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   Copyright 2019, Shanti Chellaram <shanti@shanti.im>.
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--- a/README.md
+++ b/README.md
@ -0,0 +1,14 @@
 raft_erl
 =====
 An escript
 Build
 -----
    $ rebar3 escriptize
 Run
 ---
    $ _build/default/bin/raft_erl
--- a/rebar.config
+++ b/rebar.config
@ -0,0 +1,13 @@
 {erl_opts, [no_debug_info]}.
 {deps, []}.
 {escript_incl_apps,
 [raft_erl]}.
 {escript_main_app, raft_erl}.
 {escript_name, raft_erl}.
 {escript_emu_args, "%%! +sbtu +A1\n"}.
 %% Profiles
 {profiles, [{test,
             [{erl_opts, [debug_info]}
            ]}]}.
--- a/rebar.lock
+++ b/rebar.lock
@ -0,0 +1 @@
 [].
--- a/src/paxos_agent.erl
+++ b/src/paxos_agent.erl
@ -0,0 +1,109 @@
 -module(paxos_agent).
 -behavior(gen_statem).
 -export([callback_mode/0, init/1, handle_event/2]).
 callback_mode() -> [handle_event_function, state_enter].
 init(Opts) ->
 	ok.
 % persistent state
 -record(
   pstate, {
     name, % the identifier of this agent
     outcome = [], % the last decree written in the ledger, or []
     lastTried = -1, % the number of the last ballot begun by this agent
     prevBal = -1, % the number of the last ballot in which this agent voted
     prevDec = [], % the decree of the last ballot in which this agent voted
     nextBal -1 % the number of the last ballot in which this agent agreed to participate, or -1
    }).
 % transient state
 -record(
   tstate, {
     status = idle, % one of idle, trying, or polling
     prevVotes = [], % set of votes received in LastVote messages for the current ballot (the one in lastTried)
     quorum, % if status = polling, the set of priests forming quorum of current ballot; otherwise meaningless
     voters, % if status = polling, the set of quorum members from whom p has received 'voted' messages
     decree, % if status = polling, the decree of the current ballot
    }).
 % Note: Guaranteed not to Lose Progress
 % Note: Guaranteed to Proceed if N/2+1 nodes are available.
 % Ballots are a { BallotId :: integer, OwnerName :: string }
 try_new_ballot(Tstate, Pstate) ->
    {LastTriedId, _LastTriedOwner} = Pstate#pstate.lastTried,
    Ballot = {LastTriedId+1, Pstate#pstate.name},
    NewPstate = Pstate#pstate{lastTried=Ballot},
    NewTstate = Tstate#tstate{status=trying, prevVotes=[]},
    {ok, {NewTstate, NewPstate}}.
 send_next_ballot(Tstate, Pstate)
  when Tstate#tstate.status == trying ->
    send(any_priest, {next_ballot, Pstate#pstate.lastTried}),
    {ok, Tstate, Pstate}.
 recv_next_ballot({next_ballot, Ballot}, Tstate, #pstate{nextBal=NextBal} = Pstate)
  when Ballot > NextBal ->
    NewPstate = Pstate#pstate{nextBal=Ballot},
    {ok, {Tstate, NewPstate}}.
 send_last_vote(Tstate, Pstate)
  when Pstate#pstate.nextBal > Pstate#pstate.prevBal -> % send the last decree this voted for
    Vote = {Pstate#pstate.name, Pstate#pstate.prevBal, Pstate#pstate.prevDec},
    send(owner(Pstate#pstate.nextBal), {last_vote, Pstate#pstate.nextBal, Vote}),
    {ok, Tstate, Pstate}.
 recv_last_vote({last_vote, Ballot, Vote}, Tstate, Pstate)
  when Tstate#tstate.status == trying andalso Ballot == Pstate#pstate.lastTried ->
    NewTstate=Tstate#tstate{prevVotes=[Vote|Tstate#tstate.prevVotes]}, % TODO: this should be union, not append
    {ok, NewTstate, Pstate}.
