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Modules shared by rabbitmq-server and rabbitmq-erlang-client
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Files
src/credit_flow.erl
%% The contents of this file are subject to the Mozilla Public License
%% Version 1.1 (the "License"); you may not use this file except in
%% compliance with the License. You may obtain a copy of the License
%% at http://www.mozilla.org/MPL/
%%
%% Software distributed under the License is distributed on an "AS IS"
%% basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See
%% the License for the specific language governing rights and
%% limitations under the License.
%%
%% The Original Code is RabbitMQ.
%%
%% The Initial Developer of the Original Code is GoPivotal, Inc.
%% Copyright (c) 2007-2017 Pivotal Software, Inc. All rights reserved.
%%
-module(credit_flow).
%% Credit flow is controlled by a credit specification - a
%% {InitialCredit, MoreCreditAfter} tuple. For the message sender,
%% credit starts at InitialCredit and is decremented with every
%% message sent. The message receiver grants more credit to the sender
%% by sending it a {bump_credit, ...} control message after receiving
%% MoreCreditAfter messages. The sender should pass this message in to
%% handle_bump_msg/1. The sender should block when it goes below 0
%% (check by invoking blocked/0). If a process is both a sender and a
%% receiver it will not grant any more credit to its senders when it
%% is itself blocked - thus the only processes that need to check
%% blocked/0 are ones that read from network sockets.
%%
%% Credit flows left to right when process send messags down the
%% chain, starting at the rabbit_reader, ending at the msg_store:
%% reader -> channel -> queue_process -> msg_store.
%%
%% If the message store has a back log, then it will block the
%% queue_process, which will block the channel, and finally the reader
%% will be blocked, throttling down publishers.
%%
%% Once a process is unblocked, it will grant credits up the chain,
%% possibly unblocking other processes:
%% reader <--grant channel <--grant queue_process <--grant msg_store.
%%
%% Grepping the project files for `credit_flow` will reveal the places
%% where this module is currently used, with extra comments on what's
%% going on at each instance. Note that credit flow between mirrors
%% synchronization has not been documented, since this doesn't affect
%% client publishes.
-define(DEFAULT_INITIAL_CREDIT, 200).
-define(DEFAULT_MORE_CREDIT_AFTER, 100).
-define(DEFAULT_CREDIT,
case get(credit_flow_default_credit) of
undefined ->
Val = rabbit_misc:get_env(rabbit, credit_flow_default_credit,
{?DEFAULT_INITIAL_CREDIT,
?DEFAULT_MORE_CREDIT_AFTER}),
put(credit_flow_default_credit, Val),
Val;
Val -> Val
end).
-export([send/1, send/2, ack/1, ack/2, handle_bump_msg/1, blocked/0, state/0]).
-export([peer_down/1]).
%%----------------------------------------------------------------------------
-export_type([bump_msg/0]).
-opaque(bump_msg() :: {pid(), non_neg_integer()}).
-type(credit_spec() :: {non_neg_integer(), non_neg_integer()}).
-spec send
(pid()) -> 'ok';
(credit_spec()) -> 'ok'.
-spec ack(pid()) -> 'ok'.
-spec ack(pid(), credit_spec()) -> 'ok'.
-spec handle_bump_msg(bump_msg()) -> 'ok'.
-spec blocked() -> boolean().
-spec peer_down(pid()) -> 'ok'.
%%----------------------------------------------------------------------------
%% process dict update macro - eliminates the performance-hurting
%% closure creation a HOF would introduce
-define(UPDATE(Key, Default, Var, Expr),
begin
%% We deliberately allow Var to escape from the case here
%% to be used in Expr. Any temporary var we introduced
%% would also escape, and might conflict.
Var = case get(Key) of
undefined -> Default;
V -> V
end,
put(Key, Expr)
end).
%% If current process was blocked by credit flow in the last
%% STATE_CHANGE_INTERVAL milliseconds, state/0 will report it as "in
%% flow".
-define(STATE_CHANGE_INTERVAL, 1000000).
-ifdef(CREDIT_FLOW_TRACING).
-define(TRACE_BLOCKED(SELF, FROM), rabbit_event:notify(credit_flow_blocked,
[{process, SELF},
{process_info, erlang:process_info(SELF)},
{from, FROM},
{from_info, erlang:process_info(FROM)},
{timestamp,
os:system_time(
milliseconds)}])).
-define(TRACE_UNBLOCKED(SELF, FROM), rabbit_event:notify(credit_flow_unblocked,
[{process, SELF},
{from, FROM},
{timestamp,
os:system_time(
milliseconds)}])).
-else.
-define(TRACE_BLOCKED(SELF, FROM), ok).
-define(TRACE_UNBLOCKED(SELF, FROM), ok).
-endif.
%%----------------------------------------------------------------------------
%% There are two "flows" here; of messages and of credit, going in
%% opposite directions. The variable names "From" and "To" refer to
%% the flow of credit, but the function names refer to the flow of
%% messages. This is the clearest I can make it (since the function
%% names form the API and want to make sense externally, while the
%% variable names are used in credit bookkeeping and want to make
%% sense internally).
%% For any given pair of processes, ack/2 and send/2 must always be
%% called with the same credit_spec().
send(From) -> send(From, ?DEFAULT_CREDIT).
send(From, {InitialCredit, _MoreCreditAfter}) ->
?UPDATE({credit_from, From}, InitialCredit, C,
if C == 1 -> block(From),
0;
true -> C - 1
end).
ack(To) -> ack(To, ?DEFAULT_CREDIT).
ack(To, {_InitialCredit, MoreCreditAfter}) ->
?UPDATE({credit_to, To}, MoreCreditAfter, C,
if C == 1 -> grant(To, MoreCreditAfter),
MoreCreditAfter;
true -> C - 1
end).
handle_bump_msg({From, MoreCredit}) ->
?UPDATE({credit_from, From}, 0, C,
if C =< 0 andalso C + MoreCredit > 0 -> unblock(From),
C + MoreCredit;
true -> C + MoreCredit
end).
blocked() -> case get(credit_blocked) of
undefined -> false;
[] -> false;
_ -> true
end.
state() -> case blocked() of
true -> flow;
false -> case get(credit_blocked_at) of
undefined -> running;
B -> Now = erlang:monotonic_time(),
Diff = erlang:convert_time_unit(Now - B,
native,
micro_seconds),
case Diff < ?STATE_CHANGE_INTERVAL of
true -> flow;
false -> running
end
end
end.
peer_down(Peer) ->
%% In theory we could also remove it from credit_deferred here, but it
%% doesn't really matter; at some point later we will drain
%% credit_deferred and thus send messages into the void...
unblock(Peer),
erase({credit_from, Peer}),
erase({credit_to, Peer}),
ok.
%% --------------------------------------------------------------------------
grant(To, Quantity) ->
Msg = {bump_credit, {self(), Quantity}},
case blocked() of
false -> To ! Msg;
true -> ?UPDATE(credit_deferred, [], Deferred, [{To, Msg} | Deferred])
end.
block(From) ->
?TRACE_BLOCKED(self(), From),
case blocked() of
false -> put(credit_blocked_at, erlang:monotonic_time());
true -> ok
end,
?UPDATE(credit_blocked, [], Blocks, [From | Blocks]).
unblock(From) ->
?TRACE_UNBLOCKED(self(), From),
?UPDATE(credit_blocked, [], Blocks, Blocks -- [From]),
case blocked() of
false -> case erase(credit_deferred) of
undefined -> ok;
Credits -> _ = [To ! Msg || {To, Msg} <- Credits],
ok
end;
true -> ok
end.