mnesia的普通transaction写过程（四）事务提交准备
上一篇博文介绍了mnesia的锁请求过程，在请求到锁后，mnesia:write将更新写入临时ets表，此后mnesia:write将完成其使命，重新回到mnesia_tm:apply_fun函数中，此后将继续分析。
apply_fun(Fun, Args, Type) ->
Result = apply(Fun, Args),
case t_commit(Type) of
do_commit ->
{atomic, Result};
do_commit_nested ->
{nested_atomic, Result};
{do_abort, {aborted, Reason}} ->
{'EXIT', {aborted, Reason}};
{do_abort, Reason} ->
{'EXIT', {aborted, Reason}}
end.
Type参数为async
t_commit(Type) ->
{_Mod, Tid, Ts} = get(mnesia_activity_state),
Store = Ts#tidstore.store,
if
Ts#tidstore.level == 1 ->
intercept_friends(Tid, Ts),
%% N is number of updates
case arrange(Tid, Store, Type) of
{N, Prep} when N > 0 ->
multi_commit(Prep#prep.protocol,
majority_attr(Prep),
Tid, Prep#prep.records, Store);
{0, Prep} ->
multi_commit(read_only,
majority_attr(Prep),
Tid, Prep#prep.records, Store)
end;
...
end.
在进行事务提交前，需要为每个事务参与结点计算其更新内容。
arrange(Tid, Store, Type) ->
Nodes = get_elements(nodes,Store),
Recs = prep_recs(Nodes, []),
Key = ?ets_first(Store),
N = 0,
Prep =
case Type of
async -> #prep{protocol = sym_trans, records = Recs};
sync -> #prep{protocol = sync_sym_trans, records = Recs}
end,
case catch do_arrange(Tid, Store, Key, Prep, N) of
{'EXIT', Reason} ->
dbg_out("do_arrange failed ~p ~p~n", [Reason, Tid]),
case Reason of
{aborted, R} ->
mnesia:abort(R);
_ ->
mnesia:abort(Reason)
end;
{New, Prepared} ->
{New, Prepared#prep{records = reverse(Prepared#prep.records)}}
end.
临时ets表的nodes就是之前请求锁的目的结点，它们都是事务参与结点，此时事务提交类型为异步，其事务提交协议为sym_trans，及异步同构事务。
prep_recs([N | Nodes], Recs) ->
prep_recs(Nodes, [#commit{decision = presume_commit, node = N} | Recs]);
prep_recs([], Recs) ->
Recs.
从临时ets表中取得所有的表操作记录，根据这些表操作，为每个事务参与结点构建一个事务提交结构commit并进行填充。
do_arrange(Tid, Store, {Tab, Key}, Prep, N) ->
Oid = {Tab, Key},
Items = ?ets_lookup(Store, Oid), %% Store is a bag
P2 = prepare_items(Tid, Tab, Key, Items, Prep),
do_arrange(Tid, Store, ?ets_next(Store, Oid), P2, N + 1);
prepare_items(Tid, Tab, Key, Items, Prep) when Prep#prep.prev_tab == Tab ->
Types = Prep#prep.prev_types,
Snmp = Prep#prep.prev_snmp,
Recs = Prep#prep.records,
Recs2 = do_prepare_items(Tid, Tab, Key, Types, Snmp, Items, Recs),
Prep#prep{records = Recs2};
prepare_items(Tid, Tab, Key, Items, Prep) ->
Types = val({Tab, where_to_commit}),
case Types of
[] -> mnesia:abort({no_exists, Tab});
{blocked, _} ->
unblocked = req({unblock_me, Tab}),
prepare_items(Tid, Tab, Key, Items, Prep);
_ ->
Majority = needs_majority(Tab, Prep),
Snmp = val({Tab, snmp}),
Recs2 = do_prepare_items(Tid, Tab, Key, Types,
Snmp, Items, Prep#prep.records),
Prep2 = Prep#prep{records = Recs2, prev_tab = Tab,
majority = Majority,
prev_types = Types, prev_snmp = Snmp},
check_prep(Prep2, Types)
end.
构建事务提交的准备结构，将同一个表的操作合并在一起。
do_prepare_items(Tid, Tab, Key, Types, Snmp, Items, Recs) ->
Recs2 = prepare_snmp(Tid, Tab, Key, Types, Snmp, Items, Recs), % May exit
prepare_nodes(Tid, Types, Items, Recs2, normal).
prepare_nodes(Tid, [{Node, Storage} | Rest], Items, C, Kind) ->
{Rec, C2} = pick_node(Tid, Node, C, []),
Rec2 = prepare_node(Node, Storage, Items, Rec, Kind),
[Rec2 | prepare_nodes(Tid, Rest, Items, C2, Kind)];
prepare_nodes(_Tid, [], _Items, CommitRecords, _Kind) ->
CommitRecords.
prepare_nodes根据表的where_to_commit属性进行处理，该表属性为一个{Node,StorageType}列表。
pick_node(Tid, Node, [Rec | Rest], Done) ->
if
Rec#commit.node == Node ->
{Rec, Done ++ Rest};
true ->
pick_node(Tid, Node, Rest, [Rec | Done])
end;
pick_node({dirty,_}, Node, [], Done) ->
{#commit{decision = presume_commit, node = Node}, Done};
pick_node(_Tid, Node, [], _Done) ->
mnesia:abort({bad_commit, {missing_lock, Node}}).
pick_node为一个事务参与结点整理出表的操作，形成{NodeCommit, Ops}元组
prepare_node(Node, Storage, [Item | Items], Rec, Kind) when Kind == snmp ->
Rec2 = Rec#commit{snmp = [Item | Rec#commit.snmp]},
prepare_node(Node, Storage, Items, Rec2, Kind);
prepare_node(Node, Storage, [Item | Items], Rec, Kind) when Kind /= schema ->
Rec2 =
case Storage of
ram_copies ->
Rec#commit{ram_copies = [Item | Rec#commit.ram_copies]};
disc_copies ->
Rec#commit{disc_copies = [Item | Rec#commit.disc_copies]};
disc_only_copies ->
Rec#commit{disc_only_copies = [Item | Rec#commit.disc_only_copies]}
end,
prepare_node(Node, Storage, Items, Rec2, Kind);
prepare_node(_Node, _Storage, Items, Rec, Kind)
when Kind == schema, Rec#commit.schema_ops == [] ->
Rec#commit{schema_ops = Items};
prepare_node(_Node, _Storage, [], Rec, _Kind) ->
Rec.
prepare_node根据事务参与结点的存储类型，进一步将表操作填充到事务参与结点的commit结构的具体存储类型中