# How to wait a message on multiple channels?
**URL:** https://forum.crystal-lang.org/t/how-to-wait-a-message-on-multiple-channels/7421
**Category:** Help & Support
**Created:** [November 20, 2024, 6:54pm UTC](https://forum.crystal-lang.org/t/how-to-wait-a-message-on-multiple-channels/7421 "2024-11-20T18:54:31Z")
**Posts on this page:** 11
**Page:** 1
### Author:  [@amigrave](https://forum.crystal-lang.org/u/amigrave)
#### Post date: [November 20, 2024, 6:54pm UTC](https://forum.crystal-lang.org/t/how-to-wait-a-message-on-multiple-channels/7421/1 "2024-11-20T18:54:31Z")
Hi all.
How can I have an Array of Channels with which I want the current Fiber to wait/listen for a message on all those channels at once and exit the flow either if one of the channel received a message or if a timeout (previously defined) occurred
?
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### Author:  [@Blacksmoke16](https://forum.crystal-lang.org/u/Blacksmoke16)
#### Post date: [November 20, 2024, 6:58pm UTC](https://forum.crystal-lang.org/t/how-to-wait-a-message-on-multiple-channels/7421/2 "2024-11-20T18:58:48Z")
I’d look into [select - Crystal](https://crystal-lang.org/reference/1.14/syntax_and_semantics/select.html).
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### Author:  [@amigrave](https://forum.crystal-lang.org/u/amigrave)
#### Post date: [November 21, 2024, 6:39am UTC](https://forum.crystal-lang.org/t/how-to-wait-a-message-on-multiple-channels/7421/3 "2024-11-21T06:39:54Z")
Thanks for your answer, however select seems only appropriate when working with a defined amount of channels, in my case I have a reference to an array of channels which is unknown at compile time.
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### Author:  [@sol.vin](https://forum.crystal-lang.org/u/sol.vin)
#### Post date: [November 21, 2024, 8:53am UTC](https://forum.crystal-lang.org/t/how-to-wait-a-message-on-multiple-channels/7421/4 "2024-11-21T08:53:48Z")
When working with an array of channels, you may need to spin your own solution to whatever behavior you want to achieve. For example: You may just want to get the messages waiting on the channels, if you care about the order you might do something like:
```crystal
array_of_channels.map(&.receive?)
```
but if you care about a timeout you may need to do something like
```crystal
array_of_channels.each do |channel|
select
when item = channel.receive?
#process item
when timeout(5.seconds)
#Handle timeout
end
end
```
However, this might still have a behavior you don’t want, it waits up to 5 seconds for each channel. Therefore you may need to do something like:
```crystal
wg = WaitGroup.new(array_of_channels.size)
array_of_channels.each do |channel|
spawn do
begin
select
when item = channel.receive?
#process item
when timeout(5.seconds)
#Handle timeout
end
end
ensure
wg.done
end
end
wg.wait #Waits for all the channels to finish.
```
This paradigm only works if you can process the data received from the channel independently, not if you need to process the data all at once. There are a lot of ways to skin a cat on this one.
Let us know more about what you are doing and we can recommend some potential ways to make your code work.
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### Author:  [@amigrave](https://forum.crystal-lang.org/u/amigrave)
#### Post date: [November 22, 2024, 12:05am UTC](https://forum.crystal-lang.org/t/how-to-wait-a-message-on-multiple-channels/7421/5 "2024-11-22T00:05:58Z")
Thanks for your response.
I think none of those solutions will match my use case.
I want to wait for any of the channels to receive a message (I’m using an Array of `Channel(Nil)`) , hence I want the waiting to be interrupted either as soon as one Channel receives a `Nil` , either if a timeout is reached while waiting.
Is there a solution for that ?
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### Author:  [@jgaskins](https://forum.crystal-lang.org/u/jgaskins)
#### Post date: [November 22, 2024, 12:06am UTC](https://forum.crystal-lang.org/t/how-to-wait-a-message-on-multiple-channels/7421/6 "2024-11-22T00:06:23Z")
Does your use case preclude putting all these items into one channel?
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### Author:  [@bcardiff](https://forum.crystal-lang.org/u/bcardiff)
#### Post date: [November 22, 2024, 1:13am UTC](https://forum.crystal-lang.org/t/how-to-wait-a-message-on-multiple-channels/7421/7 "2024-11-22T01:13:39Z")
I think you want to use the `:nodoc:` method `Channel.non_blocking_select`.
See usages in [crystal/spec/std/channel\_spec.cr at master · crystal-lang/crystal · GitHub](https://github.com/crystal-lang/crystal/blob/master/spec/std/channel_spec.cr#L577)
And in [crystal/spec/std/channel\_spec.cr at master · crystal-lang/crystal · GitHub](https://github.com/crystal-lang/crystal/blob/master/spec/std/channel_spec.cr#L557) you can see how to create a `timeout_select_action`.
Note, is an internal api as we didn’t envision arbitrary amount of channels to be a common idiom when designing the public api.
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### Author:  [@amigrave](https://forum.crystal-lang.org/u/amigrave)
#### Post date: [November 22, 2024, 10:45am UTC](https://forum.crystal-lang.org/t/how-to-wait-a-message-on-multiple-channels/7421/8 "2024-11-22T10:45:06Z")
> Does your use case preclude putting all these items into one channel?
@jgaskins Technically no, but design-wise I think so. If I do that I will end up with 5000 to 10000 Fibers receiving a message at the same time, all checking if this message is for them, while only a very few of them are concerned.
I’m afraid that this design could induce a cpu bound solution which I’m actually trying to avoid, considering that I’m currently evaluating Crystal for the first time in a production context to replace a Python gevented solution that becomes a bottleneck.
> I think you want to use the `:nodoc:` method `Channel.non_blocking_select` .
Thanks for your response @bcardiff . I will have a look to this. Does it imply to implicitly or explicitly spawn one Fiber per channel to receive from ?
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### Author:  [@straight-shoota](https://forum.crystal-lang.org/u/straight-shoota)
#### Post date: [November 22, 2024, 11:05am UTC](https://forum.crystal-lang.org/t/how-to-wait-a-message-on-multiple-channels/7421/9 "2024-11-22T11:05:36Z")
Maybe you could tell us a bit more about what you’re actually trying to do?
From the initial description I gathered you’d want one fiber to wait for a signal from either of a number of other fibers. But it seems there is more going on. Hearing about the full context would allow to help you better find a solution for your problem.
---
### Author:  [@bcardiff](https://forum.crystal-lang.org/u/bcardiff)
#### Post date: [November 22, 2024, 12:35pm UTC](https://forum.crystal-lang.org/t/how-to-wait-a-message-on-multiple-channels/7421/10 "2024-11-22T12:35:58Z")
> [@amigrave](#):
>
> Does it imply to implicitly or explicitly spawn one Fiber per channel to receive from ?
No.
`non_blocking_select` is what is used by the `select` statement. The compiler generates a call to the former from the later. Syntax wise there is no way to have a dynamic number of `when`s, but the `non_blocking_select` works on `Indexable` (so `Array` can be used)
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### Author:  [@amigrave](https://forum.crystal-lang.org/u/amigrave)
#### Post date: [November 22, 2024, 6:27pm UTC](https://forum.crystal-lang.org/t/how-to-wait-a-message-on-multiple-channels/7421/11 "2024-11-22T18:27:41Z")
Well I tried to be as less verbose as possible to prevent loosing people’s time but it seems it’s too late for that.
That being said I realised that I might have another option that I should try before going further in the direction I was headed to. I’ll come back to tell if it was paying of and I might end-up competing two models to see which one is better.