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 inthe official erlang tarball. If you are on Debian/Ubuntu, install theerlang-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 pyerlplugin will add Erlang functions to the Python plugin.

Activating Erlang support

You only need to set two options to enable Erlang support in yourErlang-enabled uWSGI build. The erlang option sets the Erlang node name ofyour 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. Ifyou do not specify it, the value is taken from the ~/.erlang.cookiefile.

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 toyour uWSGI server machine and define a new WSGI module – let’s call iterhello.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.

Sending messages to Erlang nodes

One of the most powerful features of Erlang is the inter-node message passingsystem. uWSGI can communicate with Erlang nodes as well. Lets define a newErlang 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. Usingpids to identify processes is problematic. Now you can send messages withuwsgi.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 thename, pass a 3-tuple of Python elements to be interpreted as a Pid. If yourErlang server returns messages to your requests you can receive them withuwsgi.erlang_recv_message(). Remember that even if Erlang needs aprocess 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 uWSGIserver. To receive Erlang messages you have to register “Erlang processes” inyour 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 configuredas an Erlang node.

Connection persistence

On high-loaded sites opening and closing connections for every Erlanginteraction is overkill. Open a connection on your app initialization withuwsgi.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 anErlang module to expose all the database interaction you need and useuwsgi.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 aspecial modifier for EWGI apps.

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