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%% -------------------------------------------------------------------
%%
%% riak_core: Core Riak Application
%%
%% Copyright (c) 2007-2010 Basho Technologies, Inc. All Rights Reserved.
%%
%% This file is provided to you under the Apache License,
%% Version 2.0 (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.apache.org/licenses/LICENSE-2.0
%%
%% Unless required by applicable law or agreed to in writing,
%% software distributed under the License is distributed on an
%% "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
%% KIND, either express or implied. See the License for the
%% specific language governing permissions and limitations
%% under the License.
%%
%% -------------------------------------------------------------------
%% @doc A simple Erlang implementation of vector clocks as inspired by Lamport logical clocks.
%%
%% @reference Leslie Lamport (1978). "Time, clocks, and the ordering of events
%% in a distributed system". Communications of the ACM 21 (7): 558-565.
%%
%% @reference Friedemann Mattern (1988). "Virtual Time and Global States of
%% Distributed Systems". Workshop on Parallel and Distributed Algorithms:
%% pp. 215-226
-module(vclock).
-export([fresh/0,descends/2,merge/1,get_counter/2,get_timestamp/2,
increment/2,increment/3,all_nodes/1,equal/2,prune/3,timestamp/0]).
-ifdef(TEST).
-include_lib("eunit/include/eunit.hrl").
-endif.
-export_type([vclock/0, timestamp/0, vclock_node/0]).
-opaque vclock() :: [vc_entry()].
% The timestamp is present but not used, in case a client wishes to inspect it.
-type vc_entry() :: {vclock_node(), {counter(), timestamp()}}.
% Nodes can have any term() as a name, but they must differ from each other.
-type vclock_node() :: term().
-type counter() :: integer().
-type timestamp() :: integer().
% @doc Create a brand new vclock.
-spec fresh() -> vclock().
fresh() ->
[].
% @doc Return true if Va is a direct descendant of Vb, else false -- remember, a vclock is its own descendant!
-spec descends(Va :: vclock()|[], Vb :: vclock()|[]) -> boolean().
descends(_, []) ->
% all vclocks descend from the empty vclock
true;
descends(Va, Vb) ->
[{NodeB, {CtrB, _T}}|RestB] = Vb,
case lists:keyfind(NodeB, 1, Va) of
false ->
false;
{_, {CtrA, _TSA}} ->
(CtrA >= CtrB) andalso descends(Va,RestB)
end.
% @doc Combine all VClocks in the input list into their least possible
% common descendant.
-spec merge(VClocks :: [vclock()]) -> vclock() | [].
merge([]) -> [];
merge([SingleVclock]) -> SingleVclock;
merge([First|Rest]) -> merge(Rest, lists:keysort(1, First)).
merge([], NClock) -> NClock;
merge([AClock|VClocks],NClock) ->
merge(VClocks, merge(lists:keysort(1, AClock), NClock, [])).
merge([], [], AccClock) -> lists:reverse(AccClock);
merge([], Left, AccClock) -> lists:reverse(AccClock, Left);
merge(Left, [], AccClock) -> lists:reverse(AccClock, Left);
merge(V=[{Node1,{Ctr1,TS1}=CT1}=NCT1|VClock],
N=[{Node2,{Ctr2,TS2}=CT2}=NCT2|NClock], AccClock) ->
if Node1 < Node2 ->
merge(VClock, N, [NCT1|AccClock]);
Node1 > Node2 ->
merge(V, NClock, [NCT2|AccClock]);
true ->
({_Ctr,_TS} = CT) = if Ctr1 > Ctr2 -> CT1;
Ctr1 < Ctr2 -> CT2;
true -> {Ctr1, erlang:max(TS1,TS2)}
end,
merge(VClock, NClock, [{Node1,CT}|AccClock])
end.
% @doc Get the counter value in VClock set from Node.
-spec get_counter(Node :: vclock_node(), VClock :: vclock()) -> counter().
get_counter(Node, VClock) ->
case lists:keyfind(Node, 1, VClock) of
{_, {Ctr, _TS}} -> Ctr;
false -> 0
end.
% @doc Get the timestamp value in a VClock set from Node.
-spec get_timestamp(Node :: vclock_node(), VClock :: vclock()) -> timestamp() | undefined.
get_timestamp(Node, VClock) ->
case lists:keyfind(Node, 1, VClock) of
{_, {_Ctr, TS}} -> TS;
false -> undefined
end.
