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Signal handling

In general, signals need only be sent to the master process. However, the signals Unicorn uses internally to communicate with the worker processes are documented here as well. With the exception of TTIN/TTOU, signal handling matches the behavior of nginx so it should be possible to easily share process management scripts between Unicorn and nginx.

One example init script is distributed with unicorn: unicorn.bogomips.org/examples/init.sh

Master Process

Worker Processes

Note: as of unicorn 4.8, the master uses a pipe to signal workers instead of kill(2) for most cases. Using signals still (and works and remains supported for external tools/libraries), however.

Sending signals directly to the worker processes should not normally be needed. If the master process is running, any exited worker will be automatically respawned.

Procedure to replace a running unicorn executable

You may replace a running instance of unicorn with a new one without losing any incoming connections. Doing so will reload all of your application code, Unicorn config, Ruby executable, and all libraries. The only things that will not change (due to OS limitations) are:

  1. The path to the unicorn executable script. If you want to change to a different installation of Ruby, you can modify the shebang line to point to your alternative interpreter.

The procedure is exactly like that of nginx:

  1. Send USR2 to the master process

  2. Check your process manager or pid files to see if a new master spawned successfully. If you're using a pid file, the old process will have ".oldbin" appended to its path. You should have two master instances of unicorn running now, both of which will have workers servicing requests. Your process tree should look something like this:

    unicorn master (old)
    \_ unicorn worker[0]
    \_ unicorn worker[1]
    \_ unicorn worker[2]
    \_ unicorn worker[3]
    \_ unicorn master
       \_ unicorn worker[0]
       \_ unicorn worker[1]
       \_ unicorn worker[2]
       \_ unicorn worker[3]
  3. You can now send WINCH to the old master process so only the new workers serve requests. If your unicorn process is bound to an interactive terminal (not daemonized), you can skip this step. Step 5 will be more difficult but you can also skip it if your process is not daemonized.

  4. You should now ensure that everything is running correctly with the new workers as the old workers die off.

  5. If everything seems ok, then send QUIT to the old master. You're done!

    If something is broken, then send HUP to the old master to reload the config and restart its workers. Then send QUIT to the new master process.


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