mnesia是erlang自带的分布式数据库，基于ets和dets实现的。mnesia兼顾了dets的持久性和ets的高性能，可以自动在多个erlang节点间同步数据库。最关键的是，mnesia实现了事务机制。

mnesia数据库有一个schema表，保存着数据库相关的一些信息，例如如何将表保存到磁盘、如何加载这些表、在哪些节点间同步数据。构建集群，关键是处理这个schema表。

调用函数mnesia:create_schema(ListOfNodes)可以创建schema，这必须在启动mnesia之前调用。缺省情况下，schema会保存在节点启动的当前目录，可以在应用启动时修改这个参数erl -name SomeName -mnesia dir where/to/store/table，或者执行函数application:set_env(mnesia, dir, "where/to/store/table").

初始化时构建mnesia集群

必须保证每一个Node都没有schema，如果有，调用mnesia:delete_schema删除。

在本地启动两个erlang 节点：

erl -name a@127.0.0.1 -mnesia dir '"path1"' -s mnesia
erl -name b@127.0.0.1 -mnesia dir '"path2"' -s mnesia

mnesia启动后会创建schema的，所以需要先删除schema，再重新创建。

(a@127.0.0.1)1> mnesia:system_info().
===> System info in version "4.11", debug level = none <===
opt_disc. Directory "path1" is NOT used.
use fallback at restart = false
running db nodes   = ['a@127.0.0.1']
stopped db nodes   = []
master node tables = []
remote             = []
ram_copies         = [schema]
disc_copies        = []
disc_only_copies   = []
[{'a@127.0.0.1',ram_copies}] = [schema]
2 transactions committed, 0 aborted, 0 restarted, 0 logged to disc
0 held locks, 0 in queue; 0 local transactions, 0 remote
0 transactions waits for other nodes: [](b@127.0.0.1)1> mnesia:system_info().
===> System info in version "4.11", debug level = none <===
opt_disc. Directory "path2" is NOT used.
use fallback at restart = false
running db nodes   = ['b@127.0.0.1']
stopped db nodes   = []
master node tables = []
remote             = []
ram_copies         = [schema]
disc_copies        = []
disc_only_copies   = []
[{'b@127.0.0.1',ram_copies}] = [schema]
2 transactions committed, 0 aborted, 0 restarted, 0 logged to disc
0 held locks, 0 in queue; 0 local transactions, 0 remote
0 transactions waits for other nodes: []

分别在两个节点上运行下面的命令删除schema

mnesia:stop(),
mnesia:delete_schema([node()])

在任意一个节点上（选择的是a@127.0.0.1）运行创建schema

mnesia:create_schema([node(), 'b@127.0.0.1']).

之后就可以看到在两个节点上看到新的schema了

(a@127.0.0.1)7> mnesia:system_info().
===> System info in version "4.11", debug level = none <===
opt_disc. Directory "/Users/zhaoxiaosen/work" is used.
use fallback at restart = true
running db nodes   = []
stopped db nodes   = ['b@127.0.0.1','a@127.0.0.1'](b@127.0.0.1)5> mnesia:system_info().
===> System info in version "4.11", debug level = none <===
opt_disc. Directory "/Users/zhaoxiaosen" is used.
use fallback at restart = true
running db nodes   = []
stopped db nodes   = ['b@127.0.0.1','a@127.0.0.1']

最后分别启动mnesia。

给已有的mnesia集群添加node

新启动一个节点c@127.0.0.1

erl -name c@127.0.0.1 -mnesia dir '"path3"' -s mnesia

1. 在a节点上运行mnesia:change_config(extra_db_nodes, ['c@127.0.0.1']).，将c节点连接到集群上，这时，c只是复制了schema
2. 在c节点上更改存储方式，mnesia:change_table_copy_type(schema, 'c@127.0.0.1', disc_copies).
3. 将所有表都同步到c节点上

[{Tb, mnesia:add_table_copy(Tb, node(), Type)}|| {Tb, [{'a@node', Type}]} <- [{T, mnesia:table_info(T, where_to_commit)}|| T <- mnesia:system_info(tables)]].

这样，新的节点就加入到mnesia集群上了。

上面的例子中，在启动节点时启动了mnesia，主要是为了测试删除schema

实例代码

%%启动
init_once() ->mnesia:stop(),mnesia:create_schema([node()]),mnesia:start(),mnesia:create_table(tab1, [{disc_copies, [node()]},{attributes, record_info(fields, config)}]),mnesia:create_table(tab2, [{ram_copies, [node()]},{attributes, record_info(fields, watcher)}]),mnesia:create_table(tab3, [{ram_copies, [node()]},{attributes, record_info(fields, version)}]).
%%更改存储方式
change_table_copy_type([]) ->ok;
change_table_copy_type([Node | T]) ->mnesia:change_table_copy_type(schema, Node, disc_copies),mnesia:change_table_copy_type(tab1, Node, disc_copies),mnesia:change_table_copy_type(tab2, Node, ram_copies),mnesia:change_table_copy_type(tab3, Node, ram_copies),change_table_copy_type(T).
%%集群添加节点
sync_data(Node) when is_atom(Node) ->sync_data([Node]);
sync_data(Nodes) when is_list(Nodes) ->mnesia:change_config(extra_db_nodes, Nodes),change_table_copy_type(Nodes),add_table_copy(Nodes);
sync_data(_) ->ok.
%%同步数据
add_table_copy([]) ->ok;
add_table_copy([Node | T]) ->[mnesia:add_table_copy(Tb, Node, Type)|| {Tb, [{_, Type}]} <- [{T1, mnesia:table_info(T1, where_to_commit)}|| T1 <- mnesia:system_info(tables)]],add_table_copy(T).
%%事务执行，Q为操作命令
do(Q) ->F = fun() -> qlc:e(Q) end,mnesia:transaction(F).

其它常用函数

wait_for_tables(TabList, Timeout) -> ok | {timeout, BadTabList} | {error, Reason}

Some applications need to wait for certain tables to be accessible to do useful work. mnesia:wait_for_tables/2 either hangs until all tables in TabList are accessible, or until timeout is reached.