mod_lbmethod_byrequests - Apache HTTP Server Version 2.4









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Apache HTTP Server Version 2.4



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Apache Module mod_lbmethod_byrequests

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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.

Topics

 Request Counting Algorithm
Directives
This module provides no
            directives.
Bugfix checklisthttpd changelogKnown issuesReport a bugSee also

mod_proxy
mod_proxy_balancer
Comments


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:

    worker
        a
        b
        c
        d
lbfactor
        25
        25
        25
        25
lbstatus
        0
        0
        0
        0


    And b gets disabled, the following schedule is produced:

    worker
        a
        b
        c
        d
lbstatus
        -50
        0
        25
        25
lbstatus
        -25
        0
        -25
        50
lbstatus
        0
        0
        0
        0
(repeat)


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

    worker
        a
        b
        c
        d
lbfactor
        25
        25
        25
        25


    Has the exact same behavior as:

    worker
        a
        b
        c
        d
lbfactor
        1
        1
        1
        1


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

    worker
        a
        b
        c
lbfactor
        1
        4
        1


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

    The following asymmetric configuration works as one would expect:

    worker
        a
        b
lbfactor
        70
        30
 
lbstatus
        -30
        30
lbstatus
        40
        -40
lbstatus
        10
        -10
lbstatus
        -20
        20
lbstatus
        -50
        50
lbstatus
        20
        -20
lbstatus
        -10
        10
lbstatus
        -40
        40
lbstatus
        30
        -30
lbstatus
        0
        0
(repeat)


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



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