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Question

Is there a way to update unread data that been sent to a channel with more up to date data?

I have a goroutine (producer) with a channel that's provides progress updates to another goroutine (consumer). In some scenarios, the progress can update much faster than the consumer consumes the update messages.

This causes me issues as I can either:

• Block on sending data to the channel. This means that if the consumer is slow to read data, the progress updating goroutine totally blocks - which it shouldn't.
• Don't block on sending and skip over progress updates when the channel is full. This means the consumer is always reading old, out of data data.

As an example, I might have something like this:

Progress reporting goroutine: Posts "1%" to channel Progress reporting goroutine: Posts "2%" to channel Progress reporting goroutine: Posts "3%" to channel Progress consuming goroutine: Reads "1%", "2%" and "3%" from channel. "1% and "2%" are outdated information.

Is there any way to update unread channel data? Or is there a better way of going about this issue?

Answer 1

You can store some value in a global variable protected with RWMutex It keeps progress. Generator updates it. Consumer reads and shows.

Also you can make a non-blocking writing to channel with length of 1:

var c = make(chan struct{}, 1)
select {
case c <- struct{}{}:
default:
}

This way sender either adds one element to the channel either do nothing if it’s full.

Reader treats this empty struct as a signal - it should take updated value in the global variable.

Another way: Updatable channel

var c = make(chan int, 1)
select {
case c <- value:  // channel was empty - ok

default: // channel if full - we have to delete a value from it with some precautions to not get locked in our own channel
    switch {
    case <- c: // read stale value and put a fresh one
         c <- value
    default: // consumer have read it - so skip and not get locked
     }
}