Attempt for CQRS in erlang

console.erl

Erlang R13B04 (erts-5.7.5) [smp:8:8] [rq:8] [async-threads:0]

Eshell V5.7.5  (abort with ^G)
1> cd('/blah/erlang/CQRS').
C:/blah/erlang/CQRS
ok
2> ls().
item.erl     item.hrl     
ok
3> c(item).    
./item.erl:65: Warning: function start_link/1 is unused
./item.erl:67: Warning: function init/1 is unused
./item.erl:70: Warning: function handle_call/3 is unused
{ok,item}
4> eunit:test(item,[verbose]).
======================== EUnit ========================
module 'item'
  item: item_activation_test_ (Activating for the first time should work)...ok
  item: item_activation_test_ (Activating twice in a row should fail)...ok
  item: item_deactivation_test_ (Deactivating before activating should fail)...ok
  item: item_deactivation_test_ (Deactivating after activating should work)...ok
  item: item_deactivation_test_ (Deactivating a deactivated item should fail)...ok
  item: item_deactivation_test_ (Deactivating an active item with a balance should fail)...ok
  item: deposit_amount_test_ (Depositing an amount to an unactivated item should fail)...ok
  item: deposit_amount_test_ (Depositing an amount to an activated item should succeed)...ok
  item: withdraw_amount_test_ (Withdrawing an amount from an unactivated item should fail)...ok
  item: withdraw_amount_test_ (Withdrawing an amount from an activated item with a balance large enough should succeed)...ok
  item: withdraw_amount_test_ (Withdrawing an amount for the second time where the total exceeds the balance should fail)...ok
  [done in 0.172 s]
=======================================================
  All 11 tests passed.
ok
5> 

item.erl

-module(item).
-author('Tom Janssens <tom@corebvba.be>').
%%-behavior(gen_server).
-include("item.hrl").

%% COMMAND HANDLERS
%% todo: figure out proper routing so I can avoid this boilerplate code
handle(#state{id=StateId},#activate_item{id=CommandId}) when StateId =/= CommandId ->
  {unhandled,command};
handle(#state{id=StateId},#deactivate_item{id=CommandId}) when StateId =/= CommandId ->
  {unhandled,command};
handle(#state{id=StateId},#deposit_amount{id=CommandId}) when StateId =/= CommandId ->
  {unhandled,command};
handle(#state{id=StateId},#withdraw_amount{id=CommandId}) when StateId =/= CommandId ->
  {unhandled,command};
  
handle(#state{active=false}, #activate_item{id=Id, name=Name}) -> 
  {ok, [#item_activated{id=Id, name=Name}]};

handle(#state{active=true}, #activate_item{id=_Id}) -> 
  {error, "you can not activate an item if it is already active"};

handle(#state{balance=Balance}, #deactivate_item{id=_Id}) when Balance > 0 ->
  {error, "You can not deactivate this item as it is still in stock"};

handle(#state{active=true}, #deactivate_item{id=Id}) ->
  {ok, [#item_deactivated{id=Id}]};

handle(#state{active=false,id=Id}, #deactivate_item{id=Id}) ->
  {error, "you can not deactivate an item if it is not active"};

handle(#state{active=false,id=Id}, #deactivate_item{id=Id}) ->
  {error, "you can not perform any actions on an inactive item"};

handle(#state{active=true}, #deposit_amount{id=Id, amount=Amount}) ->
  {ok, [#amount_deposited{id=Id, amount=Amount}]};

handle(#state{active=false}, #deposit_amount{id=_Id, amount=_Amount}) ->
  {error, "you can not perform any actions on an inactive item"};
  
handle(#state{balance=Balance},#withdraw_amount{id=_Id, amount=Amount}) when Balance < Amount ->
  {error, "the amount you want to withdraw is larger then the balance"};
  
handle(_State,#withdraw_amount{id=Id, amount=Amount}) ->
  {ok, [#amount_withdrawn{id=Id, amount=Amount}]}.
  
  
%% EVENT HANDLERS   
on(State,#item_activated{id=Id}) when State#state.id == Id -> 
  State#state{active=true};
  
on(State,#item_deactivated{id=Id}) when State#state.id == Id -> 
  State#state{active=false};
  
on(State,#amount_deposited{id=Id,amount=Amount}) when State#state.id == Id -> 
  State#state{balance=State#state.balance+Amount};

on(State,#amount_withdrawn{id=Id,amount=Amount}) when State#state.id == Id -> 
  State#state{balance=State#state.balance-Amount};
  
on(State,_) ->
  State.
  
%% GEN_SERVER BEHAVIOR - does not work ATM
start_link(Id) -> gen_server:start_link({local,Id},?MODULE,[Id],[]).

init(Id) ->
  {ok,load(Id,[])}.

handle_call(Command,_From,State) ->
  {Result,Details} = handle(State,Command),
  case Result of
    ok ->
	  {reply,Result,load(State,Details)};
	error ->
	  {reply,Result,State};
	unhandled ->
	  {noreply,State}
  end.

