Hot restart of the main loop when the source code changes

General Usage
1

Hi, everybody!
I’m currently working on the Julia backend for Plotly Dash and I’m faced with the following task. In Python and R, Dash has the following feature: if the server is running in debug mode, changes to the app’s source code will restart the server. This way, you don’t need to press ctrl-C in the terminal after each change and restart the application.
I’m trying to implement this for Julia and the first my experiments gave this draft solution (spied on Python’s flask):

function test_hot_restart(main_loop::Function)
    if haskey(ENV, "SUBPROCESS")
        app_path = abspath(Base.PROGRAM_FILE)
        files = FileStat[]
        parse_includes(app_path, files) #parse files recursively with  Base.parse_input_line to find include Exprs
        
        @async main_loop()

        poll_until_changed(files) #Waiting for changes in any of the found files
        exit(3)
    else
        julia_str = joinpath(Sys.BINDIR, Base.julia_exename())
        new_env = deepcopy(ENV)
        new_env["SUBPROCESS"] = "1"
        cmd = Cmd(`$(julia_str) $(app_path)`, env = new_env, ignorestatus = true)
        while true
            p = run(cmd)
            p.exitcode != 3 && break
        end
    end
end

This solution seems to work, but perhaps someone can offer a more robust one?

P.S.
For a better understanding of the problem, the source file that I need to track for changes looks in the simplest case something like this:

using Dash, DashHtmlComponents
app = dash()
app.layout = html_div("test")
run_server(app; debug = true)

the main loop is started inside run_server (by HTTP.serve call). So I don’t see a way to use Revise.jl here. In addition, there is a requirement that the hot restart mode is enabled in runtime by the debug argument, and not by a macro or imported an extra package in user side.

1 Like
2

Hey @waralex, were you able to find a more robust solution here? I’m running up against a similar problem and was hoping to find a nice Revise based solution

3

Hi @caleb-allen!
Unfortunately, I didn’t find it :frowning: In Dash, it now works like this - it looks at all the included files and restarts the server when any of it change.

1 Like
4

In “FemotCleaner” (a bot that updates Julia code from 0.7 to 1.0) there was a Revise.revise() call in the main loop:

From what I recall, it worked pretty well, assuming the changes were Revise compatible.

2 Likes
5

@kristoffer.carlsson this is great! I’ve tried using Revise.revise() from within code, but I hadn’t thought to use Base.invokelatest. I’ll give this a shot!

6

@kristoffer.carlsson that did the trick! invokelatest was the key. Thanks for the suggestion!

1 Like