path to rules> -l <path to log>

The rule configuration file format is described below in Revalidation Rules.

The plugin regularly (every 60 seconds) checks its rules configuration file for changes. If the file has been modified since its last scan, the contents are read and the in-memory rules list is updated. Thus, new rules may be added and existing ones modified without requiring a service restart every time (as long as the delay of up to 60 seconds is acceptable).

Revalidation Rules

Inside your revalidation rules configuration, each rule line is defined as a regular expression followed by an integer which expresses the epoch time at which the rule will expire:

<regular expression> <rule expiry, as seconds since epoch>

Blank lines and lines beginning with a # character are ignored.

Matching Expression

PCRE style regular expressions are supported and should be used to match against the complete remapped URL of cache objects (not the original client-side URL), including protocol scheme and origin server domain.

Rule Expiration

Every rule must have an expiration associated with it. The rule expiration is expressed as an integer of seconds since epoch (equivalent to the return value of time(2)), after which the forced revalidation will no longer occur.

Caveats

Matches Only Post-Remapping

The regular expressions in revalidation rules see only the final, remapped URL in a transaction. As such, they cannot be used to distinguish between two client-facing URLs which are mapped to the same origin object. This is due to the fact that the plugin uses TS_HTTP_CACHE_LOOKUP_COMPLETE_HOOK.

Removing Rules

While new rules are added dynamically (the configuration file is checked every 60 seconds for changes), rule lines removed from the configuration file do not currently lead to that rule being removed from the running plugin. In these cases, if the rule must be taken out of service, a service restart may be necessary.

Examples

The following rule would cause the cache object whose origin server is origin.tld and whose path is /images/foo.jpg to be revalidated by force in Traffic Server until 6:47:27 AM on Saturday, November 14th, 2015 (UTC):

http://origin\.tld/images/foo\.jpg 1447483647

Note the escaping of the . metacharacter in the rule’s regular expression.