Hot code replacement in erlang

Question

I am working on my first real project in erlang, however, this code is simplified for brevity. I want to be able to load a newer version of a file into my project remotely while it's running. I've read about using a behavior like gen_server or gen_fsm which has this for free. While that might achieve the result, I want to use this to learn how to do it, not just get it done. I've read the docs about code replacement, and LYSE's bit about Hot Code Loving, among other things, but I haven't been able to find anything that works for what I'm doing, so here is the basic idea.

-module(reloading).

-export([loop/0]).

loop() ->
    receive
        upgrade ->
            ?MODULE:loop();
        hello ->
            io:format("This is a test~n"),
            loop();
        _ ->
            loop()
    end.

I am simply looping with the idea that I can send the message upgrade and it will load a newer version of the code.

$ erl
Erlang R15B01 (erts-5.9.1) [source] [64-bit] [async-threads:0] [hipe] [kernel-poll:false]

Eshell V5.9.1  (abort with ^G)
1> c(reloading).
{ok,reloading}
2> Loop = spawn(reloading, loop, []).
<0.39.0>
3> Loop ! hello.
This is a test
hello

At this point I change 10 line to io:format("I have changed this!~n"),

4> Loop ! upgrade.
upgrade
5> Loop ! hello.  
This is a test
hello

I am expecting this hello call to print I have changed this! not This is a test. I know I can simply call c(reloading). and have this work the way expected, but I'm looking to send the actual project a message rather than manually updating the code. So where is my disconnect? What am I doing wrong, that I should be doing in order to hot load this code? As mentioned before, I am looking for a non-OTP solution for the sake of education.

Answer 1

For the sake of having an explicit answer, I am posting this.

Using @rvirding's suggestion of using the code module, I've modified it to look like this:

-module(reloading).

-export([loop/0]).

loop() ->
    receive
        upgrade ->
            code:purge(?MODULE),
            compile:file(?MODULE),
            code:load_file(?MODULE),
            ?MODULE:loop();
        hello ->
            io:format("This is a test~n"),
            loop();
        _ ->
            loop()
    end.

First code:purge the old ?MODULE, then compile:file the new file, and finally, code:load_file the new ?MODULE. This works as I originally intended.

$ erl
Erlang R15B01 (erts-5.9.1) [source] [64-bit] [async-threads:0] [hipe] [kernel-poll:false]

Eshell V5.9.1  (abort with ^G)
1> Loop = spawn(reloading, loop, []).
<0.34.0>
2> Loop ! hello.
This is a test
hello

Change line to io:format("I have changed this!~n"),

3> Loop ! upgrade.                   
upgrade
4> Loop ! hello.  
I have changed this!
hello

Answer 2

While erlang can handle two versions of a module and calling a function with mod:func(...) will always call the latest version of a module (if the function is exported) you still have to load the new version of the module into Erlang system. You can't expect it automagically detect that you happen to have a new version of the module somewhere, find it, compile it and load it.

N.B. compiling and loading are two separate things. So c(mod). both compiles and loads the module, while l(mod). just loads the object code (.beam file) of the already compiled module. The Erlang compiler is called from the module compile and it just compiles and generates a .beam file while the code loading is handled by the module code.