From 8770fc98e1c3fb42a3341f64107e77df76b2b7ca Mon Sep 17 00:00:00 2001
From: Evgeniy Khramtsov <ekhramtsov@process-one.net>
Date: Sun, 23 Apr 2017 11:54:56 +0300
Subject: [PATCH] Use round-robin algorithm when selecting worker from DB pool

---
 src/ejabberd_redis.erl    | 2 +-
 src/ejabberd_riak_sup.erl | 7 ++-----
 src/ejabberd_sql_sup.erl  | 4 +++-
 src/randoms.erl           | 7 ++++++-
 4 files changed, 12 insertions(+), 8 deletions(-)

diff --git a/src/ejabberd_redis.erl b/src/ejabberd_redis.erl
index bd85f0ee5..7757c6df3 100644
--- a/src/ejabberd_redis.erl
+++ b/src/ejabberd_redis.erl
@@ -516,7 +516,7 @@ log_error(Cmd, Reason) ->
 
 -spec get_rnd_id() -> pos_integer().
 get_rnd_id() ->
-    randoms:uniform(2, ejabberd_redis_sup:get_pool_size()).
+    randoms:round_robin(ejabberd_redis_sup:get_pool_size() - 1) + 2.
 
 -spec get_result([{error, atom() | binary()} | {ok, iodata()}]) ->
 			{ok, [redis_reply()]} | {error, binary()}.
diff --git a/src/ejabberd_riak_sup.erl b/src/ejabberd_riak_sup.erl
index f5c8f7e4f..a01f3538a 100644
--- a/src/ejabberd_riak_sup.erl
+++ b/src/ejabberd_riak_sup.erl
@@ -30,7 +30,7 @@
 -author('alexey@process-one.net').
 
 -export([start_link/0, init/1, get_pids/0,
-	 transform_options/1, get_random_pid/0, get_random_pid/1,
+	 transform_options/1, get_random_pid/0,
 	 host_up/1, config_reloaded/0, opt_type/1]).
 
 -include("ejabberd.hrl").
@@ -199,10 +199,7 @@ get_pids() ->
     [ejabberd_riak:get_proc(I) || I <- lists:seq(1, get_pool_size())].
 
 get_random_pid() ->
-    get_random_pid(p1_time_compat:system_time()).
-
-get_random_pid(Term) ->
-    I = erlang:phash2(Term, get_pool_size()) + 1,
+    I = randoms:round_robin(get_pool_size()) + 1,
     ejabberd_riak:get_proc(I).
 
 transform_options(Opts) ->
diff --git a/src/ejabberd_sql_sup.erl b/src/ejabberd_sql_sup.erl
index 09aceafb4..d778e32b7 100644
--- a/src/ejabberd_sql_sup.erl
+++ b/src/ejabberd_sql_sup.erl
@@ -98,7 +98,9 @@ get_pids(Host) ->
 get_random_pid(Host) ->
     case get_pids(Host) of
       [] -> none;
-      Pids -> lists:nth(erlang:phash(p1_time_compat:unique_integer(), length(Pids)), Pids)
+      Pids ->
+	    I = randoms:round_robin(length(Pids)) + 1,
+	    lists:nth(I, Pids)
     end.
 
 add_pid(Host, Pid) ->
diff --git a/src/randoms.erl b/src/randoms.erl
index ea21b4a1d..35a5d7580 100644
--- a/src/randoms.erl
+++ b/src/randoms.erl
@@ -27,7 +27,8 @@
 
 -author('alexey@process-one.net').
 
--export([get_string/0, uniform/0, uniform/1, uniform/2, bytes/1]).
+-export([get_string/0, uniform/0, uniform/1, uniform/2, bytes/1,
+	 round_robin/1]).
 
 -define(THRESHOLD, 16#10000000000000000).
 
@@ -51,3 +52,7 @@ bytes(N) ->
 bytes(N) ->
     crypto:rand_bytes(N).
 -endif.
+
+-spec round_robin(pos_integer()) -> non_neg_integer().
+round_robin(N) ->
+    erlang:unique_integer([monotonic, positive]) rem N.