%% ------------------------------------------------------------------- %% %% 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, fresh/2, descends/2, dominates/2, descends_dot/2, pure_dot/1, merge/1, get_counter/2, get_timestamp/2, get_dot/2, valid_dot/1, 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, dot/0, pure_dot/0]). -type vclock() :: [dot()]. -type dot() :: {vclock_node(), {counter(), timestamp()}}. -type pure_dot() :: {vclock_node(), counter()}. % 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() -> []. -spec fresh(vclock_node(), counter()) -> vclock(). fresh(Node, Count) -> [{Node, {Count, timestamp()}}]. % @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 does the given `vclock()' descend from the given `dot()'. The %% `dot()' can be any vclock entry returned from %% `get_entry/2'. returns `true' if the `vclock()' has an entry for %% the `actor' in the `dot()', and that the counter for that entry is %% at least that of the given `dot()'. False otherwise. Call with a %% valid entry or you'll get an error. %% %% @see descends/2 %% @see get_entry/3 %% @see dominates/2 -spec descends_dot(vclock(), dot()) -> boolean(). descends_dot(Vclock, Dot) -> descends(Vclock, [Dot]). %% @doc in some cases the dot without timestamp data is needed. -spec pure_dot(dot()) -> pure_dot(). pure_dot({N, {C, _TS}}) -> {N, C}. %% @doc true if `A' strictly dominates `B'. Note: ignores %% timestamps. In Riak it is possible to have vclocks that are %% identical except for timestamps. When two vclocks descend each %% other, but are not equal, they are concurrent. See source comment %% for more details. (Actually you can have indentical clocks %% including timestamps, that represent different events, but let's %% not go there.) %% -spec dominates(vclock(), vclock()) -> boolean(). dominates(A, B) -> %% In a sane world if two vclocks descend each other they MUST be %% equal. In riak they can descend each other and have different %% timestamps(!) How? Deleted keys, re-written, then restored is %% one example. See riak_kv#679 for others. This is why we must %% check descends both ways rather than checking descends(A, B) %% and not equal(A, B). Do not "optimise" this to dodge the second %% descends call! I know that the laws of causality say that each %% actor must act serially, but Riak breaks that. descends(A, B) andalso not descends(B, A). % @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 Get the entry `dot()' for `vclock_node()' from `vclock()'. -spec get_dot(Node :: vclock_node(), VClock :: vclock()) -> {ok, dot()} | undefined. get_dot(Node, VClock) -> case lists:keyfind(Node, 1, VClock) of false -> undefined; Entry -> {ok, Entry} end. %% @doc is the given argument a valid dot, or entry? -spec valid_dot(dot()) -> boolean(). valid_dot({_, {Cnt, TS}}) when is_integer(Cnt), is_integer(TS) -> true; valid_dot(_) -> false. % @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])). get_entry_test() -> VC = vclock:fresh(), VC1 = increment(a, increment(c, increment(b, increment(a, VC)))), ?assertMatch({ok, {a, {2, _}}}, get_dot(a, VC1)), ?assertMatch({ok, {b, {1, _}}}, get_dot(b, VC1)), ?assertMatch({ok, {c, {1, _}}}, get_dot(c, VC1)), ?assertEqual(undefined, get_dot(d, VC1)). valid_entry_test() -> VC = vclock:fresh(), VC1 = increment(c, increment(b, increment(a, VC))), [begin {ok, E} = get_dot(Actor, VC1), ?assert(valid_dot(E)) end || Actor <- [a, b, c]], ?assertNot(valid_dot(undefined)), ?assertNot(valid_dot("huffle-puff")), ?assertNot(valid_dot([])). -endif.