sifter

Consul watches act in a bit of a interesting way - when you reload the program - they all fire.

It's a fairly well known limitation that doesn't have a fix at the moment.

If it's a really lightweight process then it's not a big deal - but if it's not so lightweight - then it can be a bit of a problem.

If you take a look at what the watch passes on STDIN during a Consul reload - you will see that if it's not actually firing it just passes:

[]\n

So - sifter is a small Go binary that helps protect against event watches firing repeatedly:

{
  "watches": [
    {
      "type": "event",
      "name": "chef-client",
      "handler": "sifter run -e 'chef-client'"
    }
  ]
}

When this gets loaded into Consul - instead of launching copies and copies of chef-client processes - which is not awesome I promise - it will just say:

location='blank' elapsed='226.912µs' exec='chef-client'

When you actually send the event - the logs will show this - and the event will fire:

decoded event='chef-client' ltime='1137'
key='sifter/chef-client/i-c972d41e' value='1137'
location='complete' elapsed='47.448987142s' exec='chef-client'

Afterwards - if it sees the same event:

location='duplicate' elapsed='8.269276ms' exec='chef-client'

Real World Usage

During a cluster wide rollout of Consul changes - here are a number of events that were prevented from firing because they were duplicate or blank:

I only want Consul watches to fire when they're supposed to.

Development

brew install forego if you want to start up Redis and Consul to test.

Have added some watches in the config/ folder as well as example Consul output - input to sifter in the test/ folder.