mnesia锁数据时机

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mnesia的锁，分为读锁、写锁等，本次总结一下，在一个事务中，什么操作会触发锁。

做如下实验：

实验一：事务中，先读后写，读写之间又10s中等待；

write(Name) ->
Fun = fun() ->
[{person, Name, Age}] = mnesia:read(person, Name),
io:format("read ok, Name=~p, Age=~p~n", [Name, Age]),
timer:sleep(10000),
mnesia:write({person, Name, Age+1})
end,
{atomic, ok} = mnesia:sync_transaction(Fun).

test2() ->

erlang:spawn(?MODULE, write, ["Andy"]),
% timer:sleep(1000),
erlang:spawn(?MODULE, write, ["Andy"]),
io:format("over!~n"),
timer:sleep(10000),
ok.

结论：

1. 读数据时，会触发读锁

2. 读锁不影响进程2的读表操作

实验二：事务中，先读后写，写后等待10s，再结束事务；

write(Name) ->
Fun = fun() ->
[{person, Name, Age}] = mnesia:read(person, Name),
io:format("read ok, Name=~p, Age=~p~n", [Name, Age]),
Result = mnesia:write({person, Name, Age+1}),
timer:sleep(10000),
Result
end,
{atomic, ok} = mnesia:sync_transaction(Fun).

test2() ->

erlang:spawn(?MODULE, write, ["Andy"]),
timer:sleep(1000),
erlang:spawn(?MODULE, write, ["Andy"]),
io:format("over!~n"),
timer:sleep(10000),
ok.

结论：

1. 写数据时，触发写锁，直到事务结束，才会释放锁

2. 写锁，会延迟进程2的读表操作

实验三：修改实验一的读表语句，手动修改锁为写锁；

write(Name) ->
Fun = fun() ->
[{person, Name, Age}] = mnesia:read(person, Name, write),
io:format("read ok, Name=~p, Age=~p~n", [Name, Age]),
timer:sleep(10000),
mnesia:write({person, Name, Age+1})
end,
{atomic, ok} = mnesia:sync_transaction(Fun).

test2() ->

erlang:spawn(?MODULE, write, ["Andy"]),
% timer:sleep(1000),
erlang:spawn(?MODULE, write, ["Andy"]),
io:format("over!~n"),
timer:sleep(10000),
ok.

结论：

1. 同样印证了，写锁会延迟其他进程的读写操作，直到事务结束，才会释放锁

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