 start_polling_majority_set_q(Q, Tstate, Pstate)
  when Tstate#tstate.status == trying andalso Q ->
    NewTstate = Tstate#tstate{
                  status=polling,
                  quorum=Q,
                  voters=[],
                  decree=case max(Tstate#tstate.prevVotes) of
                             {PriestName, {-1, _}, PrevDecree} -> [];
                             {PriestName, PrevBallot, PrevDecree} -> PrevDecree
                         end,
                 },
    {ok, NewTstate, Pstate}.
 send_begin_ballot(Tstate, Pstate)
  when Tstate#tstate.status == polling ->
    send(random(Tstate#tstate.quorum), {begin_ballot, Pstate#pstate.lastTried, Tstate#tstate.decree}),
    {ok, Tstate, Pstate}.
 recv_begin_ballot({begin_ballot, Ballot, Decree}, Tstate, Pstate)
  when Ballot == Pstate#pstate.nextBal andalso Ballot > Pstate#pstate.prevBal ->
    NewPstate = Pstate#pstate{prevBal=Ballot, nextBal=Decree},
    % if there is a Ballot B in _B_ with ballotId = Ballot, choose that node and let the new value of _B_ include the union of Bc (I believe this means commit?)
    {ok, Tstate, NewPstate}.
 send_voted(Tstate, Pstate)
  when Pstate#pstate.prevBal /= -1 ->
    send(owner(Pstate#pstate.prevBal), {voted, Pstate#pstate.prevBal, Pstate#pstate.name}),
    {ok, Tstate, Pstate}.
 recv_voted({voted, Ballot, Priest}, Tstate, Pstate)
  when Tstate#tstate.status == polling andalso Ballot == Pstate#pstate.lastTried ->
    {ok, Tstate#tstate{votes=[Priest|Tstate#tstate.voters]}, Pstate}. % this could be a transition point
 succeed(Tstate, Pstate) %todo: subset needs to be implemented as an if
  when Tstate#tstate.status == polling andalso subset(quorum, voters) andalso Pstate#pstate.outcome == [] ->
    NewPstate = Pstate#pstate{outcome = Tstate#tstate.decree}, % this is the commit step, i think
    {ok, Tstate, NewPstate}.
 send_success(Tstate, Pstate)
  when Pstate#pstate.outcome /= blank ->
    send(any_priest(), {success, Pstate#pstate.outcome}),
    {ok, Tstate, Pstate}.
 recv_success({success, Outcome}, Tstate, Pstate)
  when Pstate#pstate.outcome /= blank ->
    {ok, Tstate, Pstate#pstate{outcome=Outcome}}. % follower commit step
--- a/src/raft_agent.erl
+++ b/src/raft_agent.erl
@ -0,0 +1,261 @@
 -module(raft_agent).
 -behavior(gen_statem).
 %% g
 %-export([start_link/0]).
 %% gen_statem callbacks
 -export([callback_mode/0, init/1, handle_event/4]).
 %% Client API
 -export([operate/2]).
 operate(ServerRef, Fun) ->
    gen_statem:call(ServerRef, {client_req, raft_log:op(Fun, [])}, 500).
 % persisted state should be saved to disk after modification and before a callback returns
 -record(state, {
          id,term=0,voted_for=[],log,cluster=[], %persisted state
          ldr_next_index, ldr_match_index, %volatile state
          votes_gathered=0
         }).
 %% gen_statem callbacks
 callback_mode() -> [handle_event_function, state_enter].
 init(Args) ->
    State0 = parse_args(Args),
    State1 = State0#state{log=raft_log:new(#{})},
    {ok, follower, State1}.
