How can I do hot code reload when developing with Cowboy?

Question

I'm developing with Cowboy and erlang.mk and currently my flow is : 1. change code in vi, save, run make 2. close Cowboy, start Cowboy again (i'm running Cowboy in console mode for tracing/debugging purposes)

Is there a way to make Cowboy reload and restart itself automatically with as little lag as possible? I understand I could add Cowboy stop and start to my makefile but maybe there is a better/more responsive way?

I saw there is sync package from rustyio but it seems overly complicated to have to hook it into my app directly.

Answer 1

Take a look at p6hot_deploy.erl

It's what my company uses for simplistic auto-redeploy. it scans the filesystem once a second looking for modules where the date has changed and reloads them.

so your process will be: edit, save, make

This module will see the file change and force a reload

it's not the fancy OTP way with releases and stuff, but a more low level simple code swap.

note: you'll have to change this to work for you as it depends on my logging macros and stuff, but you should get the gist by looking at it.

The key part is this function:

reload(Module) ->
    case code:purge(Module) of
        true -> ?lwarn("Process killed as result of reloading: ~p",[Module]);
        false -> ok
    end,
    code:load_file(Module).

it attempts to cycle in the new code in a safe way.

Answer 2

You can combine relx and rusty/sync for code reload while still running Erlang release. It works like charm and there are tutorials on how to do such setup.

Answer 3

I use rebar all the time. In rebar.config, I have

 {deps,[
        {sync,         ".*", {git, "git://github.com/rustyio/sync", {tag,"master"}}},
 }

I use a .erlang file, typically placed in src/.erlang with the following two lines

 code:add_path("../deps/sync/ebin").
 sync:go().

Now, whenever I save a file, it is reloaded. I see no reason why this shouldn't work for Cowboy

Answer 4

Rustyio is indeed a bit complicated for casual (sideproject) use. Most methods focus on rebar3.

For erlang.mk users, the reload.mk plugin works well. Just add it in the Makefile deps, run programs with make run RELOADABLE=1 and reload with make reload.