% @doc Increment VClock at Node.
-spec increment(Node :: vclock_node(), VClock :: vclock()) -> vclock().
increment(Node, VClock) ->
increment(Node, timestamp(), VClock).
% @doc Increment VClock at Node.
-spec increment(Node :: vclock_node(), IncTs :: timestamp(),
VClock :: vclock()) -> vclock().
increment(Node, IncTs, VClock) ->
{{_Ctr, _TS}=C1,NewV} = case lists:keytake(Node, 1, VClock) of
false ->
{{1, IncTs}, VClock};
{value, {_N, {C, _T}}, ModV} ->
{{C + 1, IncTs}, ModV}
end,
[{Node,C1}|NewV].
% @doc Return the list of all nodes that have ever incremented VClock.
-spec all_nodes(VClock :: vclock()) -> [vclock_node()].
all_nodes(VClock) ->
[X || {X,{_,_}} <- VClock].
-define(DAYS_FROM_GREGORIAN_BASE_TO_EPOCH, (1970*365+478)).
-define(SECONDS_FROM_GREGORIAN_BASE_TO_EPOCH,
(?DAYS_FROM_GREGORIAN_BASE_TO_EPOCH * 24*60*60)
%% == calendar:datetime_to_gregorian_seconds({{1970,1,1},{0,0,0}})
).
% @doc Return a timestamp for a vector clock
-spec timestamp() -> timestamp().
timestamp() ->
%% Same as calendar:datetime_to_gregorian_seconds(erlang:universaltime()),
%% but significantly faster.
{MegaSeconds, Seconds, _} = os:timestamp(),
?SECONDS_FROM_GREGORIAN_BASE_TO_EPOCH + MegaSeconds*1000000 + Seconds.
% @doc Compares two VClocks for equality.
-spec equal(VClockA :: vclock(), VClockB :: vclock()) -> boolean().
equal(VA,VB) ->
lists:sort(VA) =:= lists:sort(VB).
% @doc Possibly shrink the size of a vclock, depending on current age and size.
-spec prune(V::vclock(), Now::integer(), BucketProps::term()) -> vclock().
prune(V,Now,BucketProps) ->
%% This sort need to be deterministic, to avoid spurious merge conflicts later.
%% We achieve this by using the node ID as secondary key.
SortV = lists:sort(fun({N1,{_,T1}},{N2,{_,T2}}) -> {T1,N1} < {T2,N2} end, V),
prune_vclock1(SortV,Now,BucketProps).
% @private
prune_vclock1(V,Now,BProps) ->
case length(V) =< get_property(small_vclock, BProps) of
true -> V;
false ->
{_,{_,HeadTime}} = hd(V),
case (Now - HeadTime) < get_property(young_vclock,BProps) of
true -> V;
false -> prune_vclock1(V,Now,BProps,HeadTime)
end
end.
% @private
prune_vclock1(V,Now,BProps,HeadTime) ->
% has a precondition that V is longer than small and older than young
case (length(V) > get_property(big_vclock,BProps)) orelse
((Now - HeadTime) > get_property(old_vclock,BProps)) of
true -> prune_vclock1(tl(V),Now,BProps);
false -> V
end.
get_property(Key, PairList) ->
case lists:keyfind(Key, 1, PairList) of
{_Key, Value} ->
Value;
false ->
undefined
end.
%% ===================================================================
%% EUnit tests
%% ===================================================================
-ifdef(TEST).
% doc Serves as both a trivial test and some example code.
example_test() ->
A = vclock:fresh(),
B = vclock:fresh(),
A1 = vclock:increment(a, A),
B1 = vclock:increment(b, B),
true = vclock:descends(A1,A),
true = vclock:descends(B1,B),
false = vclock:descends(A1,B1),
A2 = vclock:increment(a, A1),
C = vclock:merge([A2, B1]),
C1 = vclock:increment(c, C),
true = vclock:descends(C1, A2),
true = vclock:descends(C1, B1),
false = vclock:descends(B1, C1),
false = vclock:descends(B1, A1),
ok.
prune_small_test() ->
% vclock with less entries than small_vclock will be untouched
Now = riak_core_util:moment(),
OldTime = Now - 32000000,
SmallVC = [{<<"1">>, {1, OldTime}},
{<<"2">>, {2, OldTime}},
{<<"3">>, {3, OldTime}}],
Props = [{small_vclock,4}],
?assertEqual(lists:sort(SmallVC), lists:sort(prune(SmallVC, Now, Props))).
