Einhorn: the language-independent shared socket manager
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Einhorn: the language-independent shared socket manager


Let's say you have a server process which processes one request at a time. Your site is becoming increasingly popular, and this one process is no longer able to handle all of your inbound connections. However, you notice that your box's load number is low.

So you start thinking about how to handle more requests. You could rewrite your server to use threads, but threads are a pain to program against (and maybe you're writing in Python or Ruby where you don't have true threads anyway). You could rewrite your server to be event-driven, but that'd require a ton of effort, and it wouldn't help you go beyond one core. So instead, you decide to just run multiple copies of your server process.

Enter Einhorn. Einhorn makes it easy to run (and keep alive) multiple copies of a single long-lived process. If that process is a server listening on some socket, Einhorn will open the socket in the master process so that it's shared among the workers.

Einhorn is designed to be compatible with arbitrary languages and frameworks, requiring minimal modification of your application. Einhorn is simple to configure and run.


Install from Rubygems as:

$ gem install einhorn

Or build from source by:

$ gem build einhorn.gemspec

And then install the built gem.


Einhorn is the language-independent shared socket manager. Run einhorn -h to see detailed usage. At a high level, usage looks like the following:

einhorn [options] program

Einhorn will open one or more shared sockets and run multiple copies of your process. You can seamlessly reload your code, dynamically reconfigure Einhorn, and more.


To set Einhorn up as a master process running 3 copies of sleep 5:

$ einhorn -n 3 sleep 5

You can communicate your running Einhorn process via einhornsh:

$ einhornsh
Welcome gdb! You are speaking to Einhorn Master Process 11902
Enter 'help' if you're not sure what to do.

Type "quit" or "exit" to quit at any time
> help
You are speaking to the Einhorn command socket. You can run the following commands:

Server sockets

If your process is a server and listens on one or more sockets, Einhorn can open these sockets and pass them to the workers. You can specify the addresses to bind by passing one or more -b ADDR arguments:

einhorn -b my-command
einhorn -b,r -b my-command

Each address is specified as an ip/port pair, possibly accompanied by options:

ADDR := (IP:PORT)[<,OPT>...]

In the worker process, the opened file descriptors will be represented as file descriptor numbers in a series of environment variables named EINHORN_FD_1, EINHORN_FD_2, etc. (respecting the order that the -b options were provided in), with the total number of file descriptors in the EINHORN_FD_COUNT environment variable:

EINHORN_FD_1="6" #

EINHORN_FD_1="6" #,r
EINHORN_FD_2="7" #

Valid opts are:

r, so_reuseaddr: set SO_REUSEADDR on the server socket
n, o_nonblock: set O_NONBLOCK on the server socket

You can for example run:

$ einhorn -b,r -m manual -n 4 -- example/time_server

Which will run 4 copies of

EINHORN_FD_1=6 EINHORN_FD_COUNT=1 example/time_server

Where file descriptor 6 is a server socket bound to and with SO_REUSEADDR set. It is then your application's job to figure out how to accept() on this file descriptor.

Command socket

Einhorn opens a UNIX socket to which you can send commands (run help in einhornsh to see what admin commands you can run). Einhorn relies on file permissions to ensure that no malicious users can gain access. Run with a -d DIRECTORY to change the directory where the socket will live.

Note that the command socket uses a line-oriented YAML protocol, and you should ensure you trust clients to send arbitrary YAML messages into your process.

Seamless upgrades

You can cause your code to be seamlessly reloaded by upgrading the worker code on disk and running

$ einhornsh
> upgrade

Once the new workers have been spawned, Einhorn will send each old worker a SIGUSR2. SIGUSR2 should be interpreted as a request for a graceful shutdown.


After Einhorn spawns a worker, it will only consider the worker up once it has received an ACK. Currently two ACK mechanisms are supported: manual and timer.

