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VMOD_DIRECTORS(3)                                            VMOD_DIRECTORS(3)

NAME

vmod_directors - Varnish Directors Module

SYNOPSIS

import directors [from "path"] ;

DESCRIPTION

vmod_directors enables backend load balancing in Varnish. The module implements a set of basic load balancing techniques, and also serves as an example on how one could extend the load balancing capabilities of Varnish. To enable load balancing you must import this vmod (directors). Then you define your backends. Once you have the backends declared you can add them to a director. This happens in executed VCL code. If you want to emulate the previous behavior of Varnish 3.0 you can just initialize the directors in vcl_init, like this: sub vcl_init { new vdir = directors.round_robin(); vdir.add_backend(backend1); vdir.add_backend(backend2); } As you can see there is nothing keeping you from manipulating the directors elsewhere in VCL. So, you could have VCL code that would add more backends to a director when a certain URL is called. Note that directors can use other directors as backends.

CONTENTS

o Object fallback o VOID fallback.add_backend(BACKEND) o BACKEND fallback.backend() o VOID fallback.remove_backend(BACKEND) o Object hash o VOID hash.add_backend(BACKEND, REAL) o BACKEND hash.backend(STRING_LIST) o VOID hash.remove_backend(BACKEND) o Object random o VOID random.add_backend(BACKEND, REAL) o BACKEND random.backend() o VOID random.remove_backend(BACKEND) o Object round_robin o VOID round_robin.add_backend(BACKEND) o BACKEND round_robin.backend() o VOID round_robin.remove_backend(BACKEND) OBJECT ROUND_ROBIN Description Create a round robin director. This director will pick backends in a round robin fashion. Example new vdir = directors.round_robin(); VOID round_robin.add_backend(BACKEND) Prototype VOID round_robin.add_backend(BACKEND) Description Add a backend to the round-robin director. Example vdir.add_backend(backend1); vdir.add_backend(backend2); VOID round_robin.remove_backend(BACKEND) Prototype VOID round_robin.remove_backend(BACKEND) Description Remove a backend from the round-robin director. Example vdir.remove_backend(backend1); vdir.remove_backend(backend2); BACKEND round_robin.backend() Prototype BACKEND round_robin.backend() Description Pick a backend from the director. Example set req.backend_hint = vdir.backend();

OBJECT FALLBACK

Description Create a fallback director. A fallback director will try each of the added backends in turn, and return the first one that is healthy. Example new vdir = directors.fallback(); VOID fallback.add_backend(BACKEND) Prototype VOID fallback.add_backend(BACKEND) Description Add a backend to the director. Note that the order in which this is done matters for the fallback director. Example vdir.add_backend(backend1); vdir.add_backend(backend2); VOID fallback.remove_backend(BACKEND) Prototype VOID fallback.remove_backend(BACKEND) Description Remove a backend from the director. Example vdir.remove_backend(backend1); vdir.remove_backend(backend2); BACKEND fallback.backend() Prototype BACKEND fallback.backend() Description Pick a backend from the director. Example set req.backend_hint = vdir.backend();

OBJECT RANDOM

Description Create a random backend director. The random director distributes load over the backends using a weighted random probability distribution. Example new vdir = directors.random(); VOID random.add_backend(BACKEND, REAL) Prototype VOID random.add_backend(BACKEND, REAL) Description Add a backend to the director with a given weight. Each backend backend will receive approximately 100 * (weight / (sum(all_added_weights))) per cent of the traffic sent to this director. Example # 2/3 to backend1, 1/3 to backend2. vdir.add_backend(backend1, 10.0); vdir.add_backend(backend2, 5.0); VOID random.remove_backend(BACKEND) Prototype VOID random.remove_backend(BACKEND) Description Remove a backend from the director. Example vdir.remove_backend(backend1); vdir.remove_backend(backend2); BACKEND random.backend() Prototype BACKEND random.backend() Description Pick a backend from the director. Example set req.backend_hint = vdir.backend();

OBJECT HASH

Description Create a hashing backend director. The director chooses the backend server by computing a hash/digest of the string given to .backend(). Commonly used with client.ip or a session cookie to get sticky sessions. Example new vdir = directors.hash(); VOID hash.add_backend(BACKEND, REAL) Prototype VOID hash.add_backend(BACKEND, REAL) Description Add a backend to the director with a certain weight. Weight is used as in the random director. Recommended value is 1.0 unless you have special needs. Example vdir.add_backend(backend1, 1.0); vdir.add_backend(backend2, 1.0); VOID hash.remove_backend(BACKEND) Prototype VOID hash.remove_backend(BACKEND) Description Remove a backend from the director. Example vdir.remove_backend(backend1); vdir.remove_backend(backend2); BACKEND hash.backend(STRING_LIST) Prototype BACKEND hash.backend(STRING_LIST) Description Pick a backend from the backend director. Use the string or list of strings provided to pick the backend. Example # pick a backend based on the cookie header from the client set req.backend_hint = vdir.backend(req.http.cookie);

COPYRIGHT

This document is licensed under the same licence as Varnish itself. See LICENCE for details. o Copyright (c) 2013-2015 Varnish Software AS VMOD_DIRECTORS(3)

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