god: watch file descriptors

by Gregor Uhlenheuer on December 14, 2014

A while ago at work we had some trouble with the C# driver for MySQL leaking file descriptors when connecting via HTTPS. Every request of the service using the MySQL driver leaked one file descriptor resulting in rapidly exceeding the maximum allowed file descriptors of the process. Afterwards the web service was effectively broken as it wasn’t able to accept any connections/requests at all.

After hours of investigating we finally found and reported the bug in the MySQL driver itself. Nevertheless this behavior inspired me to write a tiny extension for the process monitoring tool god.

god watch

The custom watch extends the basic PollCondition behavior which basically polls the number of open file descriptors of the watched process and restarts the service when a configured threshold is exceeded.

This is what the extension looks like:

module God
    module Conditions

        # Condition Symbol :file_descriptors
        # Type: Poll
        # Trigger when the process own more than a specified amount of
        # open file descriptors.
        # Parameters
        #   Required
        #     +pid_file+ is the pid file of the process in question. Automatically
        #                populated for Watches.
        #     +above+    is the amount of maximum allowed open file descriptors
        #   Optional
        #     +times+    number of checks that have to fail to be triggered
        # Examples
        #   Trigger if the process owns more than 256 file descriptors in
        #   at least 3 of the last 5 checks (from a Watch):
        #   on.condition(:file_descriptors) do |c|
        #     c.above = 256
        #     c.times = [3, 5]
        #   end
        #   Non-Watch Tasks must specify a PID file:
        #   on.condition(:file_descriptors) do |c|
        #     c.above = 512
        #     c.pid_file = "/var/run/service.3000.pid"
        #   end
        class FileDescriptors < PollCondition
            attr_accessor :above, :pid_file, :times

            def initialize
                self.above = nil
                self.times = [1, 1]

            def prepare
                if self.times.kind_of?(Integer)
                    self.times = [self.times, self.times]

                @timeline = Timeline.new(self.times[1])

            def reset

            def pid
                self.pid_file ? File.read(self.pid_file).strip.to_i : self.watch.pid

            def valid?
                valid = true
                valid &= complain("Attribute 'pid_file' must be specified", self) if self.pid_file.nil? && self.watch.pid_file.nil?
                valid &= complain("Attribute 'above' must be specified", self) if self.above.nil?

            def test
                fds = Dir["/proc/#{self.pid}/fd/*"].size

                if @timeline.select { |x| x > self.above }.size >= self.times.first
                    output = @timeline.map { |x| "#{x > self.above ? '*' : ''}#{x}" }.join(", ")
                    self.info = "max file descriptors reached: [#{output}]"

                    return true
                    return false


As described in the documentation comments you would can easily integrate the watch in your god configuration using the file_descriptors condition:

The above setup would cause the process to be restarted as soon as it exceeds 512 open file descriptors.

This post is tagged with ruby, god and linux