 %% Message Events that need to be handled:
 %%
 %% Election Timeout Elapsed (follower and candidate state only)
 %% Received AppendEntries RPC
 %% Received AppendEntries ACK
 %% Received RequestVote RPC
 %% Received RequestVote ACK
 %% 
 %% Received ClientRequest RPC -> Commit Transition / Query-NoOp
 %% [Committed ClientEntry] -> Respond to Client
 %% All States
 %% Receiving an out-of-date RPC or ACK
 %handle_event(cast, {_Type, Term, _Payload}=Msg, _Role, #state{term=CurrentTerm}=Data)
 %  when Term > CurrentTerm ->
 %    {next_state, follower, Data#state{term=Term, voted_for=[]}};
 %% Follower State
 handle_event(enter, follower, follower, #state{}) ->
    {keep_state_and_data, [action_reset_election()]};
 handle_event(timeout, election, follower, #state{}=Data) ->
    {next_state, candidate, Data};
 %% Candidate State
 handle_event(enter, _PrevState, candidate, #state{term=Term,id=Id}=Data0) ->
    Data1 = Data0#state{term=Term+1, voted_for=Id, votes_gathered=1},
    lists:foreach(fun(Node) -> send(Node, rpc(request_vote, Data1)) end, peers(Data1)),
    {keep_state, Data1};
 % if we are out of date, revert to follower
 handle_event(cast, {rpc, _RpcType, Term, _Payload}=EventData, _State, #state{term=CurrentTerm}=Data) when Term > CurrentTerm ->
    {next_state, follower, Data#state{term=Term, voted_for=[]}, [{next_event, cast, EventData}]};
 handle_event(cast, {rpc, request_vote, Term, {Candidate, _LastIdx, _LastTerm}}, _State, #state{term=CurrentTerm}) when Term < CurrentTerm ->
    send(Candidate, {ack, request_vote, CurrentTerm, false}),
    keep_state_and_data;
 handle_event(cast, {rpc, request_vote, CurrentTerm, {Candidate, CLastIndex, CLastTerm}}, follower, #state{term=CurrentTerm,log=Log,voted_for=VotedFor}=Data) ->
    LastIndex = raft_log:last_index(Log),
    LastTerm = raft_log:term_at(Log, LastIndex),
    Result = 
    if
        not (VotedFor == [] orelse VotedFor == Candidate) ->
            false;
        LastTerm > CLastTerm ->
            false;
        LastIndex > CLastIndex ->
            false;
        true ->
            true
    end,
    send(Candidate, {ack, request_vote, CurrentTerm, Result}),
    case Result of
        true ->
            {keep_state, Data#state{voted_for=Candidate}};
        false ->
            keep_state_and_data
    end;
 handle_event(cast, {rpc, append_entries, Term, {LeaderId, _PrevLogIndex, _PrevLogTerm, [] = _Entries, _LeaderCommit}}, _Role, #state{id=Id, term=Term}) ->
    send(LeaderId, {ack, append_entries, Term, Id, true}),
    keep_state_and_data;
 handle_event(cast, {rpc, append_entries, Term, {LeaderId, PrevLogIndex, PrevLogTerm, Entries, LeaderCommit}}, _Role, #state{id=Id, term=Term, log=Log}=Data) ->
    {Result, NewLog} =
    case raft_log:term_at(Log, PrevLogIndex) of
        PrevLogTerm ->
            {false, Log};
        _ ->
            Log1 = raft_log:drop_after(Log, PrevLogIndex),
            Log2 = raft_log:append_all(Log1, Entries),
            Log3 = raft_log:commit(Log2, min(LeaderCommit, raft_log:last_index(Log2))),
            {true, Log3}
    end,
    send(LeaderId, {ack, append_entries, Term, Id, Result}),
    {keep_state, Data#state{log=NewLog}};
 handle_event(cast, {ack, request_vote, Term, true}, candidate, #state{term=Term,votes_gathered=Votes,cluster=Cluster}=Data) ->
    NewVotes = Votes+1,
    NewData = Data#state{votes_gathered=NewVotes},
    if 
        NewVotes >= length(Cluster) div 2 ->
            {next_state, leader, NewData};
        true ->
            {keep_state, NewData}
    end;
 %% Leader Mode!
