Note: Always Up is a feature that was innovated and engineered by UNIXy back in 2011 and was deployed on some of our clients’ high traffic servers and clusters since. A detailed article was written on this technology: http://blog.unixy.net/2010/11/3-state-throttle-web-server/
Always Up is a new feature in the cPanel Varnish Plugin starting from version 2.4.0. This feature addresses a serious issue that affects most busy Web servers especially at times when one could be unavailable or away from their desk. As you know, Web traffic is highly erratic. A well behaving server with normal traffic could become unresponsive and crash within minutes due to a traffic spike (slashdot, reddit, viral, news, events, etc) or a runaway process within the system itself (backup, batch job, etc). Always Up is highly dynamic in the sense that it doesn’t kick in and go into effect until server vitals hit a specific programmable thresholds. Thresholds you get to set (in WHM -> Varnish -> Always Up). It’s dynamic because it’s a throttle-based caching mechanism that gets more aggressive as the load climbs up.
Once the system triggers Always Up, it sets in motion aggressive caching measures so the server is able to cope with the extra traffic as gracefully as possible. And as soon as traffic subsides, it reverts back to its original pristine state. Always Up requires the input of three pairs of variables (ex 30:1800, 60:3600, 180:7400).
Each variable (ex: 30:1800) contains two numbers, which are separated by a colon. The first part of the variable designates the threshold 5-minute load average number at which Always Up intervenes to modify caching aggressiveness and set the caching in motion in the system. So as soon as the 5-minute load average is registered in the system, a systemic caching TTL corresponding to that threshold is set in the system.
So in the example above of 30:1800, 60:3600, 180:7400, a 30 5-minute load average will trigger a systemic caching level value of 1800 seconds. If the load continues to climb or is at 60 load average, the TTL becomes 3600 seconds. And so on. The TTL is reverted as the load decreases back down in lock step fashion.