vmod_saintmode

Varnish Saintmode Module

Author:	Dag Haavi Finstad
Date:	2014-10-28
Version:	1.0
Manual section:	3

SYNOPSIS

import saintmode;

DESCRIPTION

This VMOD provides saintmode functionality for Varnish Cache 4.1.0 and newer. The code is in part based on Poul-Henning Kamp's saintmode implementation in Varnish 3.0.

Saintmode lets you deal with a backend that is failing in random ways for specific requests. It maintains a blacklist per backend, marking the backend as sick for specific objects. When the number of objects marked as sick for a backend reaches a set threshold, the backend is considered sick for all requests. Each blacklisted object carries a TTL, which denotes the time it will stay blacklisted.

Saintmode in Varnish 4.1.0 is implemented as a director VMOD. We instantiate a saintmode object and give it a backend as an argument. The resulting object can then be used in place of the backend, with the effect that it also has added saintmode capabilities.

Example:

backend b0 {
      .host = "foo";
      .port = "8080";
}

backend b1 {
      .host = "bar";
      .port = "8080";
}

sub vcl_init {
      # Instantiate sm0, sm1 for backends b0, b1
      new sm0 = saintmode.saintmode(b0, 10);
      new sm1 = saintmode.saintmode(b1, 10);

      # Add both to a director. Use sm0, sm1 in place of b0, b1
      new mydir = directors.random();
      mydir.add_backend(sm0.backend(), 1);
      mydir.add_backend(sm1.backend(), 1);
}

sub vcl_backend_fetch {
      set bereq.backend = mydir.backend();
}

sub vcl_backend_response {
      if (beresp.status == 500) {
              # This marks the backend as sick for this specific
              # object for the next 20s.
              saintmode.blacklist(20s);
              return (retry);
      }
}

FUNCTIONS

saintmode.saintmode

Prototype

saintmode.saintmode(BACKEND b, INT threshold)

Description

Constructs a saintmode director object. The threshold argument sets the saintmode threshold, which is the maximum number of items that can be blacklisted before the whole backend is regarded as sick. Corresponds with the saintmode_threshold parameter of Varnish 3.0.

Example

sub vcl_init {
        new sm = saintmode.saintmode(b, 10);
}

saintmode.backend()

Prototype

BACKEND saintmode.backend()

Description

Used for assigning the backend from the saintmode object.

Example

sub vcl_backend_fetch {
        set bereq.backend = sm.backend();
}

blacklist()

Prototype

VOID blacklist(DURATION expires)

Description

Marks the backend as sick for a specific object. Used in vcl_backend_response. Corresponds to the use of beresp.saintmode in Varnish 3.0. Only available in vcl_backend_response.

Example

sub vcl_backend_response {
        if (beresp.http.broken-app) {
                saintmode.blacklist(20s);
                return (retry);
        }

}

INSTALLATION

The source tree is based on autotools to configure the building, and does also have the necessary bits in place to do functional unit tests using the varnishtest tool.

Usage:

./configure

Make targets:

• make - builds the vmod
• make install - installs your vmod
• make check - runs the unit tests in src/tests/*.vtc

COPYRIGHT

This document is licensed under the same license as the libvmod-saintmode project. See LICENSE for details.

• Copyright (c) 2011-2015 Varnish Software