Integrating uWSGI with Erlang

Warning

Erlang support is broken as of 1.9.20. A new solution is being worked on.

The uWSGI server can act as an Erlang C-Node and exchange messages and RPC with Erlang nodes.

Building

First of all you need the ei libraries and headers. They are included in the official erlang tarball. If you are on Debian/Ubuntu, install the erlang-dev package. Erlang support can be embedded or built as a plugin. For embedding, add the erlang and pyerl plugins to your buildconf.

embedded_plugins = python, ping, nagios, rpc, fastrouter, http, ugreen, erlang, pyerl

or build both as plugins

python uwsgiconfig --plugin plugins/erlang
python uwsgiconfig --plugin plugins/pyerl

The Erlang plugin will allow uWSGI to became a Erlang C-Node. The pyerl plugin will add Erlang functions to the Python plugin.

Activating Erlang support

You only need to set two options to enable Erlang support in your Erlang-enabled uWSGI build. The erlang option sets the Erlang node name of your uWSGI server. It may be specified in simple or extended format:

  • nodename@ip
  • nodename@address
  • nodename

The erlang-cookie option sets the cookie for inter-node communications. If you do not specify it, the value is taken from the ~/.erlang.cookie file.

To run uWSGI with Erlang enabled:

uwsgi --socket :3031 --erlang testnode@192.168.173.15 --erlang-cookie UUWSGIUWSGIU -p 2

A simple RPC hello world example

  • Define a new erlang module that exports only a simple function.

    -module(uwsgitest).
    -export([hello/0]).
    
    hello() ->
        'hello world !'.
    
  • Launch the erl shell specifying the nodename and (eventually) the cookie:

    erl -name testnode@192.168.173.1
    
  • Compile the uwsgitest Erlang module

    c(uwsgitest).
    {ok,uwsgitest}
    
  • … and try to run the hello function:

    uwsgitest:hello().
    'hello world !'
    

Great - now that our Erlang module is working, we are ready for RPC! Return to your uWSGI server machine and define a new WSGI module – let’s call it erhello.py.

import uwsgi

def application(env, start_response):
    testnode = uwsgi.erlang_connect("testnode@192.168.173.1")
    start_response('200 OK', [('Content-Type', 'text/plain')])
    yield uwsgi.erlang_rpc(testnode, "uwsgitest", "hello", [])
    uwsgi.erlang_close(testnode)

or the fast-style

import uwsgi

def application(env, start_response):
    start_response('200 OK', [('Content-Type', 'text/plain')])
    yield uwsgi.erlang_rpc("testnode@192.168.173.1", "uwsgitest", "hello", [])

Now relaunch the uWSGI server with this new module:

uwsgi --socket :3031 --erlang testnode@192.168.173.15 --erlang-cookie UUWSGIUWSGIU -p 2 -w erhello

Point your browser to your uWSGI enabled webserver and you should see the output of your erlang RPC call.

Python-Erlang mappings

The uWSGI server tries to translate Erlang types to Python objects according to the table below.

Python Erlang note
str binary  
unicode atom limited by internal atom size
int/long int  
list list  
tuple tuple  
3-tuple pid  

Sending messages to Erlang nodes

One of the most powerful features of Erlang is the inter-node message passing system. uWSGI can communicate with Erlang nodes as well. Lets define a new Erlang module that simply will echo back whatever we send to it.

-module(uwsgiecho).
-export([start/0, loop/0, echo/1]).

echo(Message) ->
        {i_am_echo , Message}.

loop() ->
        receive
                Message1 ->
                        io:format("received a message~n"),
                        { useless, 'testnode@192.168.173.15' } ! echo(Message1)
        end,
        loop().

start() ->
        register(echoer, spawn(uwsgiecho, loop, [])).

Remember to register your process with the Erlang register function. Using pids to identify processes is problematic. Now you can send messages with uwsgi.erlang_send_message().

uwsgi.erlang_send_message(node, "echoer", "Hello echo server !!!" )

The second argument is the registered process name. If you do not specify the name, pass a 3-tuple of Python elements to be interpreted as a Pid. If your Erlang server returns messages to your requests you can receive them with uwsgi.erlang_recv_message(). Remember that even if Erlang needs a process name/pid to send messages, they will be blissfully ignored by uWSGI.

Receiving erlang messages

Sometimes you want to directly send messages from an Erlang node to the uWSGI server. To receive Erlang messages you have to register “Erlang processes” in your Python code.

import uwsgi

def erman(arg):
    print "received an erlang message:", arg

uwsgi.erlang_register_process("myprocess", erman)

Now from Erlang you can send messages to the “myprocess” process you registered:

{ myprocess, 'testnode@192.168.173.15' } ! "Hello".

RPC

You can call uWSGI uWSGI RPC Stack functions directly from Erlang.

rpc:call('testnode@192.168.173.15', useless, myfunction, []).

this will call the “myfunction” uWSGI RPC function on a uWSGI server configured as an Erlang node.

Connection persistence

On high-loaded sites opening and closing connections for every Erlang interaction is overkill. Open a connection on your app initialization with uwsgi.erlang_connect() and hold on to the file descriptor.

What about Mnesia?

We suggest you to use Mnesia when you need a high-availability site. Build an Erlang module to expose all the database interaction you need and use uwsgi.erlang_rpc() to interact with it.

Can I run EWGI applications on top of uWSGI?

For now, no. The best way to do this would be to develop a plugin and assign a special modifier for EWGI apps.

But before that happens, you can wrap the incoming request into EWGI form in Python code and use uwsgi.erlang_rpc() to call your Erlang app.