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-module(partitions).
-author('brad@cloudant.com').
%% API
-export([fullmap/2, fullmap/3, hash/1, install_fullmap/4]).
-export([for_key/2, all_parts/1]).
-export([shard_name/2]).
-define(RINGTOP, trunc(math:pow(2,160))). % SHA-1 space
-include("../../couch/src/couch_db.hrl").
-include("../../dynomite/include/membership.hrl").
-include_lib("eunit/include/eunit.hrl").
%%====================================================================
%% API
%%====================================================================
%% @doc build a full partition map
fullmap(DbName, Options) ->
{ok, Nodes} = mem3:nodes(),
fullmap(DbName, Nodes, Options).
fullmap(DbName, Nodes, Options) ->
{N,Q} = db_init_constants(Options),
NewNodes = ordered_nodes(DbName, Nodes),
Pmap = pmap(Q, NewNodes),
int_fullmap(DbName, N, Pmap, NewNodes).
%% @spec hash(term()) -> Digest::binary()
%% @doc uses SHA-1 as its hash
hash(Item) when is_binary(Item) ->
crypto:sha(Item);
hash(Item) ->
crypto:sha(term_to_binary(Item)).
install_fullmap(DbName, Fullmap, FullNodes, Options) ->
{N,Q} = db_init_constants(Options),
Doc = {[{<<"_id">>,DbName},
{<<"map">>, jsonify(<<"map">>, Fullmap)},
{<<"nodes">>, jsonify(<<"nodes">>, FullNodes)},
{<<"n">>,N},
{<<"q">>,Q}]},
write_db_doc(Doc).
for_key(DbName, Key) ->
<<HashKey:160/integer>> = hash(Key),
Head = #shard{
name = '_',
node = '_',
dbname = DbName,
range = ['$1','$2'],
ref = '_'
},
% TODO these conditions assume A < B, which we don't require
Conditions = [{'<', '$1', HashKey}, {'<', HashKey, '$2'}],
case ets:select(partitions, [{Head, Conditions, ['$_']}]) of
[] ->
erlang:error(database_does_not_exist);
Shards ->
Shards
end.
all_parts(DbName) ->
case ets:lookup(partitions, DbName) of
[] ->
erlang:error(database_does_not_exist);
Else ->
Else
end.
%%====================================================================
%% Internal functions
%%====================================================================
%% @doc get cluster constants from options or config
db_init_constants(Options) ->
{const(n, Options), const(q, Options)}.
%% @doc get individual constant
const(Const, Options) ->
ListResult = case couch_util:get_value(Const, Options) of
undefined -> couch_config:get("cluster", atom_to_list(Const));
Val -> Val
end,
list_to_integer(ListResult).
%% @doc hash the dbname, and return the corresponding node for seeding a ring
seednode(DbName, Nodes) ->
<<HashInt:160/integer>> = hash(DbName),
Size = partition_range(length(Nodes)),
Factor = (HashInt div Size),
lists:nth(Factor+1, Nodes).
%% @doc take the list of nodes, and rearrange it, starting with the node that
%% results from hashing the Term
ordered_nodes(Term, Nodes) ->
SeedNode = seednode(Term, Nodes),
{A, B} = lists:splitwith(fun(N) -> N /= SeedNode end, Nodes),
lists:append(B,A).
%% @doc create a partition map
pmap(NumPartitions, Nodes) ->
Increment = ?RINGTOP div NumPartitions,
Parts = parts(?RINGTOP, Increment, 0, []),
make_map(Nodes, Nodes, Parts, []).
%% @doc makes a {beg, end} list of partition ranges
%% last range may have an extra few values, because Increment is created
%% with Ringtop 'div' NumPartitions above.
parts(Top, _, Beg, Acc) when Beg > Top -> Acc;
parts(Top, Increment, Beg, Acc) ->
End = case Beg + 2*Increment of
Over when Over > Top -> Top;
_ -> Beg + Increment - 1
end,
NewAcc = [{Beg, End} | Acc],
parts(Top, Increment, End+1, NewAcc).
%% @doc create a full map, which is a pmap with N-1 replication partner nodes
%% added per partition
int_fullmap(DbName, N, Pmap, Nodes) ->
Full = lists:foldl(fun({Node,{B,E} = Part}, AccIn) ->
Primary = [#shard{dbname=DbName, node=Node, range=[B,E],
name=shard_name(B,DbName)}],
Partners = partners(DbName, N, Node, Nodes, Part),
lists:append([Primary, Partners, AccIn])
end, [], Pmap),
lists:reverse(Full).
partners(DbName, N, Node, Nodes, {Beg,End}) ->
{A, [Node|B]} = lists:splitwith(fun(Nd) -> Nd /= Node end, Nodes),
Nodes1 = lists:append(B,A),
Partners = lists:sublist(Nodes1, N-1), % N-1 replication partner nodes
lists:map(fun(Partner) ->
#shard{dbname=DbName, node=Partner, range=[Beg,End],
name=shard_name(Beg,DbName)}
end, Partners).
%% @doc size of one partition in the ring
partition_range(Q) ->
trunc( ?RINGTOP / Q ). % SHA-1 space / Q
%% @doc assign nodes to each of the partitions. When you run out of nodes,
%% start at the beginning of the node list again.
%% The provided node list starts with the seed node (seednode fun)
make_map(_,_,[], Acc) ->
lists:keysort(2,Acc);
make_map(AllNodes, [], Parts, Acc) ->
% start back at beginning of node list
make_map(AllNodes, AllNodes, Parts, Acc);
make_map(AllNodes, [Node|RestNodes], [Part|RestParts], Acc) ->
% add a node/part combo to the Acc
make_map(AllNodes, RestNodes, RestParts, [{Node,Part}|Acc]).
jsonify(<<"map">>, Map) ->
lists:map(fun(#shard{node=Node, range=[Beg,End]}) ->
{[{<<"node">>, Node}, {<<"b">>, Beg}, {<<"e">>, End}]}
end, Map);
jsonify(<<"nodes">>, Nodes) ->
lists:map(fun({Order, Node, Options}) ->
{[{<<"order">>, Order}, {<<"node">>, Node}, {<<"options">>, Options}]}
end, Nodes).
write_db_doc(EDoc) ->
{ok, Db} = couch_db:open(<<"dbs">>, []),
try
update_db_doc(Db, couch_doc:from_json_obj(EDoc))
catch {conflict, _} ->
?LOG_ERROR("conflict writing db doc, must be a race", [])
after
couch_db:close(Db)
end.
update_db_doc(Db, #doc{id=Id, body=Body} = Doc) ->
case couch_db:open_doc(Db, Id, []) of
{not_found, _} ->
{ok, _} = couch_db:update_doc(Db, Doc, []);
{ok, #doc{body=Body}} ->
ok;
{ok, OldDoc} ->
{ok, _} = couch_db:update_doc(Db, OldDoc#doc{body=Body}, [])
end.
shard_name(Part, DbName) when is_list(DbName) ->
shard_name(Part, ?l2b(DbName));
shard_name(Part, DbName) ->
PartHex = ?l2b(showroom_utils:int_to_hexstr(Part)),
<<"x", PartHex/binary, "/", DbName/binary, "_", PartHex/binary>>.
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