Proposal: ChannelWriter<T>.DroppedWrites counter

[declspec]

As far as I'm aware, when operating a BoundedChannel<T> with a BoundedChannelFullMode of any of the Drop* variants there is no way to tell when and how many writes start being dropped. In all cases the call to TryWrite will succeed and the producer will be none the wiser.

It would be nice feature to have a DroppedWrites counter (or something that performs a similar function) that increments whenever a channel drops a pending write from the queue. Something like the following:

var channel = Channel.CreateBounded<object>(new BoundedChannelOptions(10) {
    FullMode = BoundedChannelFullMode.DropOldest
});

for (var i = 0; i < 20; ++i) {
    Debug.Assert(channel.Writer.TryWrite((object)i));
}

Debug.WriteLine(channel.Writer.DroppedWrites); // should output '10' at this point.

[matttrawicki]

👍 I find the Channels very intriguing but unusable as implemented. I need to know when the channel is full and writes are being blocked/dropped. Even a simple Count property would be a useful step.

[stephentoub]

Even a simple Count property would be a useful step.

That now exists: #312

[matttrawicki]

@stephentoub Thank you! I really did try to look before asking, but I missed it. 😥

[declspec]

As an alternative maybe an overload to TryWrite that adds an out parameter for communicating the action that was taken, something along the lines of:

bool TryWrite(T data, out BoundedChannelFullAction fullAction);

public enum BoundedChannelFullAction {
    None, // Channel isn't full yet
    Wait, // When BoundedChannelFullMode.Wait was specified, this could actually just be merged into `DroppedWrite` as the return value from `TryWrite` would disambiguate them
    DroppedWrite, // When BoundedChannelFullMode.DropWrite was specified
    DroppedOldest, // When BoundedChannelFullMode.DropOldest was specified
    DroppedNewest // When BoundedChannelFullModel.DropNewest was specified
}

Even just writing that out I'm not sure if it's a very nice API, but it would be nice to be able to get some extra info out of the TryWrite call, given that we can control its behaviour with BoundedChannelFullMode.

Or, without an out parameter:

public BoundedChannelWriteResult Write(T data);

public bool TryWrite(T data) {
    return Write(data) != BoundedChannelWriteResult.Failed;
}

public enum BoundedChannelWriteResult {
    Failed,
    Written,
    DroppedWrite,
    DroppedOldest,
    DroppedNewest
}

[springy76]

I don't see any counters on netcore 3.1.4

[stephentoub]

I'm going to close this as a dup of #36522. Such a callback can trivially be used to implement a counter if that's what's needed. Thanks!

[declspec]

@stephentoub Yep that'll work perfectly, thanks.