How does the perceptive algorithm ramp up the load on new servers in a pool?

The perceptive algorithm introduces traffic to a new server (or a server that has returned from a failed state) gently. When a new server is added to a pool, the algorithm tries it with a single request, and if it receives a reply, gradually increases the number of requests it sends the new server until it is receiving the same proportion of the load as other equivalent nodes in the pool.

The algorithm used to ramp up the load is adaptive, so it isn't possible to make statements of the sort "the load will be increased from 0 to 100% of its fair share over 2 minutes"; the rate at which the load is increased is dependent on the responsiveness of the server. So, for example, a new web server serving a small quantity of static content will very quickly be ramped up to full speed, whereas a Java application server that compiles JSPs the first time they are used (and so is slow to respond to begin with) will be ramped up more slowly.

Julian [Zeus Dev Team] 27 June 2006