Developing on the go

One benefit to using erlang compared to more traditional languages is the development cycle.  Generally you write code, compile, debug, write, compile, debug, until you have something you want to use.  In between compile and debug you run the program.  In traditional languages you shut down the application then restart it to debug again.  In erlang we can skip the ‘restart’ and simply load the new code in.  This is assuming no bugs in the code didn’t cause the entire application to crash horribly.
So basically, what is going on is, if you have a logic problem or what not in your application that you wish to fix.  Outside of your running application, you edit the appropriate .erl files and recompile.  For example, your application consists of a.erl and b.erl.  In b.erl you have a function called mogwai.
So in our example.  a.erl calls b:mogwai, and b:mogwai has some sort of error in it that does not cause the application to crash but you want to fix, regardless.  You fix b:mogwai’s error and you want to load the new codebase in your running application.  Recompile b.erl then in the shell to your application you simple do:

nl(b).

This loads the new version of b into your application and now calls to b:mogwai will use the current version of the function.  For certain applications this certinaly provides a more elegant development cycle.  I think this style also alters the structure of ones code.  For instance, if one writes an application knowing that errors in the code can be fixed on the fly, no longer do they necesarly have to exit nicely.  Rather, the application can provide a means of restarting the portions that have crashed.  The application can continue working without the crashed process/code or stall until the required portion can be brought back.  I’m under the impression this is a feature supervision trees offer you.  A supervisor simply restarts a process if it crashes, and reports it.  By restarting, a new codebase can be loaded that fixes the error.

One final note on how code replacement works.  Erlang only attempts to load a new module if the call is in the form of: module:function.  For instance, if you have a process in a loop something like:

loop() ->
  receive
    Something ->
      loop()
  end.

If you reload the module that this loop is defined in, the call the ‘loop()’ will not load the new codebase.  The common idiom is something a long the lines of:

loop() ->
  receive
  restart ->
    ?MODULE:loop();
  Something ->
    loop()
  end.

This will load the new codebase.  On a final note, any function called as module:function must be exported.
Erlang certinaly provides some interesting features.  Certinaly somethign like code replacement is possible in other languages, such as python (which provides a reload function to reload a module) I think erlang provides a more elegant solution.  For instance I don’t think Python provides a means of a module to reload itself, especially in the middle of an event loop.
However this is not a contest between code replacement in various languages.  I have modified my irc bot to allow code replacement more seemlessly, however I have not allowed a decent means of bringing an irc bot back if there is an error which causes a crash.  I’ll have the new code online later if anyone is interested.

Comments

I have modified my irc bot to allow code replacement more seemlessly, however I have not allowed a decent means of bringing an irc bot back if there is an error which causes a crash.  I’ll have the new code online later if anyone is interested.

Posted by vista error on 24 Jan 2010 at 12:18

What happens if there are bugs that make the system crash? I would love to have the new code so that there isn’t an error which cases a crash.  In meantime is it best to install some sort of disaster recovery software in case it crashes?  I have been shopping around and can’t decide what to get.  What are your opinions on this one: http://www.neverfailgroup.com?  Or do you have any other suggestions.  Thanks, and I look forward to getting the new code.

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