Example of mnesia fragmentation including activating dactivating it and adding and removing fragments

Author: drozzy

.gitignore

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+Mnesia.nonode@nohost
+*.beam

README.md

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+Mnesia Table Fragmentation
+==========================
+
+Following these two resources:
+
+- https://erlangcentral.org/wiki/index.php/Mnesia_Table_Fragmentation
+- http://erlang.org/doc/apps/mnesia/Mnesia_chap5.html#id79999

frags.erl

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+%% Start the shell as:
+%% erl -mnesia
+%% to enable mnesia support.
+-module(frags).
+-export([create/0, activate/0, deactivate/0, add_frag/0, del_frag/0]).
+-export([add_person/1, populate/1]).
+-record(person, {name,
+                 age = 0,
+                 address = unknown,
+                 salary = 0,
+                 children = []}).
+
+
+add_person(Name) ->
+	T = fun() ->
+		mnesia:write(#person{name=Name})
+	end,
+	mnesia:activity(transaction, T, [], mnesia_frag).
+
+%% Populate the db with N test data
+populate(N) ->
+	[add_person(integer_to_list(X)) || X <- lists:seq(1,N)].
+
+create() ->
+	application:stop(mnesia),
+	mnesia:create_schema([node()|nodes()]),
+	application:ensure_all_started(mnesia),
+	create_tables(tables()),
+	ok.
+
+%% Adds new fragment to the table!
+add_frag() ->
+	mnesia:change_table_frag(person, {add_frag, [node()]}).
+
+%% Removes an existing frag from the table
+del_frag() ->
+	mnesia:change_table_frag(person, del_frag).
+
+%% Activate table frag for this table
+activate() ->
+	mnesia:change_table_frag(person, {activate,[]}).
+
+%% Deactivate table fragmentation for this table
+deactivate() ->
+	mnesia:change_table_frag(person, deactivate).
+%% Private
+create_tables([]) -> ok;
+create_tables([{Table, Options}|Rest]) ->
+	mnesia:create_table(Table, Options),
+	create_tables(Rest).
+
+tables() ->
+	[{person, 
+		[{frag_properties, [{node_pool, [node()]},
+		  {n_fragments, 2}, {n_disc_copies, 1}]},
+		 {attributes, record_info(fields, person)}]}].