 handle_event(enter, leader, leader, Data) ->
    handle_event(enter, candidate, leader, Data);
 handle_event(enter, candidate, leader, Data) ->
    % send to all members
    Peers = peers(Data),
    LastLogIndex = raft_log:last_index(Data#state.log),
    NextIndex = maps:from_list(lists:map(fun (Peer) -> {Peer, LastLogIndex + 1} end, Peers)),
    MatchIndex = maps:from_list(lists:map(fun (Peer) -> {Peer, 0} end, Peers)),
    NewData = Data#state{ldr_match_index=MatchIndex,ldr_next_index=NextIndex},
    SendHeartbeat = fun(Server) -> send(Server, rpc(append_entries, Server, NewData)) end,
    lists:foreach(SendHeartbeat, Peers),
    {keep_state, NewData, [action_heartbeat()]};
 handle_event({call, From}, {client_req, {op, Op, Args}}, leader, #state{term=Term,log=Log}=Data) -> 
    Parent = self(),
    ProxyOp =
        fun(State) ->
                {State, Response} = apply(Op, [State | Args]),
                gen_statem:cast(Parent, {client_resp, From, Response}),
                State
        end,
    NewLog = raft_log:append(Log, Term, {op, ProxyOp, []}),
    {keep_state, Data#state{log=NewLog}, [{next_event, internal, check_followers}]};
 handle_event(cast, {client_resp, To, Result}, leader, _Data) ->
    gen_statem:reply(To, Result),
    keep_state_and_data;
 handle_event(internal, check_followers, leader, #state{cluster=Cluster,log=Log,ldr_next_index=NextIndices}=Data) ->
    LastIndex = raft_log:last_index(Log),
    AllPeers = peers(Data),
    CheckFn = fun(Peer) ->
                      #{Peer := NextIndex} = NextIndices,
                      LastIndex >= NextIndex
              end,
    OutOfDatePeers = lists:filter(CheckFn, AllPeers),
    SendAppendEntries = fun(Peer) -> send(Peer, rpc(append_entries, Peer, Data)) end,
    lists:foreach(SendAppendEntries, OutOfDatePeers),
    keep_state_and_data;
 handle_event(cast, {ack, append_entries, Term, FollowerId, false}, leader, #state{ldr_next_index=NextIndices,log=Log}=Data) ->
    NewNextIndices = maps:update_with(FollowerId, fun(Value) -> Value-1 end, NextIndices),
    NewData = Data#state{ldr_next_index=NewNextIndices},
    send(FollowerId, rpc(append_entries, NewData)),
    {keep_state, NewData};
 handle_event(cast, {ack, append_entries, Term, FollowerId, true}, leader, #state{ldr_match_index=MatchIndices, ldr_next_index=NextIndices,log=Log,cluster=Cluster}=Data) ->
    %io:format("LEADER HEARTBEAT: ~p~n", [Data]),
    NewNextIndices = NextIndices#{FollowerId => raft_log:last_index(Log)},
    NewMatchIndices = MatchIndices#{FollowerId => raft_log:last_index(Log)},
    % if SOME INDEX is replicated to a majority of followers, commit that index (the highest SOME INDEX)
    % That index will be the median of the indices in NextIndices
    SortedIndices = lists:sort(maps:values(NewMatchIndices)),
    MedianIndex = length(Cluster) div 2,
    MajorityCommitted = lists:nth(MedianIndex + 1, SortedIndices),
    NewLog = raft_log:commit(Log, MajorityCommitted),
    {keep_state, Data#state{log=NewLog, ldr_next_index=NewNextIndices, ldr_match_index=NewMatchIndices}};
 %% Mostly Ignore votes we gather after being elected
 handle_event(cast, {ack, request_vote, Term, Success}, leader, #state{term=Term,votes_gathered=Votes}=Data) ->
    case Success of
        true ->
            {keep_state, Data#state{votes_gathered=Votes+1}};
        false ->
            keep_state_and_data
    end;
 handle_event(state_timeout, heartbeat, leader, #state{}) ->
    repeat_state_and_data;
 handle_event(timeout, election, candidate, #state{}) ->
    repeat_state_and_data.
