Apache Module mod_lbmethod_byrequests

Summary

This module does not provide any configuration directives of its own.It requires the services ofmod_proxy_balancer, andprovides thebyrequests load balancing method.

Topics

• Request Counting Algorithm

Directives

This module provides no directives.

Bugfix checklist

• httpd changelog
• Known issues
• Report a bug

See also

• mod_proxy
• mod_proxy_balancer
• Comments

Request Counting Algorithm

Enabled vialbmethod=byrequests, the idea behind this scheduler is that we distribute the requests among the various workers to ensure that each gets their configured share of the number of requests. It works as follows:

lbfactor ishow much we expect this worker to work, orthe workers' work quota. This is a normalized value representing their "share" of the amount of work to be done.

lbstatus ishow urgent this worker has to work to fulfill its quota of work.

Theworker is a member of the load balancer, usually a remote host serving one of the supported protocols.

We distribute each worker's work quota to the worker, and then look which of them needs to work most urgently (biggest lbstatus). This worker is then selected for work, and its lbstatus reduced by the total work quota we distributed to all workers. Thus the sum of all lbstatus does not change(*) and we distribute the requests as desired.

If some workers are disabled, the others will still be scheduled correctly.

for each worker in workers    worker lbstatus += worker lbfactor    total factor    += worker lbfactor    if worker lbstatus > candidate lbstatus        candidate = workercandidate lbstatus -= total factor

If a balancer is configured as follows:

Andb gets disabled, the following schedule is produced:

That is it schedules:acdacdacd ... Please note that:

Has the exact same behavior as:

This is because all values oflbfactor are normalized with respect to the others. For:

workerb will, on average, get 4 times the requests thata andc will.

The following asymmetric configuration works as one would expect:

That is after 10 schedules, the schedule repeats and 7a are selected with 3b interspersed.