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  1. 191
      LICENSE
  2. 14
      README.md
  3. 13
      rebar.config
  4. 1
      rebar.lock
  5. 109
      src/paxos_agent.erl
  6. 261
      src/raft_agent.erl
  7. 14
      src/raft_erl.app.src
  8. 43
      src/raft_erl.erl
  9. 64
      src/raft_log.erl
  10. 41
      src/storage.erl

191
LICENSE
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Apache License
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http://www.apache.org/licenses/
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Copyright 2019, Shanti Chellaram <shanti@shanti.im>.
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README.md
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raft_erl
=====
An escript
Build
-----
$ rebar3 escriptize
Run
---
$ _build/default/bin/raft_erl

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rebar.config
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{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]}
]}]}.

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rebar.lock
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[].

109
src/paxos_agent.erl
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-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

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src/raft_agent.erl
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-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).

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src/raft_erl.app.src
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{application, raft_erl,
[{description, "An escript"},
{vsn, "0.1.0"},
{registered, []},
{applications,
[kernel,
stdlib
]},
{env,[]},
{modules, []},
{licenses, ["Apache 2.0"]},
{links, []}
]}.

43
src/raft_erl.erl
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-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
%%====================================================================

64
src/raft_log.erl
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@ -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.

41
src/storage.erl
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@ -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}.
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