 %% 
 %% Private Functions
 %%
 %% A message
 rpc(append_entries, To, #state{ldr_next_index=NextIndices,term=Term,id=Id,log=Log}) ->
    #{To := NextIndex} = NextIndices,
    LastIndex = raft_log:last_index(Log),
    PrevLogIndex = NextIndex - 1,
    PrevLogTerm = raft_log:at(Log, PrevLogIndex),
    Entries = raft_log:sublist(Log, NextIndex),
    {rpc, append_entries, Term, {Id, PrevLogIndex, PrevLogTerm, Entries, raft_log:commit_index(Log)}}.
 rpc(heartbeat, #state{term=Term,id=Id,log=Log}) ->
    PrevLogIndex = 0,
    PrevLogTerm = 0,
    % TODO: populate entries
    Entries = [],
    {rpc, append_entries, Term, {Id, PrevLogIndex, PrevLogTerm, Entries, raft_log:commit_index(Log)}};
 rpc(request_vote, #state{term=Term,id=Id,log=Log}) ->
    LastIndex = raft_log:last_index(Log),
    {rpc, request_vote, Term, {Id, LastIndex, raft_log:term_at(Log, LastIndex)}}.
 % min and max in millseconds
 -define(election_min, 150).
 -define(election_max, 300).
 election_timeout() ->
    ?election_min + rand:uniform(?election_max-?election_min+1) - 1.
 heartbeat_timeout() ->
    50.
 %% gen_statem action to handle event transition
 action_reset_election() ->
    {timeout, election_timeout(), election}.
 action_heartbeat() ->
    {state_timeout, heartbeat_timeout(), heartbeat}.
 %action_retry_rpc(RpcData) ->
 %    {state_timeout, heartbeat_timeout(), {retry, RpcData}}.
 parse_args(Args) -> parse_args(Args, #state{}).
 parse_args([{id, Id}|Rest], State) ->
    parse_args(Rest, State#state{id=Id});
 parse_args([{cluster, Cluster}|Rest], State) ->
    parse_args(Rest, State#state{cluster=Cluster});
 parse_args([], State) -> State.
 peers(#state{id=Id,cluster=Cluster}) ->
    Cluster -- [{local, Id}].
 %% Update Commit Index (call this from check_followers)
 %% MOCK
 % TODO: Make this probabilistically fail
 send(To, Data) when is_atom(To) ->
 	%io:format("Sending: ~p to ~p~n", [Data, To]),
    gen_statem:cast(To,Data);
 send({local, To}, Data) ->
 	%io:format("Sending: ~p to ~p~n", [Data, To]),
    gen_statem:cast(To,Data).
--- a/src/raft_erl.app.src
+++ b/src/raft_erl.app.src
@ -0,0 +1,14 @@
 {application, raft_erl,
 [{description, "An escript"},
  {vsn, "0.1.0"},
  {registered, []},
  {applications,
   [kernel,
    stdlib
   ]},
  {env,[]},
  {modules, []},
  {licenses, ["Apache 2.0"]},
  {links, []}
 ]}.
--- a/src/raft_erl.erl
+++ b/src/raft_erl.erl
@ -0,0 +1,43 @@
 -module(raft_erl).
 %% API exports
 -export([main/1]).
 %%====================================================================
 %% API functions
 %%====================================================================
 %% escript Entry point
 main(Args) ->
    storage:start_link(),
    storage:store(lol, wut),
    {ok, Omg} = storage:load(lol),
    %Debug = [],
    Debug = [{debug, [trace,log]}],
    %Cluster = [{local, agent1}],
    Cluster = [{local, agent1}, {local, agent2}, {local, agent3}],
    lists:foreach(
      fun({local, Id}=Name) ->
        {ok, _Pid} = gen_statem:start_link(Name, raft_agent, [{id, Id}, {cluster, Cluster}],
                                           Debug)
      end,
      Cluster),
    timer:sleep(1000),
    %io:format("Raft State: ~p~n", [raft_agent:operate(agent1, get_op())]),
    erlang:halt(0).
 set_op(Value) ->
    raft_log:op(fun do_set/2, [Value]).
 get_op() ->
    raft_log:op(fun do_get/1, []).
 do_set(State, Value) ->
    {Value, ok}.
 do_get(State) ->
    {State, State}.