Manual ACK

A manual ACK (configured by providing a -m manual) requires your application to send a command to the command socket once it's ready. This is the safest ACK mechanism. If you're writing in Ruby, just do

require 'einhorn/worker'

in your worker code. If you're writing in a different language, or don't want to include Einhorn in your namespace, you can send the string

{"command":"worker:ack", "pid":PID}

to the UNIX socket pointed to by the environment variable EINHORN_SOCK_PATH. (Be sure to include a trailing newline.)

To make things even easier, you can pass a -g to Einhorn, in which case you just need to write() the above message to the open file descriptor pointed to by EINHORN_SOCK_FD.

(See lib/einhorn/worker.rb for details of these and other socket discovery mechanisms.)

Timer ACK [default]

By default, Einhorn will use a timer ACK of 1 second. That means that if your process hasn't exited after 1 second, it is considered ACK'd and healthy. You can modify this timeout to be more appropriate for your application (and even set to 0 if desired). Just pass a -m FLOAT.


If you're running a Ruby process, Einhorn can optionally preload its code, so it only has to load the code once per upgrade rather than once per worker process. This also saves on memory overhead, since all of the code in these processes will be stored only once using your operating system's copy-on-write features.

To use preloading, just give Einhorn a -p PATH_TO_CODE, and make sure you've defined an einhorn_main method.

In order to maximize compatibility, we've worked to minimize Einhorn's dependencies. It has no dependencies outside of the Ruby standard library.

Command name

You can set the name that Einhorn and your workers show in PS. Just pass -c <name>.


-b, --bind ADDR                  Bind an address and add the corresponding FD via the environment
-c, --command-name CMD_NAME      Set the command name in ps to this value
-d, --socket-path PATH           Where to open the Einhorn command socket
-e, --pidfile PIDFILE            Where to write out the Einhorn pidfile
-f, --lockfile LOCKFILE          Where to store the Einhorn lockfile
-g, --command-socket-as-fd       Leave the command socket open as a file descriptor, passed in the EINHORN_SOCK_FD environment variable. This allows your worker processes to ACK without needing to know where on the filesystem the command socket lives.
-h, --help                       Display this message
-k, --kill-children-on-exit      If Einhorn exits unexpectedly, gracefully kill all its children
-l, --backlog N                  Connection backlog (assuming this is a server)
-m, --ack-mode MODE              What kinds of ACK to expect from workers. Choices: FLOAT (number of seconds until assumed alive), manual (process will speak to command socket when ready). Default is MODE=1.
-n, --number N                   Number of copies to spin up
-p, --preload PATH               Load this code into memory, and fork but do not exec upon spawn. Must define an "einhorn_main" method
-q, --quiet                      Make output quiet (can be reconfigured on the fly)
-s, --seconds N                  Number of seconds to wait until respawning
-v, --verbose                    Make output verbose (can be reconfigured on the fly)
    --with-state-fd STATE        [Internal option] With file descriptor containing state
    --version                    Show version


Contributions are definitely welcome. To contribute, just follow the usual workflow:

  1. Fork Einhorn
  2. Create your feature branch (git checkout -b my-new-feature)
  3. Commit your changes (git commit -am 'Added some feature')
  4. Push to the branch (git push origin my-new-feature)
  5. Create new Github pull request


Einhorn came about when Stripe was investigating seamless code upgrading solutions for our API worker processes. We really liked the process model of Unicorn, but didn't want to use its HTTP functionality. So Einhorn was born, providing the master process functionality of Unicorn (and similar preforking servers) to a wider array of applications.

See https://stripe.com/blog/meet-einhorn for more background.

Stripe currently uses Einhorn in production for a number of services. Our Thin + EventMachine servers currently require patches to both Thin and EventMachine (to support file-descriptor passing). You can obtain these patches from our public forks of the respective projects. Check out example/thin_example for an example of running Thin under Einhorn.


Einhorn was developed and tested under Ruby 1.8.7.


Einhorn is a project of Stripe, led by Greg Brockman. Feel free to get in touch at info@stripe.com.