%% HELPER FUNCTIONS
load(State,Events) when is_record(State,state) ->
  lists:foldl(fun(E,NewState) -> on(NewState,E) end,State,Events);

load(Id,Events) ->
  load(#state{id=Id},Events).
  
%% SPECS
-define(TEST,1).
  
-ifdef(TEST).
-include_lib("eunit/include/eunit.hrl").
 
  item_activation_test_() -> 
  [
    {"Activating for the first time should work", fun()-> 
      State = load("item/1",[]),
      Result = handle(State,#activate_item{id="item/1",name="Test item"}),
      ?assertEqual({ok,[#item_activated{id="item/1",name="Test item"}]},Result)
    end },
    {"Activating twice in a row should fail", fun()-> 
      State = load("item/1",[#item_activated{id="item/1",name="blah"}]),
      {Status,_Message} = handle(State,#activate_item{id="item/1",name="Test item"}),
      ?assertEqual(Status,error)
    end }
  ].
  
  item_deactivation_test_() -> 
  [
    {"Deactivating before activating should fail", fun()-> 
      State = load("item/1",[]),
      {Status,_Message} = handle(State,#deactivate_item{id="item/1"}),
      ?assertEqual(Status,error)
    end },
    {"Deactivating after activating should work", fun()-> 
      State = load("item/1",[#item_activated{id="item/1"}]),
      Result = handle(State,#deactivate_item{id="item/1"}),
      ?assertEqual(Result,{ok,[#item_deactivated{id="item/1"}]})
    end },
    {"Deactivating a deactivated item should fail", fun()-> 
      State = load("item/1",[#item_activated{id="item/1",name="blah"},#item_deactivated{id="item/1"}]),
      {Status,_Message} = handle(State,#deactivate_item{id="item/1"}),
      ?assertEqual(Status,error)
    end },
    {"Deactivating an active item with a balance should fail",fun() ->
      State = load("item/1",[#item_activated{id="item/1",name="blah"},#amount_deposited{id="item/1",amount=100}]),
      {Status,_Message} = handle(State,#deactivate_item{id="item/1"}),
      ?assertEqual(Status,error)
    end }
  ].
  
  deposit_amount_test_() ->
  [
      {"Depositing an amount to an unactivated item should fail",fun()-> 
        State = load("item/1",[]),
        {Status,_Message} = handle(State,#deposit_amount{id="item/1"}),
        ?assertEqual(Status,error)
      end},
      {"Depositing an amount to an activated item should succeed",fun()-> 
        State = load("item/1",[#item_activated{id="item/1"}]),
        Result = handle(State,#deposit_amount{id="item/1",amount=100}),
        ?assertEqual(Result,{ok,[#amount_deposited{id="item/1",amount=100}]})
      end}
  ].

  withdraw_amount_test_() ->
   [
      {"Withdrawing an amount from an unactivated item should fail",fun()-> 
        State = load("item/1",[]),
        {Status,_Message} = handle(State,#withdraw_amount{id="item/1"}),
        ?assertEqual(Status,error)
      end},
      {"Withdrawing an amount from an activated item with a balance large enough should succeed",fun()-> 
        State = load("item/1",[#item_activated{id="item/1"},#amount_deposited{id="item/1",amount=200}]),
        Result = handle(State,#withdraw_amount{id="item/1",amount=100}),
        ?assertEqual(Result,{ok,[#amount_withdrawn{id="item/1",amount=100}]})
      end},
      {"Withdrawing an amount for the second time where the total exceeds the balance should fail",fun()->
        State = load("item/1",[#item_activated{id="item/1"},#amount_deposited{id="item/1",amount=120},#amount_withdrawn{id="item/1",amount=100}]),
        {Status,_Message} = handle(State,#withdraw_amount{id="item/1",amount=100}),
        ?assertEqual(Status,error)
      end }
   ].
   
-endif.

item.hrl

%% TYPE DECLARATIONS
-type item_id() :: nonempty_string().

%% INTERNAL STATE RECORD
-record(state,{ id::item_id(), active=false :: boolean(),balance=0::non_neg_integer()}).

%% COMMANDS
-record(activate_item, {id::item_id(),name::nonempty_string()}).
-record(deactivate_item, {id::item_id()}).
-record(deposit_amount,{id::item_id(),amount=1::pos_integer()}).
-record(withdraw_amount,{id::item_id(),amount=1::pos_integer()}).

%% EVENTS
-record(item_activated, {id::item_id(),name::nonempty_string()}).
-record(item_deactivated, {id::item_id()}).
-record(amount_deposited,{id::item_id(),amount=1::pos_integer()}).
-record(amount_withdrawn,{id::item_id(),amount=1::pos_integer()}).