在Erlang的gen_server中实现代码交换 [英] Achieving code swapping in Erlang's gen_server
问题描述
我正在寻找在gen_server上使用Erlang的热代码交换功能，这样我就不用重新启动它了。我该怎么办？当我搜索时，我可以找到的是一篇文章，提到我需要使用 gen_server：code_change 回调。
I am looking to make use of Erlang's hot code swapping feature on a gen_server, so that I don't have to restart it. How should I do that? When I searched, all I could find was one article which mentioned that I need to make use of gen_server:code_change callback.
但是，我找不到任何关于如何使用的文档/示例。任何帮助或资源的链接非常感谢！
However, I could not really find any documentation/examples on how to use this. Any help or links to resources greatly appreciated!
推荐答案
正如我已经提到的正常的升级方式是创建正确的.appup和.relup文件，让release_handler做什么需要做的。但是，您可以手动执行所涉及的步骤，如此处所述。对于长的答案，对不起。
As I already mentioned the normal way of upgrading is creating the proper .appup and .relup files, and let release_handler do what needs to be done. However you can manually execute the steps involved, as described here. Sorry for the long answer.
以下虚拟gen_server实现了一个计数器。旧版本（0）简单地将整数存储为状态，而新版本（1）将{tschak，Int}存储为状态。正如我所说，这是一个虚拟的例子。
The following dummy gen_server implements a counter. The old version ("0") simply stores an integer as state, while the new version ("1") stores {tschak, Int} as state. As I said, this is a dummy example.
z.erl（old）：
z.erl (old):
-module(z).
-version("0").
-export([start_link/0, boing/0]).
-behavior(gen_server).
-export([init/1, handle_call/3, handle_cast/2, handle_info/2, terminate/2, code_change/3]).
start_link() -> gen_server:start_link({local, ?MODULE}, ?MODULE, [], [{debug, [trace]}]).
boing() -> gen_server:call(?MODULE, boom).
init([]) -> {ok, 0}.
handle_call(boom, _From, Num) -> {reply, Num, Num+1};
handle_call(_Call, _From, State) -> {noreply, State}.
handle_cast(_Cast, State) -> {noreply, State}.
handle_info(_Info, State) -> {noreply, State}.
terminate(_Reason, _State) -> ok.
code_change(_OldVsn, State, _Extra) -> {ok, State}.
z.erl（new）：
z.erl (new):
-module(z).
-version("1").
-export([start_link/0, boing/0]).
-behavior(gen_server).
-export([init/1, handle_call/3, handle_cast/2, handle_info/2, terminate/2, code_change/3]).
start_link() -> gen_server:start_link({local, ?MODULE}, ?MODULE, [], [{debug, [trace]}]).
boing() -> gen_server:call(?MODULE, boom).
init([]) -> {ok, {tschak, 0}}.
handle_call(boom, _From, {tschak, Num}) -> {reply, Num, {tschak, Num+1}};
handle_call(_Call, _From, State) -> {noreply, State}.
handle_cast(_Cast, State) -> {noreply, State}.
handle_info(_Info, State) -> {noreply, State}.
terminate(_Reason, _State) -> ok.
code_change("0", Num, _Extra) -> {ok, {tschak, Num}}.
启动shell并编译旧代码。注意gen_server以调试跟踪开始。
Start the shell, and compile the old code. Notice the gen_server is started with debug trace.
1> c(z).
{ok,z}
2> z:start_link().
{ok,<0.38.0>}
3> z:boing().
*DBG* z got call boom from <0.31.0>
*DBG* z sent 0 to <0.31.0>, new state 1
0
4> z:boing().
*DBG* z got call boom from <0.31.0>
*DBG* z sent 1 to <0.31.0>, new state 2
1
按预期工作：返回Int，新状态为Int + 1。
Works as expected: returns the Int, and new state is Int+1.
现在将z.erl替换为新的，然后执行以下操作步骤。
Now replace z.erl with the new one, and execute the following steps.
5> compile:file(z).
{ok,z}
6> sys:suspend(z).
ok
7> code:purge(z).
false
8> code:load_file(z).
{module,z}
9> sys:change_code(z,z,"0",[]).
ok
10> sys:resume(z).
ok
你刚刚做了什么：5：编译新的代码。 6：挂起服务器。 7：清除较旧的代码（以防万一）。 8：加载新的代码。 9：通过[]作为额外传递给code_change，从版本0调用模块z的进程'z'中的代码更改。 10：恢复服务器。
What you just did: 5: compiled the new code. 6: suspended the server. 7: purged older code (just in case). 8: loaded the new code. 9: invoked code change in process 'z' for module 'z' from version "0" with [] passed as "Extra" to code_change. 10: resumed the server.
现在，如果您再运行一些测试，可以看到服务器使用新的状态格式：
Now if you run some more tests, you can see, that the server works with the new state format:
11> z:boing().
*DBG* z got call boom from <0.31.0>
*DBG* z sent 2 to <0.31.0>, new state {tschak,3}
2
12> z:boing().
*DBG* z got call boom from <0.31.0>
*DBG* z sent 3 to <0.31.0>, new state {tschak,4}
3
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