From 2f396cdfafca756f550a5c54d9442e8b55f8cb94 Mon Sep 17 00:00:00 2001
From: Peter Tihanyi <peter@systream.hu>
Date: Tue, 28 Nov 2023 22:06:11 +0100
Subject: [PATCH] Send only a rumor head on tick

---
 src/simple_gossip.hrl        |  7 +++++-
 src/simple_gossip_rumor.erl  | 43 ++++++++++++++++++++++--------------
 src/simple_gossip_server.erl | 26 +++++++++++++++++++---
 3 files changed, 56 insertions(+), 20 deletions(-)

diff --git a/src/simple_gossip.hrl b/src/simple_gossip.hrl
index 01fb0c0..96cf50c 100644
--- a/src/simple_gossip.hrl
+++ b/src/simple_gossip.hrl
@@ -11,9 +11,14 @@
                 data :: any(),
                 leader :: node(),
                 nodes = [] :: [node()],
-                max_gossip_per_period = 3 :: pos_integer(),
+                max_gossip_per_period = 1 :: pos_integer(),
                 gossip_period = 15000 :: pos_integer()
 }).
 
+-record(rumor_head, {gossip_version = 1 :: pos_integer(),
+                     vector_clock = simple_gossip_vclock:vclock()
+}).
+
 -type rumor() :: #rumor{}.
+-type rumor_head() :: #rumor_head{}.
 -export_type([rumor/0]).
diff --git a/src/simple_gossip_rumor.erl b/src/simple_gossip_rumor.erl
index ef61d84..161caf0 100644
--- a/src/simple_gossip_rumor.erl
+++ b/src/simple_gossip_rumor.erl
@@ -14,22 +14,23 @@
 
 %% API
 -export([new/0,
-         new/1,
-         add_node/2,
-         change_leader/1,
-         remove_node/2,
-         nodes/1,
-         pick_random_nodes/2,
-         check_node_exclude/1,
-         if_member/3,
-         if_not_member/3,
-         set_data/2,
-         data/1,
-         check_vector_clocks/2,
-         change_max_gossip_per_period/2,
-         change_gossip_interval/2,
-         calculate_new_leader/1,
-         calculate_new_leader/2]).
+  new/1,
+  head/1,
+  add_node/2,
+  change_leader/1,
+  remove_node/2,
+  nodes/1,
+  pick_random_nodes/2,
+  check_node_exclude/1,
+  if_member/3,
+  if_not_member/3,
+  set_data/2,
+  data/1,
+  check_vector_clocks/2,
+  change_max_gossip_per_period/2,
+  change_gossip_interval/2,
+  calculate_new_leader/1,
+  calculate_new_leader/2, version/1]).
 
 -type manage_node_fun() ::  fun(() -> rumor()).
 -type if_leader_node_fun() ::  fun((node()) -> rumor()).
@@ -50,6 +51,10 @@ new(#rumor{vector_clock = VectorClocks}) ->
          gossip_version = 1,
          vector_clock = VectorClocks}.
 
+-spec head(rumor()) -> rumor_head().
+head(#rumor{gossip_version = GossipVersion, vector_clock = VectorClock}) ->
+  #rumor_head{gossip_version = GossipVersion, vector_clock = VectorClock}.
+
 -spec change_max_gossip_per_period(rumor(), pos_integer()) -> rumor().
 change_max_gossip_per_period(#rumor{} = Rumor, Period) ->
   increase_version(Rumor#rumor{max_gossip_per_period = Period}).
@@ -90,6 +95,12 @@ set_data(#rumor{} = Rumor, Data) ->
 data(#rumor{data = Data}) ->
   Data.
 
