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chash.erl
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/
chash.erl
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%% -------------------------------------------------------------------
%%
%% chash: basic consistent hashing
%%
%% 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 consistent hashing implementation. The space described by the ring
%% coincides with SHA-1 hashes, and so any two keys producing the same
%% SHA-1 hash are considered identical within the ring.
%%
%% @reference Karger, D.; Lehman, E.; Leighton, T.; Panigrahy, R.; Levine, M.;
%% Lewin, D. (1997). "Consistent hashing and random trees". Proceedings of the
%% twenty-ninth annual ACM symposium on Theory of computing: 654~663. ACM Press
%% New York, NY, USA
-module(chash).
-author('Justin Sheehy <justin@basho.com>').
-author('Andy Gross <andy@basho.com>').
-export([fresh/2,update/3,lookup/2,members/1,size/1,nodes/1,
successors/2,successors/3,
predecessors/2,predecessors/3,
contains_name/2,key_of/1,
merge_rings/2]).
-define(RINGTOP, trunc(math:pow(2,160)-1)). % SHA-1 space
-include_lib("eunit/include/eunit.hrl").
% @type chash() = {NumPartitions, [NodeEntry]}
% NumPartitions = integer()
% NodeEntry = {IndexAsInt, Node}
% IndexAsInt = integer()
% Node = chash_node().
% It is not recommended that code outside this module make use
% of the structure of a chash.
% @type index() = binary().
% Indices into the ring, used as keys for object location, are binary
% representations of 160-bit integers.
% @type chash_node() = term().
% A Node is the unique identifier for the owner of a given partition.
% An Erlang Pid works well here, but the chash module allows it to
% be any term.
% @doc Create a brand new ring. The size and seednode are specified;
% initially all partitions are owned by the seednode. If NumPartitions
% is not much larger than the intended eventual number of
% participating nodes, then performance will suffer.
% @spec fresh(NumPartitions :: integer(), SeedNode :: chash_node()) -> chash()
fresh(NumPartitions, SeedNode) ->
Inc = ?RINGTOP div NumPartitions,
{NumPartitions, [{IndexAsInt, SeedNode} ||
IndexAsInt <- lists:seq(0,(?RINGTOP-1),Inc)]}.
% @doc Find the Node that owns the partition identified by IndexAsInt.
% @spec lookup(IndexAsInt :: integer(), CHash :: chash()) -> chash_node()
lookup(IndexAsInt, CHash) ->
{_NumPartitions, Nodes} = CHash,
{IndexAsInt, X} = proplists:lookup(IndexAsInt, Nodes),
X.
% @doc Return true if named Node owns any partitions in the ring, else false.
% @spec contains_name(Name :: chash_node(), CHash :: chash()) -> bool()
contains_name(Name, CHash) ->
{_NumPartitions, Nodes} = CHash,
[X || {_,X} <- Nodes, X == Name] =/= [].
% @doc Make the partition beginning at IndexAsInt owned by Name'd node.
% @spec update(IndexAsInt :: integer(), Name :: chash_node(), CHash :: chash())
% -> chash()
update(IndexAsInt, Name, CHash) ->
{NumPartitions, Nodes} = CHash,
NewNodes = lists:keyreplace(IndexAsInt, 1, Nodes, {IndexAsInt, Name}),
{NumPartitions, NewNodes}.
% @doc Given an object key, return all NodeEntries in order starting at Index.
% @spec successors(Index :: index(), CHash :: chash()) -> [NodeEntry]
successors(Index, CHash) ->
{NumPartitions, _Nodes} = CHash,
successors(Index, CHash, NumPartitions).
% @doc Given an object key, return the next N NodeEntries in order
% starting at Index.
% @spec successors(Index :: index(), CHash :: chash(), N :: integer())
% -> [NodeEntry]
successors(Index, CHash, N) ->
Num = max_n(N, CHash),
{Res, _} = lists:split(Num, ordered_from(Index, CHash)),
Res.
% @doc Given an object key, return all NodeEntries in reverse order
% starting at Index.
% @spec predecessors(Index :: index(), CHash :: chash()) -> [NodeEntry]
predecessors(Index, CHash) ->
{NumPartitions, _Nodes} = CHash,
predecessors(Index, CHash, NumPartitions).
% @doc Given an object key, return the next N NodeEntries in reverse order
% starting at Index.
% @spec predecessors(Index :: index(), CHash :: chash(), N :: integer())
% -> [NodeEntry]
predecessors(Index, CHash, N) ->
Num = max_n(N, CHash),
{Res, _} = lists:split(Num, lists:reverse(ordered_from(Index,CHash))),
Res.
