# Manage asynchronous tasks

**URL:** <https://discourse.julialang.org/t/manage-asynchronous-tasks/11620>\
**Category:** General Usage\
**Tags:** multithreading\
**Created:** [June 12, 2018, 6:36pm UTC](https://discourse.julialang.org/t/manage-asynchronous-tasks/11620 "2018-06-12T18:36:52Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![han-so1omon](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/han-so1omon/32/4398_2.png) [@han-so1omon](https://discourse.julialang.org/u/han-so1omon)\
**Post date:** [June 12, 2018, 6:36pm UTC](https://discourse.julialang.org/t/manage-asynchronous-tasks/11620/1 "2018-06-12T18:36:52Z")

</div>

Hi all:  
Question about multithreading with asynchronous tasks. There are three tasks running in parallel: main script, asynchronous1 (test server), asynchronous2 (test client). What is the best way to kill the asynchronous tasks upon InterruptException. The program currently looks as such:

> function t1()  
> serv = listen(5000)  
> while true  
> sock = accept(serv)  
> @async while isopen(sock)  
> write(sock,readline(sock))  
> end  
> sleep(0.5)  
> end  
> end

> function t2()  
> cli = connect(5000)  
> while true  
> println(cli,“hi”)  
> sleep(1)  
> end

> tserv = @async t1()  
> tcli = @async t2()  
> try  
> while true  
> sleep(1)  
> end  
> catch ex  
> Base.throwto(tserv,ex)  
> Base.throwto(tcli,ex)  
> throw(ex)  
> end

Edit: Current status  
Upon interrupting and restarting program thru REPL `include("test.jl")`, receive `ERROR (unhandled task failure): listen: address already in use (EADDRINUSE)`

Cheers

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<div class="post-metadata">

**Author:** ![hustf](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/hustf/32/2374_2.png) [@hustf](https://discourse.julialang.org/u/hustf)\
**Post date:** [June 12, 2018, 7:06pm UTC](https://discourse.julialang.org/t/manage-asynchronous-tasks/11620/2 "2018-06-12T19:06:48Z")

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Here’s an attempt to do something of the same:

```julia
"""
For the closing handshake, we won't wait indefinitely for non-responsive clients.
Returns a throwaway frame if the socket happens to be empty
"""
function readframe_nonblocking(ws)
    chnl= Channel{WebSocketFragment}(1)
    # Read, output put to Channel for type stability
    function _readinterruptable(c::Channel{WebSocketFragment})
        try
            put!(chnl, read_frame(ws))
        catch
            # Output a dummy frame that is not a control frame.
            put!(chnl, WebSocketFragment(false, false, false, false,
                                UInt8(0), false, UInt64(0),
                                Vector{UInt8}([0x0,0x0,0x0,0x0]),
                                Vector{UInt8}()))
        end
    end
    # Start reading as a task. Will not return if there is nothing to read
    rt = @schedule _readinterruptable(chnl)
    bind(chnl, rt)
    yield()
    # Define a task for throwing interrupt exception to the (possibly blocked) read task.
    # We don't start this task because it would never return
    killta = @task try;Base.throwto(rt, InterruptException());end
    # We start the killing task. When it is scheduled the second time,
    # we pass an InterruptException through the scheduler.
    try;schedule(killta, InterruptException(), error = false);end
    # We now have content on chnl, and no additional tasks.
    take!(chnl)
end

```

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<div class="post-metadata">

**Author:** ![han-so1omon](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/han-so1omon/32/4398_2.png) [@han-so1omon](https://discourse.julialang.org/u/han-so1omon)\
**Post date:** [June 12, 2018, 7:21pm UTC](https://discourse.julialang.org/t/manage-asynchronous-tasks/11620/3 "2018-06-12T19:21:11Z")

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can you explain this?

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<div class="post-metadata">

**Author:** ![hustf](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/hustf/32/2374_2.png) [@hustf](https://discourse.julialang.org/u/hustf)\
**Post date:** [June 12, 2018, 8:34pm UTC](https://discourse.julialang.org/t/manage-asynchronous-tasks/11620/4 "2018-06-12T20:34:26Z")

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Han-so1omon, sorry for a sloppy and unhelpful reply above. From your example, it seems you are experimenting with adapting examples from the docs. I didn’t take the time to adapt the example to your context, which I should have done.

t1() is hard to read because you drop the tabs. It never returns, but creates a new task every 0.5 seconds. I can’t see any reason for doing that. Is this what you want to do?

```julia
function t1()
    serv = listen(5000)
    sock = accept(serv)
    while isopen(sock)
        write(sock, readline(sock))
    end
    close(serv)
end

```

The problem I was replying to was a different one. Your time is probably much better used reading about [tasks](https://docs.julialang.org/en/stable/manual/control-flow/#man-tasks-1).

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<div class="post-metadata">

**Author:** ![han-so1omon](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/han-so1omon/32/4398_2.png) [@han-so1omon](https://discourse.julialang.org/u/han-so1omon)\
**Post date:** [June 13, 2018, 3:45am UTC](https://discourse.julialang.org/t/manage-asynchronous-tasks/11620/5 "2018-06-13T03:45:38Z")

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thank you for your answers. it went over my head, as I’m not sure the difference between `@schedule`, `@task`, and `@async` julialang macros.

yes, mostly experimenting, but open to all ideas.

My solution is similar to your second post, but for clientside. Serverside is written in C.

I occasionally end up with `ERROR (unhandled task failure): connect: connection refused (ECONNREFUSED)`. From this it is unclear if `ECONNREFUSED` causes `(unhandled task failure)` or the other way around. Any insight into this?

Edit: Maybe this is related to server not immediately responding to `connect()`