如何在 erlang gen_server 中有效地使用接收子句来解决超时错误？
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【中文标题】如何在 erlang gen_server 中有效地使用接收子句来解决超时错误？【英文标题】：How to use efficiently receive clause in erlang gen_server to resolve timeout error? 【发布时间】：2021-04-14 03:18:54 【问题描述】：有时我的循环会因为超时而返回正常，如何以正确的方式编写此代码。当超时时，它只会返回 ok 但不是我假设的实际值。在句柄调用中，我在 loop() 函数中调用函数 loop()，我正在接收带有接收子句的消息。现在，我使用 loop2 函数将这些数据发送到我的数据库，无论数据是否已成功保存，都会从数据库返回响应，并将响应返回给 loop()。但如果超时，我的循环函数会返回 ok 但不是实际值。
% @Author: ZEESHAN AHMAD
% @Date: 2020-12-22 05:06:12
% @Last Modified by: ZEESHAN AHMAD
% @Last Modified time: 2021-01-10 04:42:59
-module(getAccDataCons).
-behaviour(gen_server).
-include_lib("deps/amqp_client/include/amqp_client.hrl").
-export([start_link/0, stop/0]).
-export([init/1, handle_call/3, handle_cast/2, handle_info/2, code_change/3,
terminate/2]).
-export([get_account/0]).
start_link() ->
gen_server:start_link(local, ?MODULE, ?MODULE, [], []).
stop() ->
gen_server:cast(?MODULE, stop).
get_account() ->
gen_server:call(?MODULE, get_account).
init(_Args) ->
ok, Connection = amqp_connection:start(#amqp_params_networkhost = "localhost"),
ok, Channel = amqp_connection:open_channel(Connection),
ok, Channel.
handle_call(get_account, _From, State) ->
amqp_channel:call(State, #'exchange.declare'exchange = <<"get">>, type = <<"topic">>),
amqp_channel:call(State, #'queue.declare'queue = <<"get_account">>),
Binding =
#'queue.bind'exchange = <<"get">>,
routing_key = <<"get.account">>,
queue = <<"get_account">>,
#'queue.bind_ok' = amqp_channel:call(State, Binding),
io:format(" [*] Waiting for logs. To exit press CTRL+C~n"),
amqp_channel:call(State,#'basic.consume'queue = <<"get_account">>, no_ack = true),
Returned =loop(),
io:format("~nReti=~p",[Returned]),
reply, Returned, State;
handle_call(Message, _From, State) ->
io:format("received other handle_call message: ~p~n", [Message]),
reply, ok, State.
handle_cast(stop, State) ->
stop, normal, State;
handle_cast(Message, State) ->
io:format("received other handle_cast call : ~p~n", [Message]),
noreply, State.
handle_info(Message, State) ->
io:format("received handle_info message : ~p~n", [Message]),
noreply, State.
code_change(_OldVer, State, _Extra) ->
ok, State.
terminate(Reason, _State) ->
io:format("server is terminating with reason :~p~n", [Reason]).
loop()->
receive
#'basic.consume_ok' -> ok
end,
receive
#'basic.deliver', Msg ->
#amqp_msgpayload = Payload = Msg,
Value=loop2(Payload),
Value
after 2000->
io:format("Server timeout")
end.
loop2(Payload)->
Result = jiffy:decode(Payload),
[<<"account_id">>, AccountId] = Result,
Doc = [<<"account_id">>, AccountId],
getAccDataDb:create_AccountId_view(),
Returned=case getAccDataDb:getAccountNameDetails(Doc) of
success ->
Respo = getAccDataDb:getAccountNameDetails1(Doc),
Respo;
details_not_matched ->
user_not_exist
end,
Returned.
