This document describes Perlbals high/low priority queueing system.
Perlbal has three queues: normal, high priority and low priority.
As their names suggest, this means that usually requests get to the normal queue and are dispatched in FIFO order, with high priority requests going to a different queue that gets ahead of the normal one and a low priority queue that only gets done when the high and normal queues are empty.
In a nutshell, whenever Perlbal needs to select which request to take care of next, it first looks for requests in the high priority queue; if that one is empty, it then looks into the normal queue; and, if the normal queue is empty too, it finally looks in the low priority queue.
High priority with cookies
The parameters to configure this are high_priority_cookie and high_priority_cookie_contents; the first defines the name of the field to check for on the cookie and the second one defines the content in that field that will trigger the request going to the fast queue:
SET myservice.high_priority_cookie = name_of_the_field
SET myservice.high_priority_cookie_contents = required_content_on_that_field
Heres a clearer example:
SET myservice.high_priority_cookie = highpriority
SET myservice.high_priority_cookie_contents = yes
High priority with plugins
The plugin Perlbal::Plugin::Highpri supports making requests high priority by URI or Host. Also check make_high_priority under Perlbal::Manual::Hooks.
Sometimes if the high priority queue is really busy, the standard queue will suffer from resource starvation. The queue relief system helps prevent this. When there are queue_relief_size or more connections in the standard queue, newly available backends have a queue_relief_chance percent chance of taking a request from the standard priority queue instead of the high priority queue.
SET web_proxy.queue_relief_size = 2000
SET web_proxy.queue_relief_chance = 30 # 0-100, in percent
make_high_priority and make_low_priority in Perlbal::Manual::Hooks, Perlbal::Plugin::HighPriority.