 %%====================================================================
 %% Internal functions
 %%====================================================================
--- a/src/raft_log.erl
+++ b/src/raft_log.erl
@ -0,0 +1,64 @@
 -module(raft_log).
 -export([new/1, last_index/1, commit_index/1, applied_index/1, commit/2, sublist/2, op/2]).
 -export([append/3, append_all/3, drop_after/2]).
 -export([at/2, term_at/2, op_at/2]).
 -record(state, {oplist=[], state=[], applied_idx=0, commit_idx=0}).
 -record(op, {f=fun(X)->X end, args=[]}).
 new(Init) -> #state{oplist=[{0, op(fun first_op/2, [Init])}]}.
 op(Fun, Args) ->
    #op{f=Fun, args=Args}.
 last_index(#state{oplist=Log}) -> length(Log).
 applied_index(#state{applied_idx=Idx}) -> Idx.
 commit_index(#state{commit_idx=Idx}) -> Idx.
 commit(#state{oplist=Ops,state=State,commit_idx=OldIndex}=Log, NewIndex) when NewIndex > OldIndex ->
    io:format("COMMITTING~n"),
    % foreach operation between NewIndex and OldIndex, apply that operation to state
    ToApply = lists:sublist(Ops, OldIndex, NewIndex - OldIndex),
    NewState = lists:foldl(fun({Op, Args}, Acc) -> apply(Op, [Acc|Args]) end, State, ToApply),
    Log#state{state=NewState, commit_idx=NewIndex};
 % Ignore NewIndex <= OldIndex
 commit(Log, NewIndex) ->
 %    io:format("NOT COMMITTING~p~n Index:(~p)~n State:(~p)~n Log:(~p)~n", [self(), NewIndex, Log#state.state, Log#state.oplist]),
    Log.
 sublist(Log, Index) when Index >= length(Log#state.oplist) ->
    [];
 sublist(Log, Index) ->
    lists:nthtail(Index, Log#state.oplist).
 drop_after(Log, Index) ->
    lists:sublist(Log#state.oplist, Index).
 append(#state{oplist=Log}=State, Term, Op) ->
    State#state{oplist=Log ++ [{Term, Op}]}.
 append_all(State, _Term, []) ->
    State;
 append_all(State, Term, [Op|Tail]) ->
    append(State, Term, Op).
 %% Lookup commands
 at(#state{oplist=Log}, Index) ->
    lists:nth(Index, Log).
 term_at(Log, Index) ->
    {Term, _Op} = at(Log, Index),
    Term.
 op_at(Log, Index) ->
    {_Term, Op} = at(Log, Index),
    Op.
 %%
 first_op(_, InitialState) -> InitialState.
--- a/src/storage.erl
+++ b/src/storage.erl
@ -0,0 +1,41 @@
 -module(storage).
 -behavior(gen_server).
 %% gen_server callbacks
 -export([init/1, handle_call/3, terminate/2]).
 %% public api
 -export([start_link/0, load/1, store/2]).
 init([]) ->
 	TableRef = ets:new(storage, []),
 	{ok, TableRef}.
 terminate(_Reason, TableRef) ->
 	catch ets:delete(TableRef),
 	ok.
 % TODO: deal with hard expectation to be running as 'storage' here
 start_link() ->
 	gen_server:start_link({local, storage}, ?MODULE, [], []).
 load(Key) -> 
 	gen_server:call(storage, {load, Key}).
 store(Key, Value) -> 
 	gen_server:call(storage, {store, Key, Value}).
 %% private methods
 handle_call({load, Key}, _From, TableRef) ->
 	Reply = case ets:lookup(TableRef, Key) of
 			[{Key, Value}] -> {ok, Value};
 			[] -> error
 		end,
 	{reply, Reply, TableRef};
 handle_call({store, Key, Value}, _From, TableRef) ->
 	ets:insert(TableRef, {Key, Value}),
 	{reply, ok, TableRef}.