Orleans Broadcast Channels - Relationship between publisher and consumer is unclear

[michaelmcneilnet]

Type of issue

Missing information

Description

I am having a hard time trying to understand broadcast channels from the documentation.

One main source of confusion is this line of code in StockWorker:

ChannelId channelId = ChannelId.Create(ChannelNames.LiveStockTicker, Guid.Empty);

• There's no explanation of why Guid.Empty is used. Is Guid.Empty the key of the consuming grain? The declaration of ILiveStockGrain is not in the docs, so it's unclear what the grain's key type is. There's also no code showing how the grain reference was created, so again unclear as to what the key is.
• It's not clear where ChannelNames comes from, is it a static class not shown in the docs?
• The grain doesn't use ChannelNames.LiveStockTicker, so what's the relationship here between subscription in the grain, and producer in the background service?
• One of the bullet points states The OnSubscribed method is called when the grain subscribes to the broadcast channel. It would be good to call out when this is. I assume it's when the grain is activated, and the grain then subscribes 'under-the-hood' due to the mechanism being implicit?
• This may be a quirk of the API, but in the StockWorker constructor, ChannelNames.LiveStockTicker is used to get the channel provider, so why does it need to also be specified as part of ChannelId.Create?

A comparison between Broadcast Channels and Streams (probably in-memory streams as they seem similar to broadcast channels) would be great, as I'm not sure when to use one over the other and the pros and cons.

/cc @ReubenBond

Clarifications to the docs might also help close dotnet/orleans#8292

Page URL

https://learn.microsoft.com/en-us/dotnet/orleans/streaming/broadcast-channel

Content source URL

https://github.com/dotnet/docs/blob/main/docs/orleans/streaming/broadcast-channel.md

Document Version Independent Id

7de240a6-146f-48d8-c652-58f0ff3571db

Article author

@IEvangelist

Metadata

• ID: e43e2b09-a8fa-e0fa-99a6-42f7d927e106
• Service: dotnet-orleans

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Associated WorkItem - 500376

[Neftedollar]

Agree. I tryed it. And it looks like it doesn't works

[TimoSchmechel]

+1

the documentation for Broadcasting Streams is very confusing/misleading. The existence of the GUID in the channel ID seems to indicate this channel will only send to a grain with that specific key which is counter intuitive to the word "Broadcast"

[P-Hayk]

The documentation is unclear.
It seems Guid.Empty is used because ChannelId.Create doesn’t support a single-parameter overload. Implicit subscription likely creates a receiver grain with the same key as the streamer grain under the hood.

I need explicit subscription since my streamer and receiver should have different keys (X:Y for the streamer and X:Y:Z for the receiver). However, there doesn’t seem to be a way to use explicit subscription, which forces to use Streams instead—unless I don’t need persistence.

If you know how to avoid using streams, please advise.