Skip to content
This repository
branch: wrd-remove-cap…
Fetching contributors…

Cannot retrieve contributors at this time

file 314 lines (277 sloc) 11.487 kb
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314
%% -------------------------------------------------------------------
%%
%% 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).

-author('Justin Sheehy <justin@basho.com>').
-author('Andy Gross <andy@basho.com>').

-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.
Something went wrong with that request. Please try again.