How to Set Up Erlang Common Test for Code Coverage

Common Test code coverage and usage is documented here and here, but:

  • there are no examples given
  • documentation fails to state what the minimum required configuration is to turn on code coverage
  • the implementation has no sane defaults and by default code coverage is disabled

So, here’s the minimum sane default:

  • create a cover.spec file in your application directory with the following contents
    {incl_app, my_app, details}.
    {excl_mods, my_app, [my_app_SUITE]}.

    where my_app and my_app_SUITE are names of your application and the test suite(s) you have
  • when running using ct_run, make sure to
    • either specify -cover cover.spec option; OR
    • add this line to your main CT spec file
      {cover, "cover.spec"}.

That’s it! Next time you run CT you should have a nice code coverage report generated with it.

Oh, also, it’s probably worth mentioning that I couldn’t get erlang.mk to run ct_run with the -spec option, so I manually added it in the CT_RUN command.

Elastic Applications in Erlang

How realistic or useful are elastic applications in general, or specifically in Erlang? To demonstrate, I start with Joe Armstrong’s favorite Erlang program – the universal server:

universal_server() ->
    receive
        {become, F} ->
            F()
    end.

Once spawned, it sits and waits until it gets an instruction to become something. Then it becomes that thing it’s told to be.

First, this is really cool. Second, we should expand a bit:

  • make the wrapping process loop
  • optionally make the internal function loop
  • if internal function receives messages, make it respect an “abort” message which would make it stop executing and return the control back to its parent

So, blah blah – all of the above is only few lines of code in Erlang. I’ll call this module gen_node and the gen_node.erl file is as follows:

-module(gen_node).
-export([start/0]).

start() ->
    receive
        {become, F} ->
            F(),
            start();
        reset ->
            start();
        stop ->
            ok
    end.

and then the way we invoke it would be:

1> c(gen_node).
{ok,gen_node}
2> N = spawn(gen_node, start, []).
<0.40.0>

OK, the process is spawned and waiting, let’s give it something to do. Let’s define 2 functions: one that doubles the given number, and another that squares it. They both communicate via messages:

3> Double = fun Double() ->
    receive
      reset -> ok;
      {From, Args} -> From ! Args+Args, Double()
    end
   end.
#Fun<erl_eval.44.106461118>
4> Square = fun Square() ->
    receive
      reset -> ok;
      {From, Args} -> From ! Args*Args, Square()
    end
   end.
#Fun<erl_eval.44.106461118>

Now, let’s tell the spawned process to become “Double”, then reset it and tell it to become “Square”:

5> N ! {become, Double}.
{become,#Fun<erl_eval.44.106461118>}
6> N ! {self(), 16}.
{<0.33.0>,16}
7> flush().
Shell got 32
ok
8> N ! reset.
reset
9> N ! {become, Square}.
{become,#Fun<erl_eval.44.106461118>}
10> N ! {self(), 16}.
{<0.33.0>,16}
11> flush().
Shell got 256
ok

So, this is cool because now we have a generic computing node that we can tell to transform into any arbitrary processor and communicate to it via messages.

Now, finally, let’s wrap this up in an OTP application using gen_server so the worker processes are also supervised:

https://github.com/unix1/gen_node

Now the interaction becomes:

1> Double = fun Double() ->
    receive
      {_, reset} -> ok;
      {From, Args} -> From ! Args+Args, Double()
    end
   end.
#Fun<erl_eval.44.106461118>
2> Square = fun Square() ->
    receive
      {_, reset} -> ok;
      {From, Args} -> From ! Args*Args, Square()
    end
   end.
#Fun<erl_eval.44.106461118>
3> application:start(gen_node).
ok
4> {ok, N, _} = gen_node:start_server().
{ok,<0.42.0>,#Ref<0.0.0.45>}
5> gen_node:become(N, Double).
ok
6> gen_node:send(N, 16).
32
7> gen_node:reset(N).
ok
8> gen_node:become(N, Square).
ok
9> gen_node:send(N, 16).
256

It worked!

So, what’s next?

This is obviously just a concept code. To be usable something needs to track the states and types of nodes, and something else to intelligently control what they are doing and how the behavior should adapt. Possibilities could include:

  • predicting application-specific demand and adjusting resources: e.g. queue X is filling up, while queue Y processes are waiting too long – so make an adjustment in real-time
  • defining capacities of systems by giving different weights to each type of operation, or calculating resources needed to make certain computations
  • more fully using existing resources, or distributing load where the right resources are available

Questions: is this applicable and interesting? Boring? Already been done? Do you have any ideas which direction this should take?

Any constructive feedback is fully welcome.

PHP Sessions in Erlang Mnesia

Motivation

  • create a very simple first Erlang/OTP application
  • link to conventional web development

I decided one of the simplest things to do would be to create a session storage for PHP in Mnesia. But that, in doing so, I would also create an extremely simple key value store wrapper around Mnesia which would track access times.

Disclaimer

Do NOT use this in production, or anywhere where it’s important. If you do, you’re crazy because:

  • it has not been tested in production or production-like environment
  • as I mentioned, this is my first Erlang application
  • connection is made to a single Erlang node, and there is no failover mechanism if that fails
  • session garbage collection is not implemented

Good Stuff

OK, the disclaimer is out of the way, let’s get to the good stuff! Here’s what I did and how.

Tools used

  • Erlang runtime and development headers
  • PHP >=5.3 (>=5.4 preferred) binary and development headers
  • Apache
  • mypeb
  • kvstore
  • lib360

Architecture

[ PHP ] <–> [ mypeb ] <–> [ Erlang ] <–> [ kvstore ] <–> [ Mnesia ]

Instructions

  • download and install prerequisites as best done in your environment
    • Erlang runtime and development headers
    • Apache
    • PHP >=5.3 binary and development headers
  • download and compile mypeb
    • git clone git://github.com/videlalvaro/mypeb.git
    • cd mypeb
    • phpize
    • ./configure
    • make
    • sudo make install
    • php -m | grep peb
    • Note 1: the last command is a test to verify peb module loads successfully
    • Note 2: you might need to specify --with-erllib and --with-erlinc options with ./configure command if they are not automatically found
  • download, compile and run kvstore
  • download lib360
    • git clone git@github.com:unix1/spoof.git
  • add sample PHP script to your web server and test everything
    • download sample gist
    • replace the path in require_once() to wherever you downloaded lib360
    • replace the your-erlang-cookie-here with the contents of ~/.erlang.cookie file
    • access the file through the web server and have fun!

Missing

These are things that I thought of but didn’t bother implementing that I might someday add:

  • make kvstore more configurable and easy to install/start/stop
    • kvstore currently fails to stop (you have to abort)
    • it could use rebar and more automated scripts instead of manual commands
    • support for configurable multiple table names
    • make access time tracking optional
    • implement other storage backends
  • account for failover when primary node connection fails from PHP
  • PHP session garbage collection
  • implement kvstore access through lib360 database layer

Suggestions/Contributions/Feedback

If you have any feedback, feel free to give it via github, blog comments, etc. as appropriate. Enjoy!