March 23, 2016

Wait for a port to be listening in Salt

Sometimes, you want to wait for a service to be running before running other states. Usually this can be done with a service.running state, which is then required by other states. For example, a mysql_database.present state can require the mysql service state, and it won't be ran before the mysql service has been started.

However, sometimes the service can start up but still not be ready to serve requests. I faced this problem with InfluxDB - there would be up to a 1 second delay between service influxdb start and InfluxDB actually listening on all the ports. Because of this, influxdb_user.present states would fail if the service had been restarted due to configuration changes, because the connection to port 8086 would fail.

The solution: Using until and nc/netcat.

  pkg.installed: []
    - name: influxdb
    - name: until nc -z localhost 8086; do sleep 1; done
    - timeout: 10
    - onchanges:
      - service: influxdb

    - name: example
    - passwd: example
    - require:
      - cmd: influxdb

What's happening here is that whenever the service gets restarted, that counts as a change in the service.running state. That triggers the state, which will be executed synchronosusly - in other words, it'll block other states from being executed until it completes. Then, our states that require the port to be listening simply add a requirement for the state.

In Salt 2016.3, you don't even need the cmd: in front of the requirement.

Even if your service doesn't listen on a port, this approach can still be used. All you need is to find some sort of shell comand that either blocks or exits with 1 (or higher) if your service is down, but exits with 0 when your service is up and running and fully operational. Simply replace the nc command in the example above with your command.