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Highway of Life
Asked: 2013-03-30 13:47:21 +0800 CST

Varnish configure a backend to have multiple probes?

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Is it possible in Varnish 3 to configure a backend to have multiple probes?

I have multiple varnish servers and multiple backend servers running Drupal. I've configured a basic 1 second interval healthcheck PHP file to verify the health of the application server, but I would also like to use a healthcheck for the Drupal stack, however this check would need to run less often than the basic healthcheck. e.g.: Every 5 seconds it could check the Drupal stack.

Is it possible to use 2 separate probes on a single backend for such a case?

Theoretical configuration:

probe healthcheck {
    .url = "/healthcheck.php";
    .interval = 1s;
    .timeout = 100 ms;
    .window = 5;
    .threshold = 5;
}

probe drupalcheck {
    .url = "/index.php";
    .interval = 5s;
    .timeout = 5s;
    .window = 5;
    .threshold = 5;
}

backend apache_1 {.host = "server01"; .port = "8080"; .probe = healthcheck; .probe = drupalcheck}
backend apache_2 {.host = "server02"; .port = "8080"; .probe = healthcheck; .probe = drupalcheck}
backend apache_3 {.host = "server03"; .port = "8080"; .probe = healthcheck; .probe = drupalcheck}
varnish

2 Answers

  1. Quick answer: no.

    Longer answer: You can specify as many backends to a single actual backend server, and each can have it's own probe. You can then route traffic to them as you want.

    You could also call something like a 'probe-check' script on your application server via the probe call, which could do it's own status checks. You're just calling a php/python/etc script anyway, so it can be a script you created.

    There's more (not totally relevant) here: https://www.varnish-cache.org/lists/pipermail/varnish-misc/2011-October/021298.html

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  2. The probe just checks the header returned. Thus you can have ohne script /probe.php which first sends a request to /healthcheck.php and afterwards another one to index.php.

    Inside /probe.php you can use $headers = get_headers($url,1); and analyze for a 40X or 50X return code on each of the urls.

    At the end:

    if($atleastOneOfTheUrlsReturnedSomeThingBad){
  header('HTTP/1.0 404 Not Found');
}
else{
  header( "HTTP/1.1 200 OK" );
}
exit();
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