% @doc Return either N or the number of partitions in the ring, whichever
% is lesser.
% @spec max_n(N :: integer(), CHash :: chash()) -> integer()
max_n(N, {NumPartitions, _Nodes}) ->
erlang:min(N, NumPartitions).
% @doc Given an object key, return all NodeEntries in order starting at Index.
% @spec ordered_from(Index :: index(), CHash :: chash()) -> [NodeEntry]
ordered_from(Index, {NumPartitions, Nodes}) ->
<<IndexAsInt:160/integer>> = Index,
Inc = ?RINGTOP div NumPartitions,
{A, B} = lists:split((IndexAsInt div Inc)+1, Nodes),
B ++ A.
% @doc Given any term used to name an object, produce that object's key
% into the ring. Two names with the same SHA-1 hash value are
% considered the same name.
% @spec key_of(ObjectName :: term()) -> index()
key_of(ObjectName) ->
crypto:sha(term_to_binary(ObjectName)).
% @doc Return all Nodes that own any partitions in the ring.
% @spec members(CHash :: chash()) -> [Node]
members(CHash) ->
{_NumPartitions, Nodes} = CHash,
lists:usort([X || {_Idx,X} <- Nodes]).
% @doc Return the entire set of NodeEntries in the ring.
% @spec nodes(CHash :: chash()) -> [NodeEntry]
nodes(CHash) ->
{_NumPartitions, Nodes} = CHash,
Nodes.
% @doc Return a randomized merge of two rings.
% If multiple nodes are actively claiming nodes in the same
% time period, churn will occur. Be prepared to live with it.
% @spec merge_rings(CHashA :: chash(), CHashB :: chash()) -> chash()
merge_rings(CHashA,CHashB) ->
{NumPartitions, NodesA} = CHashA,
{NumPartitions, NodesB} = CHashB,
{NumPartitions, [{I,randomnode(A,B)} ||
{{I,A},{I,B}} <- lists:zip(NodesA,NodesB)]}.
% @spec randomnode(NodeA :: chash_node(), NodeB :: chash_node()) -> chash_node()
randomnode(NodeA,NodeA) -> NodeA;
randomnode(NodeA,NodeB) -> lists:nth(random:uniform(2),[NodeA,NodeB]).
% @doc Return the number of partitions in the ring.
% @spec size(CHash :: chash()) -> integer()
size(CHash) ->
{_NumPartitions,Nodes} = CHash,
length(Nodes).
update_test() ->
Node = 'old@host', NewNode = 'new@host',
% Create a fresh ring...
CHash = chash:fresh(5, Node),
GetNthIndex = fun(N, {_, Nodes}) -> {Index, _} = lists:nth(N, Nodes), Index end,
% Test update...
FirstIndex = GetNthIndex(1, CHash),
ThirdIndex = GetNthIndex(3, CHash),
{5, [{_, NewNode}, {_, Node}, {_, Node}, {_, Node}, {_, Node}, {_, Node}]} = update(FirstIndex, NewNode, CHash),
{5, [{_, Node}, {_, Node}, {_, NewNode}, {_, Node}, {_, Node}, {_, Node}]} = update(ThirdIndex, NewNode, CHash).
contains_test() ->
CHash = chash:fresh(8, the_node),
?assertEqual(true, contains_name(the_node,CHash)),
?assertEqual(false, contains_name(some_other_node,CHash)).
max_n_test() ->
CHash = chash:fresh(8, the_node),
?assertEqual(1, max_n(1,CHash)),
?assertEqual(8, max_n(11,CHash)).
simple_size_test() ->
?assertEqual(8, length(chash:nodes(chash:fresh(8,the_node)))).
successors_length_test() ->
?assertEqual(8, length(chash:successors(chash:key_of(0),
chash:fresh(8,the_node)))).
inverse_pred_test() ->
CHash = chash:fresh(8,the_node),
S = [I || {I,_} <- chash:successors(chash:key_of(4),CHash)],
P = [I || {I,_} <- chash:predecessors(chash:key_of(4),CHash)],
?assertEqual(S,lists:reverse(P)).
merge_test() ->
CHashA = chash:fresh(8,node_one),
CHashB = chash:update(0,node_one,chash:fresh(8,node_two)),
CHash = chash:merge_rings(CHashA,CHashB),
?assertEqual(node_one,chash:lookup(0,CHash)).