[erlang-questions] Trace-Driven Development

Henning Diedrich hd2010@REDACTED
Sat Jun 2 21:02:10 CEST 2012

Should there be any interest I would like to discuss tracing and if it 
may be more valuable for the actor model than unit tests.

A couple of days ago, during Jesper Louis Andersen's very enlightening 
debugging and tracing tutorial at Ericsson, Joe Armstrong asked whether 
showing visual traces /first/ may be a good approach to /teaching/ 
Erlang, book-side.

I would like to propose again that trace-driven development may possibly 
be /the/ way to teach andalso /program/ Erlang.

In the instance, we were talking about visual traces, like these:


I mean it in the slightly unrealistic way that most people probably 
appreciate test-driven programming as a great idea but seldom execute it 
as dogmatically as preached.

But I realize that what I /actually/ do when programming a new app, or 
when looking for truly nasty bugs, is almost always pretty 
'trace-driven'. Writing traces into the program has become a routine 
step for me when coding medium complicated stuff that uses supervisors, 
monitors and multiple process that come to live at various times 
throughout the lifetime of an Erlang (OTP) application -- and most of 
all, during the /start up/. And I have always found surprises, which 
process comes to live when. I think Jesper related a similar story how 
they found tons of bugs that no-one had realized were there when using 
tracing (not sure any more.)

I believe that the main aggravating factor is that you don't construct 
an OTP application from scratch that often. You'll have to look things 
up again every next time. My guess is that this is true for the vast 
majority of Erlang programmers. Of course, for the ultimate expert, it 
may be hard to grasp what I am even talking about because it's all so 

But what I really find myself currently doing, before writing tests, is 
writing traces. I also haven't found out how to plan out and write 
meaningful stateful tests up front, before I even made it to architect 
the application.

My concrete order is: writing first, get to compile, put in traces, try 
to run, debug.

I am so sure meanwhile that I will need the traces that I don't wait 
anymore until I have gotten lost in searching for a bug and not even 
knowing what happens before the crash and what parts of the program may 
have silently died and what other parts may have waited for which other 

These things will be obvious for expert Erlang programmers who have set 
up applications time and again. But few people using Erlang for 
productivity reasons will /ever/ come into that position. And for 
finding bugs: that's exactly the moment when reality doesn't follow your 
fantasy and a reality check in the form of a trace is one of the first 
tools to turn to. Is it not?

So trace-driven development sounds like it could be a useful 
recommendation. And if it is, a new best practice of waterfalling your 
application into existence could maybe emerge, similar to what Joe 
describes in his book (if I remember well) how he almost always starts out.

I learned a lot in Jesper's tutorial. For example that because Erlang 
and OTP keep growing, there are now three distinct mechanisms, with 
respective modules and function calls, to trace and debug. And despite 
me using traces a lot, I hadn't used any of them, for the wrong reasons 
it turned out. (And for productivity, a simple io:format can beat 
getting tied up into using more powerful but less simple minded options.)

So I am asking this out of honest ignorance:

Instead of tracing, is there a way to write meaningful, concise tests 
that are meant or at least good for checking the sequence of process 
communications? In other words, are there better alternatives to follow 
traces with the naked eye? I do not mean a heavy construct, or an 
also-possible use, but a test-package made for that and/or a useful 
practice that someone is in fact applying?

And it need not be the visual traces as the image linked to above. A 
ping-pong output in the terminal may do (if not skewered by delays or 
re-sequences or contentions in io:format.)

For trace-driven development, what concrete procedure with which 
concrete calls and parameters to erl can be a best practice for starting 
out? I am sure it' s 'obvious' for many, but at least for me, not. What 
could be the hello world case for a trace-driven approach?

And finally, how do /you/ approach setting up a new app, finding a bug 
-- how much tracing does everyone use in these things? And is anyone 
also using io:format instead of the OTP goodness, if so, for a good reason?


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