【问题讨论】：
【参考方案1】：没有loop 和loop2 代码，很难给出答案，如果这两个函数之一检测到超时，您必须首先更改它们的行为以避免任何超时，或者将其增加到一个有效的值。如果需要超时，则确保返回值是显式的，例如 error,RequestRef,timeout 而不是 ok。
尽管如此gen_server 不应该等待太久才能得到答案，您可以修改您的代码：
在客户端进程中不要使用gen_server:call(ServerRef,Request)，您可以使用：
RequestId = send_request(ServerRef, Request),
Result = wait_response(RequestId, Timeout),
并删除loop 和/或loop2 中的超时。这样做可以控制客户端的超时，甚至可以将其设置为无穷大（这不是一个好主意！）。
或者你可以将你的功能分成两部分
gen_server:cast(ServerRef,Request,RequestRef),
% this will not wait for any answer, RequestRef is a tag to identify later
% if the request was fulfilled, you can use make_ref() to generate it
以后，或者在另一个客户端进程中（这需要至少将RequestRef传递给这个进程）检查请求的结果：
Answer = gen_server:call(ServerRef,get_answer,RequestRef),
case Answer of
no_reply -> ... % no answer yet
ok,Reply -> ... % handle the answer
end,
最后，您必须修改循环代码以处理RequestRef，将消息（再次使用gen_server:cast）连同结果和RequestRef 发送回服务器，并将此结果存储在服务器状态中。
我不认为第二种解决方案有价值，因为它与第一种解决方案或多或少相同，但都是手工制作的，它让您可以管理许多可能最终导致一种内存泄漏。
【讨论】：
我已经用 loop() 和 loop2() 更新了我的代码 @Coderops 看到我的新回复【参考方案2】：这对编辑来说太长了，我把它放在一个新的答案中。
发生超时时收到ok 的原因在于loop() 代码。在第二个接收块中，2000 毫秒后，您返回
紧接在io:format/1 语句之后。
io:format 返回 ok，这就是您在 Returned 变量中得到的内容。您应该使用
loop()->
ok = receive
#'basic.consume_ok' -> ok
end,
receive
#'basic.deliver', #amqp_msgpayload = Payload -> ok,loop2(Payload)
after 2000 ->
io:format("Server timeout"),
error,timeout
end.
使用此代码，您的客户将收到ok,Value 或error,timeout 并能够做出相应的反应。
但是这个版本还是有问题的：
- 2 秒超时可能太短，您缺少有效答案
- 当您在接收块中使用模式匹配并且不检查每个amqp_channel:call 的结果时，可能会出现许多不同的问题并显示为超时
首先让我们看看超时。对amqp_channel 的 4 次调用可能总共需要超过 2 秒才能成功完成。简单的解决方案是增加超时时间，将after 2000 更改为after 3000 或更多。
但是你会遇到两个问题：
gen_server 提供了一些接口函数来解决这类问题：'send_request'、'wait_response'和reply。这是一个基本的 gen_server 可以处理 3 种请求：
stop ... 停止服务器，对更新代码很有用。 blocking,Time,Value 服务器将在 Time ms 结束期间休眠，然后返回 Value。这模拟了你的情况，你可以调整如何 需要很长时间才能得到答案。 non_blocking,Time,Value 服务器会将作业委托给另一个进程并立即返回而无应答（因此 它可用于另一个请求）。新进程将在 Time ms 结束期间休眠，然后使用 gen_server:reply 返回值。服务器模块实现了几个用户界面：
标准的 start()、stop() blocking(Time,Value) 使用 gen_server:call 通过请求 blocking,Time,Value 调用服务器 blocking_catch(Time,Value) 与上一个相同，但捕获 gen_server:call 的结果以显示隐藏的超时 non_blocking(Time,Value,Wait) 使用 gen_server:send_request 使用请求 non_blocking,Time,Value 调用服务器并等待等待 ms 最大值的答案最后它包括2个测试功能
test([Type,Time,Value,OptionalWait]) 它产生一个进程，该进程将发送一个带有相应参数的类型的请求。答案被发送回调用进程。可以在 shell 中使用 flush() 来检索答案。 parallel_test ([Type,Time,NbRequests,OptionalWait]) 它使用相应的参数调用 NbRequests 次测试。它收集了所有 答案并使用本地函数 collect(NbRequests,Timeout) 打印出来。代码如下
-module (server_test).
