db_server.erl

-module(db_server).
-export([start/0, stop/1, start/1, stop/0]).
-import(db_persistence, [read/1, save/2]).
-import(db_operation, [select/2, add/2]).

serve_add(DB, NewRow) ->
    try add(DB, NewRow) of
        NewDB -> {ok, NewDB}
    catch
        throw:Reason -> {{error, Reason}, DB};
        error:function_clause -> {{error, "Not a triple"}, DB}
    end.

serve_select(DB, Filters) ->
    try select(DB, Filters) of
        Result -> Result
    catch
        throw:Reason -> {error, Reason};
        error:function_clause -> {error, "Invalid filters"}
    end.

serve(DB) ->
    receive
        {SenderPID, {add, Row}} ->
          {Response, NewDB} = serve_add(DB, Row),
          SenderPID ! Response,
          serve(NewDB);
        {SenderPID, {select, Filters}} ->
          Response = serve_select(DB, Filters),
          SenderPID ! Response,
          serve(DB);
        {'EXIT', _, {stop, OutFile}} ->
          ok = save(DB, OutFile)
    end.

stop(OutFile) ->
    case whereis(dbms) of
        undefined -> throw("DBMS not started");
        Pid -> exit(Pid, {stop, OutFile})
    end.
stop() ->
    stop(none).

init(InitialDB) ->
    process_flag(trap_exit, true),
    serve(InitialDB).

start(none) ->
    register(dbms, spawn(fun() -> init([]) end));
start(InFile) ->
    InitialDB = read(InFile),
    register(dbms, spawn(fun() -> init(InitialDB) end)).
start() ->
    start(none).