+-spec version(rumor_head() | rumor()) -> pos_integer().
+version(#rumor{gossip_version = Version}) ->
+  Version;
+version(#rumor_head{gossip_version = Version}) ->
+  Version.
+
 -spec remove_node(rumor(), node()) -> rumor().
 remove_node(#rumor{nodes = Nodes} = Rumor, Node) ->
   if_member(Rumor, Node,
diff --git a/src/simple_gossip_server.erl b/src/simple_gossip_server.erl
index 2b8af1b..30759ba 100644
--- a/src/simple_gossip_server.erl
+++ b/src/simple_gossip_server.erl
@@ -218,6 +218,9 @@ handle_call({join_request, #rumor{gossip_version = InGossipVsn}}, {FromPid, _},
     " Asking remote to upgrade", [Node, CurrentGossipVsn, InGossipVsn]),
   simple_gossip_event:notify(NewRumor),
   {reply, {upgrade, NewRumor}, reschedule_gossip(State#state{rumor = NewRumor})};
+handle_call(reconcile, {FromPid, _FromRef}, #state{rumor = Rumor} = State) ->
+  gossip(Rumor, node(FromPid)),
+  {reply, ok, State};
 
 handle_call(Command, From, State) ->
   proxy_call(Command, From, State).
@@ -322,6 +325,11 @@ handle_command(whisper_your_gossip, _From,
   {noreply, NewState :: state()} |
   {noreply, NewState :: state(), timeout() | hibernate} |
   {stop, Reason :: term(), NewState :: state()}).
+handle_cast({reconcile, #rumor_head{gossip_version = InVersion}, SenderNode},
+             #state{rumor = #rumor{gossip_version = Version}} = State)
+  when InVersion > Version ->
+  gen_server:call({?SERVER, SenderNode}, reconcile),
+  {noreply, State};
 handle_cast({reconcile,
              #rumor{gossip_version = InVersion} = InRumor,
              SenderNode},
@@ -341,9 +349,14 @@ handle_cast({reconcile,
       ?LOG_WARNING("Dropped newer gossip due to vclock check", []),
       {noreply, State}
   end;
-handle_cast({reconcile, Rumor, SenderNode}, State) ->
+handle_cast({reconcile, #rumor_head{gossip_version = InVersion}, _SenderNode},
+            #state{rumor = #rumor{gossip_version = Version}} = State)
+  when InVersion =:= Version ->
+  {noreply, State};
+handle_cast({reconcile, InRumor, SenderNode}, #state{rumor = Rumor} = State) ->
   ?LOG_DEBUG("Gossip ~p dropped due to lower version, From ~p",
-             [Rumor#rumor.gossip_version, SenderNode]),
+             [simple_gossip_rumor:version(InRumor), SenderNode]),
+  gossip(Rumor, SenderNode),
   {noreply, State};
 
 handle_cast({get_gossip_version, Requester, Ref},
@@ -403,7 +416,10 @@ gossip_random_node(#rumor{nodes = []}) ->
   ok;
 gossip_random_node(Rumor) ->
   NewNodes = remove_current_node(simple_gossip_rumor:nodes(Rumor)),
-  gossip(Rumor, simple_gossip_rumor:pick_random_nodes(NewNodes, 1)).
+  case simple_gossip_rumor:pick_random_nodes(NewNodes, 1) of
+    [] -> ok;
+    [Node] -> gossip_head(Rumor, Node)
+  end.
 
 -spec remove_current_node([node()]) -> [node()].
 remove_current_node(Nodes) ->
@@ -417,6 +433,10 @@ gossip(#rumor{max_gossip_per_period = Max} = Rumor, Nodes) when is_list(Nodes) -
 gossip(Rumor, Node) ->
   gen_server:cast({?MODULE, Node}, {reconcile, Rumor, node()}).
 
+-spec gossip_head(rumor(), node()) -> ok.
+gossip_head(Rumor, Node) ->
+  gen_server:cast({?MODULE, Node}, {reconcile, simple_gossip_rumor:head(Rumor), node()}).
+
 -spec schedule_gossip(rumor()) -> reference().
 schedule_gossip(#rumor{gossip_period = Period}) ->
   erlang:send_after(Period + rand:uniform(100), ?SERVER, tick).