vmod_directors

Varnish Directors Module

Manual section:3

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.

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);