Dispatcher Farms and Cache Invalidation

One thing to be aware of when using multiple farms configuration in the dispatcher is the anatomy of cache invalidation requests.

Cache invalidation requests are sent to the URL /dispatcher/invalidate.cache of the dispatcher. On receiving this request dispatcher checks for the CQ-Handle http header field which contains the path of the resource to be invalidated and performs the invalidation based on the configuration on the dispatcher.

When dispatcher is configured with multiple farms, it’s natural to expect that the invalidation request matches the farm based on the resource that is invalidated. But this does not happen.
Invalidation request tries to match the farm based on URL of the invalidation request which is
/dispatcher/invalidate.cache

The configuration in the matching farm identified for this URL is used for invalidation behavior. Since the invalidation URL for all the resources in AEM is the same, all invalidation requests irrespective of the resource being invalidated picks up the same matching farm.

This poses a challenge in having different invalidation configuration based on the resource being invalidated

URL Rewrite using CQ-Handle
One simple solution to handle this is to use URL rewrite on Apache to pre-pend the value from the CQ-Handle header field to the URL. This would prefix the URL with the path of the resource being flushed.

When this URL gets processed by dispatcher, it would pick up the matching farm based on the path of the resource being flushed and uses that farm configuration for invalidation