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https://github.com/stripe/einhorn

Einhorn: the language-independent shared socket manager
https://github.com/stripe/einhorn

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Einhorn: the language-independent shared socket manager

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# Einhorn: the language-independent shared socket manager



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.



## Installation



Install from Rubygems as:



    $ gem install einhorn



Or build from source by:



    $ gem build einhorn.gemspec



And then install the built gem.



## Usage



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.



## Overview



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 127.0.0.1:1234 my-command

    einhorn -b 127.0.0.1:1234,r -b 127.0.0.1:1235 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_0, EINHORN_FD_1, 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_0="6" # 127.0.0.1:1234

    EINHORN_FD_COUNT="1"



    EINHORN_FD_0="6" # 127.0.0.1:1234,r

    EINHORN_FD_1="7" # 127.0.0.1:1235

    EINHORN_FD_COUNT="2"



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 127.0.0.1:2345,r -m manual -n 4 -- example/time_server



Which will run 4 copies of



    EINHORN_FD_0=6 EINHORN_FD_COUNT=1 example/time_server



Where file descriptor 6 is a server socket bound to `127.0.0.1:2345`

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.



### ACKs



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'

    Einhorn::Worker.ack!



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`.



### Preloading



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 `.



### Re exec



You can use the `--reexec-as` option to replace the `einhorn` command with a command or script of your own. This might be useful for those with a Capistrano like deploy process that has changing symlinks. To ensure that you are following the symlinks you could use a bash script like this.



    #!/bin/bash



    cd 

    exec /usr/local/bin/einhorn "$@"



Then you could set `--reexec-as=` to the name of your bash script and it will run in place of the plain einhorn command.



### Options



    -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)

        --drop-env-var VAR_NAME      Delete VAR_NAME from the environment that is restored on upgrade

        --reexec-as=CMDLINE          Substitute CMDLINE for \"einhorn\" when upgrading

        --nice MASTER[:WORKER=0][:RENICE_CMD=/usr/bin/renice]

                                     Unix nice level at which to run the einhorn processes. If not running as root, make sure to ulimit -e as appopriate.

        --with-state-fd STATE        [Internal option] With file descriptor containing state

        --upgrade-check              [Internal option] Check if Einhorn can exec itself and exit with status 0 before loading code

    -t, --signal-timeout=T           If children do not react to signals after T seconds, escalate to SIGKILL

        --version                    Show version



## History



Einhorn came about when Stripe was investigating seamless code

upgrading solutions for our API worker processes. We really liked the

process model of [Unicorn](http://unicorn.bogomips.org/), 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.



Read more background at [Meet Einhorn](https://web.archive.org/web/20210507145921/https://stripe.com/blog/meet-einhorn).



Stripe currently uses Einhorn in production for a number of

services. You can use Conrad Irwin's thin-attach_socket gem along with

EventMachine-LE to support file-descriptor passing. Check out

`example/thin_example` for an example of running Thin under Einhorn.



## Compatibility



Einhorn runs in Ruby 2.5+.



The following libraries ease integration with Einhorn with languages other than

Ruby:



- **[go-einhorn](https://github.com/stripe/go-einhorn)**: Stripe's own library

  for *talking* to an einhorn master (doesn't wrap socket code).

- **[goji](https://github.com/zenazn/goji/)**: Go (golang) server framework. The

  [`bind`](https://godoc.org/github.com/zenazn/goji/bind) and

  [`graceful`](https://godoc.org/github.com/zenazn/goji/graceful)

  packages provide helpers and HTTP/TCP connection wrappers for Einhorn

  integration.

- **[github.com/CHH/einhorn](https://github.com/CHH/einhorn)**: PHP library

- **[thin-attach\_socket](https://github.com/ConradIrwin/thin-attach_socket)**:

  run `thin` behind Einhorn

- **[baseplate](https://reddit.github.io/baseplate/cli/serve.html)**: a

  collection of Python helpers and libraries, with support for running behind

  Einhorn



*NB: this list should not imply any official endorsement or vetting!*



## About



Einhorn was a project of [Stripe](https://stripe.com).

It is now maintained by [Contributed Systems](https://contribsys.com).