# Communication between two threads in Julia without blocking
**URL:** https://discourse.julialang.org/t/communication-between-two-threads-in-julia-without-blocking/77637
**Category:** General Usage
**Tags:** question
**Created:** [March 9, 2022, 3:07pm UTC](https://discourse.julialang.org/t/communication-between-two-threads-in-julia-without-blocking/77637 "2022-03-09T15:07:32Z")
**Posts on this page:** 5
**Page:** 1
### Author:  [@Tine](https://discourse.julialang.org/u/Tine)
#### Post date: [March 9, 2022, 3:07pm UTC](https://discourse.julialang.org/t/communication-between-two-threads-in-julia-without-blocking/77637/1 "2022-03-09T15:07:33Z")
Hi!
I am building a system that consist of multiple threads doing different work. For the purpose of this post, I am simplifying the code.
My system:
```julia
using ThreadPools
function work()
(timer) -> begin
stop = isready(running) && fetch(running) isa Stop
if stop
close(timer)
else
println("Hello!")
schedule()
end
end
end
function schedule()
Timer(work(), 2)
end
function manager()
# Schedule first work()
schedule()
# Run till a signal to stop is received
stop = isready(running) && fetch(running) isa Stop
while ! stop
if isready(sig)
take!(sig)
println("Received signal!!!")
end
stop = isready(running) && fetch(running) isa Stop
end
end
function send_signal()
sleep(8)
put!(sig, true)
end
struct Stop end
running = Channel{Stop}(1)
sig = Channel{Bool}(1)
w1 = @tspawnat 2 manager()
errormonitor(w1)
w2 = @tspawnat 3 send_signal()
errormonitor(w2)
sleep(20)
put!(running, Stop())
```
So we have one thread (`w2`) that only put an element in the Channel `sig` after 8 seconds. Then we have another thread: `w1`, which has two main jobs. First, it has to periodically call `work()`. In the example, this happen every other second. The other job it has to do is to check if a signal has come on the Channel `sig`.
The system is supposed to run as long as I want it do. In the example, it will run in 20s, before a Stop signal is sent which will stop `w1` and the timer.
The problem is that in `manager()` the process of waiting for a signal in `sig` is blocking the execution of `work()`. How can I run this simultaneously, without introducing another thread?
---
### Author:  [@goerch](https://discourse.julialang.org/u/goerch)
#### Post date: [March 9, 2022, 6:32pm UTC](https://discourse.julialang.org/t/communication-between-two-threads-in-julia-without-blocking/77637/2 "2022-03-09T18:32:59Z")
I could imagine to solve this with a shared variable guarded by a `ReentrantLock` using `trylock`…
Will try if necessary…
---
### Author:  [@Tine](https://discourse.julialang.org/u/Tine)
#### Post date: [March 10, 2022, 8:17am UTC](https://discourse.julialang.org/t/communication-between-two-threads-in-julia-without-blocking/77637/3 "2022-03-10T08:17:54Z")
Hmmm… I don’t understand how that will work…
---
### Author:  [@djholiver](https://discourse.julialang.org/u/djholiver)
#### Post date: [March 10, 2022, 9:19am UTC](https://discourse.julialang.org/t/communication-between-two-threads-in-julia-without-blocking/77637/4 "2022-03-10T09:19:48Z")
Hi,
I’m using my phone so can’t play with the above code but what your describing is a pattern often applied in Actors and there happens to be a Julia [implementation](https://github.com/JuliaActors/Actors.jl) (that I’ve not used yet) that may prove useful
Regards,
---
### Author:  [@ffevotte](https://discourse.julialang.org/u/ffevotte)
#### Post date: [March 10, 2022, 11:23am UTC](https://discourse.julialang.org/t/communication-between-two-threads-in-julia-without-blocking/77637/5 "2022-03-10T11:23:16Z")
I’m far from being an expert on this, but when a task has to wait for one of several conditions, I think it is easier to have only one `Channel` that stores all events, rather than having one `Channel` per event type.
This is a significant rewrite, only loosely based on your original example, but I believe it does what you want (for your MWE; perhaps not for your real application)
```julia
@enum Event WORK SIGNAL STOP
events = Channel{Event}(32)
timer = Timer(2, interval=2) do _
put!(events, WORK)
end
w1 = Threads.@spawn begin
sleep(8)
put!(events, SIGNAL)
end
w2 = Threads.@spawn begin
while true
evt = take!(events)
if evt == WORK
println("Hello!")
elseif evt == SIGNAL
println("Received signal!")
elseif evt == STOP
println("Terminating")
close(timer)
return
else
println("Unknown event $evt")
end
end
end
sleep(20)
put!(events, STOP)
wait(w1)
wait(w2)
```