Detecting unicorn / defining after_fork after master startup

Eric Wong normalperson at
Thu Jul 19 23:13:34 UTC 2012

Matt Sanders <matt at> wrote:
> 1. What is the best way to determine whether the app is indeed running
> inside a unicorn server?

Unicorn.listener_names will return a non-empty array of listener

> 2. Is there an established way for adding an after_fork hook after
> Unicorn has already started up?

Nothing that I know of.

> I'm aware of the ability to do this via the config file but I don't
> want my users to have to add something to their unicorn config file.
> Is there an equivalent to passenger's
> PhusionPassenger.on_event(:starting_worker_process) method?

Lately, I've been favoring the following pattern instead:

  def initialize
    @init_pid = $$

  def initialize_child_fork
    @init_pid = $$

  def call(env)
    initialize_child_fork if @init_pid != $$

The overhead is negligible and works regardless of server (even in
non-HTTP servers somebody may write).  I really don't like code
that would need to special case for all sorts of servers:

  foo_for_passenger if defined?(PhusionPassenger)
  foo_for_unicorn if defined?(Unicorn)
  foo_for_something_fastcgi if defined?(SomethingFastCGI)
  foo_for_something_scgi if defined?(SomethingSCGI)
  foo_for_something_else_that_forks if defined?(YetAnotherForkingRackServer)
  foo_for_something_else if defined?(SomethingElse)

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