prune_young_test() ->
% vclock with all entries younger than young_vclock will be untouched
Now = riak_core_util:moment(),
NewTime = Now - 1,
VC = [{<<"1">>, {1, NewTime}},
{<<"2">>, {2, NewTime}},
{<<"3">>, {3, NewTime}}],
Props = [{small_vclock,1},{young_vclock,1000}],
?assertEqual(lists:sort(VC), lists:sort(prune(VC, Now, Props))).
prune_big_test() ->
% vclock not preserved by small or young will be pruned down to
% no larger than big_vclock entries
Now = riak_core_util:moment(),
NewTime = Now - 1000,
VC = [{<<"1">>, {1, NewTime}},
{<<"2">>, {2, NewTime}},
{<<"3">>, {3, NewTime}}],
Props = [{small_vclock,1},{young_vclock,1},
{big_vclock,2},{old_vclock,100000}],
?assert(length(prune(VC, Now, Props)) =:= 2).
prune_old_test() ->
% vclock not preserved by small or young will be pruned down to
% no larger than big_vclock and no entries more than old_vclock ago
Now = riak_core_util:moment(),
NewTime = Now - 1000,
OldTime = Now - 100000,
VC = [{<<"1">>, {1, NewTime}},
{<<"2">>, {2, OldTime}},
{<<"3">>, {3, OldTime}}],
Props = [{small_vclock,1},{young_vclock,1},
{big_vclock,2},{old_vclock,10000}],
?assert(length(prune(VC, Now, Props)) =:= 1).
prune_order_test() ->
% vclock with two nodes of the same timestamp will be pruned down
% to the same node
Now = riak_core_util:moment(),
OldTime = Now - 100000,
VC1 = [{<<"1">>, {1, OldTime}},
{<<"2">>, {2, OldTime}}],
VC2 = lists:reverse(VC1),
Props = [{small_vclock,1},{young_vclock,1},
{big_vclock,2},{old_vclock,10000}],
?assertEqual(prune(VC1, Now, Props), prune(VC2, Now, Props)).
accessor_test() ->
VC = [{<<"1">>, {1, 1}},
{<<"2">>, {2, 2}}],
?assertEqual(1, get_counter(<<"1">>, VC)),
?assertEqual(1, get_timestamp(<<"1">>, VC)),
?assertEqual(2, get_counter(<<"2">>, VC)),
?assertEqual(2, get_timestamp(<<"2">>, VC)),
?assertEqual(0, get_counter(<<"3">>, VC)),
?assertEqual(undefined, get_timestamp(<<"3">>, VC)),
?assertEqual([<<"1">>, <<"2">>], all_nodes(VC)).
merge_test() ->
VC1 = [{<<"1">>, {1, 1}},
{<<"2">>, {2, 2}},
{<<"4">>, {4, 4}}],
VC2 = [{<<"3">>, {3, 3}},
{<<"4">>, {3, 3}}],
?assertEqual([], merge(vclock:fresh())),
?assertEqual([{<<"1">>,{1,1}},{<<"2">>,{2,2}},{<<"3">>,{3,3}},{<<"4">>,{4,4}}],
merge([VC1, VC2])).
merge_less_left_test() ->
VC1 = [{<<"5">>, {5, 5}}],
VC2 = [{<<"6">>, {6, 6}}, {<<"7">>, {7, 7}}],
?assertEqual([{<<"5">>, {5, 5}},{<<"6">>, {6, 6}}, {<<"7">>, {7, 7}}],
vclock:merge([VC1, VC2])).
merge_less_right_test() ->
VC1 = [{<<"6">>, {6, 6}}, {<<"7">>, {7, 7}}],
VC2 = [{<<"5">>, {5, 5}}],
?assertEqual([{<<"5">>, {5, 5}},{<<"6">>, {6, 6}}, {<<"7">>, {7, 7}}],
vclock:merge([VC1, VC2])).
merge_same_id_test() ->
VC1 = [{<<"1">>, {1, 2}},{<<"2">>,{1,4}}],
VC2 = [{<<"1">>, {1, 3}},{<<"3">>,{1,5}}],
?assertEqual([{<<"1">>, {1, 3}},{<<"2">>,{1,4}},{<<"3">>,{1,5}}],
vclock:merge([VC1, VC2])).
-endif.
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