NAME

       Perlbal::Manual::HighPriority - Perlbal's high/low priority queueing system.

   VERSION
       Perlbal 1.78.

   DESCRIPTION
       This document describes Perlbal's high/low priority queueing system.

       Queuing 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

       Perlbal can use cookies to determine if a request should go to the high priority queue (configurable).

       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

       Here's 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.

       Queue relief

       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

       SEE ALSO

       "make_high_priority" and "make_low_priority" in Perlbal::Manual::Hooks, Perlbal::Plugin::HighPriority.