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PID files

Distillery now has the ability to create a PID file during boot. A PID file is a file which contains the PID of the executable which generated it. When an application terminates, that file is removed. If it is removed while the application is running, the application terminates. If the application restarts, a new PID is written to the file.

These files are used in particular by system supervisors, such as systemd, to better monitor and interact with the services under their purvey. In general I recommend using foreground mode when running under systemd, but if you prefer to run the service as a daemon via start, and have systemd monitor that, PID file support provides better integration than previous releases.


If you are looking into this because you are using systemd, I would encourage you to instead define a service unit which leverages the foreground task, rather than start. Not only do you not need a PID file in this case, but you get automatic log management via syslog/journalctl. This feature was added because that isn’t always an option, but if you have the choice, the above is a simpler approach.


To turn on PID file generation, you have two choices:

  • Export an environment variable, PIDFILE="path/to/pidfile"
  • Set a flag in vm.args, -kernel pidfile "'path/to/pidfile'"


We need to use two level of quoting, due to how erl parses the command line arguments


We use the kernel application for this setting because the PID file manager process is started as a kernel process, so to access the configuration, it must be part of the kernel application.

If either of these are set, when the PID file manager is started, it will write the current PID to the given path, and then monitor it for changes. If it detects that the file has been deleted, then the node is terminated. If asked to terminate, it will clean up the PID file before doing so.


If you terminate a node brutally, the PID file will remain, since our process will never have a chance to execute any cleanup in that case - but shutting down that way is not recommended for a variety of reasons, one of which is the likelihood of resources being left hanging in the wind. If the PID file still exists when the release is restarted, it will be overwritten with the new PID, but it may cause some confusion for external processes like systemd.