Example of usage of an Erlang port

Example of usage of an Erlang port
(April 2011)

Table of contents

Overview

Overview

This page gives an example, designed as follows:

design

The Erlang program (test_port.erl), that is in charge of the computing, sends data via a port to the Python program (test_port.py), that is in charge of the display.

The result is like this:
display

The dots are some moving bodies, and each dot is linked to the three nearest dots.

Getting Started

In order to run this example, you need at least:

Erlang R13B03 (erts-5.7.4)
Python 2.6
test_port.erl
test_port.py

Then run:

erlc +debug_info test_port.erl
erl -run test_port start

If you want to test only the Python part, you may download sample_data.log and run this:

cat sample_data.log | python test_port.py

# or if you want profiling:
cat sample_data.log | python -m cProfile test_port.py

# or if you want to debug
python -m pdb test_port.py -i sample_data.log

Erlang port

The Erlang port is technically achieved as follows:

The Erlang machine creates the Python process
A pipe is created between the Erlang machine and the Python process
On the Python side the pipe is connected to stdin and stdout

Data

Data sent via the port to the Python displayer looks like this:

...
<0.39.0>: x=300.48, y=489.93, pid1=<0.36.0>, pid2=<0.37.0>, pid3=<0.35.0>
<0.45.0>: x=372.63, y=154.37, pid1=<0.38.0>, pid2=<0.46.0>, pid3=<0.34.0>
<0.49.0>: x=674.50, y=127.33, pid1=<0.53.0>, pid2=<0.56.0>, pid3=<0.41.0>
<0.61.0>: x=910.48, y=264.60, pid1=<0.62.0>, pid2=<0.57.0>, pid3=<0.53.0>
<0.54.0>: x=545.77, y=359.24, pid1=<0.47.0>, pid2=<0.55.0>, pid3=<0.50.0>
<0.40.0>: x=152.31, y=348.52, pid1=<0.35.0>, pid2=<0.36.0>, pid3=<0.37.0>
<0.52.0>: x=496.86, y=579.55, pid1=<0.48.0>, pid2=<0.44.0>, pid3=<0.55.0>
...

test_port.erl

The present example runs 32 moving bodies.
The bodies move around randomly, except that we try to keep them close to their original position.
Download test_port.erl

Excerpt of test_port.erl

Port = open_port({spawn, "python test_port.py"}, [stream]),
...
Data = io_lib:format("~p: x=~.2f, y=~.2f, pid1=~p, pid2=~p, pid3=~p\n", [ self(), X, Y, Pid1, Pid2, Pid3 ]),
port_command(Port, Data),

test_port.py

The present example uses Tkinter for the display.
Download test_port.py

Excerpt of test_port.py

line = sys.stdin.readline()
...
matchingObject = re.match("(.*): x=([0-9\.]*), y=([0-9\.]*), pid1=(.*), pid2=(.*), pid3=(.*)", line)
if matchingObject is not None:
    pid = matchingObject.group(1)
    x = matchingObject.group(2)
    y = matchingObject.group(3)
    pid1 = matchingObject.group(4)
    pid2 = matchingObject.group(5)
    pid3 = matchingObject.group(6)