确保在 Erlang/OTP 中切换 gen_event 处理程序时处理事件

发布于 2024-09-24 03:17:29 字数 1126 浏览 5 评论 0原文

假设我有多个版本的 gen_event 处理程序，并且想要在程序运行时更改它们：

-module(logger_all).
-behaviour(gen_event).
-export([init/1, handle_event/2, terminate/2]).
init(_Args) ->
    {ok, []}.
handle_event({Severity, ErrorMsg}, State) ->
    io:format("***~p*** ~p~n", [Severity, ErrorMsg]),
    {ok, State}.
terminate(_Args, _State) ->
    ok.

-module(logger_errors_only).
-behaviour(gen_event).
-export([init/1, handle_event/2, terminate/2]).
init(_Args) ->
    {ok, []}.
handle_event({error, ErrorMsg}, State) ->
    io:format("***Error*** ~p~n", [ErrorMsg]),
    {ok, State}.
handle_event({_, ErrorMsg}, State) ->
    {ok, State}. %% ignore everything except errors
terminate(_Args, _State) ->
    ok.

显然，我可以通过删除一个处理程序并添加另一个处理程序来切换它们：

log_errors_only() -> 
    gen_event:delete_handler(error_man, logger_all, []),
    gen_event:add_handler(error_man, logger_errors_only, []).

但这会留下一个出现竞争条件的可能性；如果 error_man 在错误的时间收到事件，则不会记录该事件。或者如果我改变操作的顺序，它将被记录两次，这也是不可取的。我如何确保它被精确处理一次？

对于这种情况，我可以只使用一个处理程序并将日志记录级别保持为 State，但假设这是不可接受的。

原文

Let's say I have several versions of a gen_event handler and want to change them around while the program is running:

-module(logger_all).
-behaviour(gen_event).
-export([init/1, handle_event/2, terminate/2]).
init(_Args) ->
    {ok, []}.
handle_event({Severity, ErrorMsg}, State) ->
    io:format("***~p*** ~p~n", [Severity, ErrorMsg]),
    {ok, State}.
terminate(_Args, _State) ->
    ok.

-module(logger_errors_only).
-behaviour(gen_event).
-export([init/1, handle_event/2, terminate/2]).
init(_Args) ->
    {ok, []}.
handle_event({error, ErrorMsg}, State) ->
    io:format("***Error*** ~p~n", [ErrorMsg]),
    {ok, State}.
handle_event({_, ErrorMsg}, State) ->
    {ok, State}. %% ignore everything except errors
terminate(_Args, _State) ->
    ok.

Obviously, I can switch them around by removing one handler and adding the other one:

log_errors_only() -> 
    gen_event:delete_handler(error_man, logger_all, []),
    gen_event:add_handler(error_man, logger_errors_only, []).

But this leaves a possibility of a race condition; if error_man receives an event just at the wrong time, it won't be logged. Or if I change the order of actions, it will be logged twice, which is also undesirable. How can I make sure it's handled precisely once?

For this case I can just have one handler and keep logging level as the State, but suppose this isn't acceptable.

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她说她爱他 2024-10-01 03:17:29

OTP团队很擅长预见这样的需求； gen_event 有一个原子地交换事件处理程序的函数：

log_errors_only() ->
    gen_event:swap_handler(error_man,
                           {logger_all, swapped_out},
                           {logger_errors_only, []}).

The OTP team is good at forseeing needs like this; gen_event has a function to swap event handlers atomically:

log_errors_only() ->
    gen_event:swap_handler(error_man,
                           {logger_all, swapped_out},
                           {logger_errors_only, []}).

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