mnesia_sync.erl

%% The contents of this file are subject to the Mozilla Public License
%% Version 1.1 (the "License"); you may not use this file except in
%% compliance with the License. You may obtain a copy of the License
%% at http://www.mozilla.org/MPL/
%%
%% Software distributed under the License is distributed on an "AS IS"
%% basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See
%% the License for the specific language governing rights and
%% limitations under the License.
%%
%% The Original Code is RabbitMQ.
%%
%% The Initial Developer of the Original Code is GoPivotal, Inc.
%% Copyright (c) 2007-2014 GoPivotal, Inc.  All rights reserved.
%%

-module(mnesia_sync).

%% mnesia:sync_transaction/3 fails to guarantee(保证) that the log is flushed to disk
%% at commit(提交). This module is an attempt to minimise(最小化) the risk of data loss by
%% performing a coalesced(合并) log fsync. Unfortunately this is performed regardless(不管)
%% of whether or not the log was appended to.

-behaviour(gen_server).

-export([sync/0]).

-export([start_link/0, init/1, handle_call/3, handle_cast/2, handle_info/2,
         terminate/2, code_change/3]).

-define(SERVER, ?MODULE).

-record(state, {waiting, disc_node}).

%%----------------------------------------------------------------------------

-ifdef(use_specs).

-spec(sync/0 :: () -> 'ok').

-endif.

%%----------------------------------------------------------------------------
%% mnesia_sync进程的启动入口函数
start_link() ->
	gen_server:start_link({local, ?SERVER}, ?MODULE, [], []).


sync() ->
	gen_server:call(?SERVER, sync, infinity).

%%----------------------------------------------------------------------------
%% 初始化回调函数
init([]) ->
	{ok, #state{disc_node = mnesia:system_info(use_dir), waiting = []}}.


handle_call(sync, _From, #state{disc_node = false} = State) ->
	{reply, ok, State};


handle_call(sync, From, #state{waiting = Waiting} = State) ->
	{noreply, State#state{waiting = [From | Waiting]}, 0};


handle_call(Request, _From, State) ->
	{stop, {unhandled_call, Request}, State}.


handle_cast(Request, State) ->
	{stop, {unhandled_cast, Request}, State}.


%% 超时后通知所有同步等待的进程，同步结束
handle_info(timeout, #state{waiting = Waiting} = State) ->
	ok = disk_log:sync(latest_log),
	[gen_server:reply(From, ok) || From <- Waiting],
	{noreply, State#state{waiting = []}};


handle_info(Message, State) ->
	{stop, {unhandled_info, Message}, State}.


terminate(_Reason, _State) ->
	ok.


code_change(_OldVsn, State, _Extra) ->
	{ok, State}.