Module versioning

Modules can export a -module_schema() attribute which contains an integer number, denoting the current module’s version. On module initialization, Module:manage_schema/2 is called which handles installation and upgrade of data.

Minimal example:

-module(mod_twitter).
-mod_title("Twitter module").
-mod_schema(3).  %% we are currently at revision 3

-export([manage_schema/2]).
.... more code here...

manage_schema(install, Context) ->
  % .. code to install your stuff here, for instance:
  #datamodel{categories=
               [
                {tweet,
                 text,
                 [{title, <<"Tweet">>}]}
               ]};

manage_schema({upgrade, 2}, Context) ->
  %% code to upgrade from 1 to 2
  ok;

manage_schema({upgrade, 3}, Context) ->
 %% code to upgrade from 2 to 3
 ok.

Note that the install function should always be kept up-to-date according to the latest schema version. When you install a module for the first time, no upgrade functions are called, but only the install clause.