In "our" proxy, we launch an external helper app the does active health checking of the origin servers. This is a HEAD request on a configurable (per origin "pool") uri (ie, http://host:port/url/blah). When an origin passes/fails a given number of checks it is marked up/down. For example, when an origin passes 2 health checks in a row, it is marked up; when it fails 3 in a row, it is marked down. We check each backend every x seconds (x usually equal to 5).
The proxy module and the health checker are linked via a simple array in shared memory. The health checker marks the index of each origin as 1 (up) or 0 (down). I have been contemplating writing a balancer module that implements this, but haven't had the time. With this method, we don't have to check every time, as we assume if the origin is up in the health checker that it is up. This works several million times a day for us :) (Also, we do "true" connection pooling to the origins, but that's another story...) -- Brian Akins Chief Operations Engineer Turner Digital Media Technologies
