Apache Module mod_lbmethod_byrequests

Description:Request Counting load balancer scheduler algorithm for mod_proxy_balancer
Status:Extension
Module Identifier:lbmethod_byrequests_module
Source File:mod_lbmethod_byrequests.c
Compatibility:Split off from mod_proxy_balancer in 2.3

Summary

This module does not provide any configuration directives of its own. It requires the services of mod_proxy_balancer, and provides the byrequests load balancing method.

Request Counting Algorithm

Enabled via lbmethod=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 is how much we expect this worker to work, or the workers' work quota. This is a normalized value representing their "share" of the amount of work to be done.

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

The worker 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 = worker

candidate lbstatus -= total factor

If a balancer is configured as follows:

workerabcd
lbfactor25252525
lbstatus0000

And b gets disabled, the following schedule is produced:

workerabcd
lbstatus-5002525
lbstatus-250-2550
lbstatus0000
(repeat)

That is it schedules: a c d a c d a c d ... Please note that:

workerabcd
lbfactor25252525

Has the exact same behavior as:

workerabcd
lbfactor1111

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

workerabc
lbfactor141

worker b will, on average, get 4 times the requests that a and c will.

The following asymmetric configuration works as one would expect:

workerab
lbfactor7030
lbstatus-3030
lbstatus40-40
lbstatus10-10
lbstatus-2020
lbstatus-5050
lbstatus20-20
lbstatus-1010
lbstatus-4040
lbstatus30-30
lbstatus00
(repeat)

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

Updated at: 9 months ago
Run-time Configuration DirectivesTable of contentMulti-Processing Modules (MPMs)