-behaviour(gen_server).
%% API
-export([start/0,stop/0,blocking/2,blocking_catch/2,non_blocking/3,test/1,parallel_test/1]).
%% gen_server callbacks
-export([init/1, handle_call/3, handle_cast/2, handle_info/2,
terminate/2, code_change/3]).
-define(SERVER, ?MODULE).
%%%===================================================================
%%% API
%%%===================================================================
start() ->
gen_server:start_link(local, ?SERVER, ?MODULE, [], []).
stop() ->
gen_server:cast(?SERVER, stop).
blocking(Time,Value) ->
gen_server:call(?SERVER, blocking,Time,Value).
blocking_catch(Time,Value) ->
catch ok,gen_server:call(?SERVER, blocking,Time,Value).
non_blocking(Time,Value,Wait) ->
ReqId = gen_server:send_request(?SERVER,non_blocking,Time,Value),
gen_server:wait_response(ReqId,Wait).
test([Type,Time,Value]) -> test([Type,Time,Value,5000]);
test([Type,Time,Value,Wait]) ->
Start = erlang:monotonic_time(),
From = self(),
F = fun() ->
R = case Type of
non_blocking -> ?MODULE:Type(Time,Value,Wait);
_ -> ?MODULE:Type(Time,Value)
end,
From ! request,Type,Time,Value,got_answer,R,after_microsec,erlang:monotonic_time() - Start
end,
spawn(F).
parallel_test([Type,Time,NbRequests]) -> parallel_test([Type,Time,NbRequests,5000]);
parallel_test([Type,Time,NbRequests,Wait]) ->
case Type of
non_blocking -> [server_test:test([Type,Time,X,Wait]) || X <- lists:seq(1,NbRequests)];
_ -> [server_test:test([Type,Time,X]) || X <- lists:seq(1,NbRequests)]
end,
collect_answers(NbRequests,Time + 1000).
%%%===================================================================
%%% gen_server callbacks
%%%===================================================================
init([]) ->
ok, #.
handle_call(blocking,Time,Value, _From, State) ->
timer:sleep(Time),
Reply = ok,Value,
reply, Reply, State;
handle_call(non_blocking,Time,Value, From, State) ->
F = fun() ->
do_answer(From,Time,Value)
end,
spawn(F),
noreply, State;
handle_call(_Request, _From, State) ->
Reply = ok,
reply, Reply, State.
handle_cast(stop, State) ->
stop,stopped, State;
handle_cast(_Msg, State) ->
noreply, State.
handle_info(_Info, State) ->
noreply, State.
terminate(_Reason, _State) ->
ok.
code_change(OldVsn, State, _Extra) ->
io:format("changing code replacing version ~p~n",[OldVsn]),
ok, State.
%%%===================================================================
%%% Internal functions
%%%===================================================================
do_answer(From,Time,Value) ->
timer:sleep(Time),
gen_server:reply(From, Value).
collect_answers(0,_Timeout) ->
got_all_answers;
collect_answers(NbRequests,Timeout) ->
receive
A -> io:format("~p~n",[A]),
collect_answers(NbRequests - 1, Timeout)
after Timeout ->
missing_answers
end.
shell 中的会话：
44> c(server_test).
ok,server_test
45> server_test:start().
ok,<0.338.0>
46> server_test:parallel_test([blocking,200,3]).
request,blocking,200,1,got_answer,ok,1,after_microsec,207872
request,blocking,200,2,got_answer,ok,2,after_microsec,415743
request,blocking,200,3,got_answer,ok,3,after_microsec,623615
got_all_answers
47> % 3 blocking requests in parallel, each lasting 200ms, they are executed in sequence but no timemout is reached
47> % All the clients get their answers
47> server_test:parallel_test([blocking,2000,3]).
request,blocking,2000,1,got_answer,ok,1,after_microsec,2063358
request,blocking,2000,2,got_answer,ok,2,after_microsec,4127740
missing_answers
48> % 3 blocking requests in parallel, each lasting 2000ms, they are executed in sequence and the last answer exceeds the gen_server timeout.
48> % The client for this request don't receive answer. The client should also manage its own timeout to handle this case
48> server_test:parallel_test([blocking_catch,2000,3]).
request,blocking_catch,2000,1,got_answer,ok,1,after_microsec,2063358
request,blocking_catch,2000,2,got_answer,ok,2,after_microsec,4127740
request,blocking_catch,2000,3,got_answer,
'EXIT',timeout,gen_server,call,[server_test,blocking,2000,3],
after_microsec,5135355
got_all_answers
49> % same thing but catching the exception. After 5 seconds the gen_server call throws a timeout exception.
49> % The information can be forwarded to the client
49> server_test:parallel_test([non_blocking,200,3]).
request,non_blocking,200,1,got_answer,reply,1,after_microsec,207872
request,non_blocking,200,2,got_answer,reply,2,after_microsec,207872
request,non_blocking,200,3,got_answer,reply,3,after_microsec,207872
got_all_answers
50> % using non blocking mechanism, we can see that all the requests were managed in parallel
50> server_test:parallel_test([non_blocking,5100,3]).
request,non_blocking,5100,1,got_answer,timeout,after_microsec,5136379
request,non_blocking,5100,2,got_answer,timeout,after_microsec,5136379
request,non_blocking,5100,3,got_answer,timeout,after_microsec,5136379
got_all_answers
51> % if we increase the answer delay above 5000ms, all requests fail in default timeout
51> server_test:parallel_test([non_blocking,5100,3,6000]).
request,non_blocking,5100,1,got_answer,reply,1,after_microsec,5231611
request,non_blocking,5100,2,got_answer,reply,2,after_microsec,5231611
request,non_blocking,5100,3,got_answer,reply,3,after_microsec,5231611
got_all_answers
52> % but thanks to the send_request/wait_response/reply interfaces, the client can adjust the timeout to an accurate value
52> % for each request
请求无法完成的下一个原因是 amqp_channel:call 之一失败。取决于你想做什么，有几个 什么都不做，让崩溃，捕获异常或管理所有情况的可能性。下一个提案使用全局捕获
handle_call(get_account,Timeout, From, State) ->
F = fun() ->
do_get_account(From,State,Timeout)
end,
spawn(F), % delegate the job to another process and free the server
noreply, State; % I don't see any change of State in your code, this should be enough
...
do_get_account(From,State,Timeout) ->
% this block of code asserts all positive return values from amqp_channel calls. it will catch any error
% and return it as error,.... If everything goes well it return ok,Answer
Reply = try
ok = amqp_channel:call(State, #'exchange.declare'exchange = <<"get">>, type = <<"topic">>),
ok = amqp_channel:call(State, #'queue.declare'queue = <<"get_account">>),
Binding = #'queue.bind'exchange = <<"get">>,
routing_key = <<"get.account">>,
queue = <<"get_account">>,
#'queue.bind_ok' = amqp_channel:call(State, Binding),
ok = amqp_channel:call(State,#'basic.consume'queue = <<"get_account">>, no_ack = true),
ok,wait_account_reply(Timeout)
catch
Class:Exception -> error,Class,Exception
end,
gen_server:reply(From, Reply).
wait_account_reply(Timeout) ->
receive
% #'basic.consume_ok' -> ok % you do not handle this message, ignore it since it will be garbaged when the process die
#'basic.deliver', #amqp_msgpayload = Payload -> extract_account(Payload)
after Timeout->
server_timeout
end.
extract_account(Payload)->
[<<"account_id">>, AccountId] = jiffy:decode(Payload),
Doc = [<<"account_id">>, AccountId],
getAccDataDb:create_AccountId_view(), % What is the effect of this function, what is the return value?
case getAccDataDb:getAccountNameDetails(Doc) of
success ->
getAccDataDb:getAccountNameDetails1(Doc);
details_not_matched ->
user_not_exist
end.
客户端应该是这样的：
get_account() ->
ReqId = gen_server:send_request(server_name,get_account,2000),
gen_server:wait_response(ReqId,2200).
【讨论】：
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