SignalR message order?

Hi.

I have a question about signalr order or messages that are received by client. Situation (server code):

...
await hubContext.Clients.All.SendAsync("EventA", "123");
await hubContext.Clients.All.SendAsync("EventB", "456");
...

Can it happen that some client will receive message 456 before message 123 (note that method name is different)?

[analogrelay]

Why would Thread.Sleep change anything? Feels unreliable.

That is indeed unreliable. Also Thread.Sleep is dangerous when used in an async context since it blocks the entire thread which can lead to thread-pool starvation. Use await Task.Delay(1) when in an async context. Anything that “blocks” without using await in an async context can cause significant performance problems.

If ordering is not guaranteed what the purpose of SendAsync api that allows you to await something?

You are awaiting successful entry of the message in to the queue of messages to be sent out to the client. If you have no interest in awaiting that, you don’t have to.

I also want to clarify something about our ordering. We do not enforce ordering at a protocol level, thus cannot guarantee that ordering is preserved in all cases. Nor does SignalR provide any facility (built-in) to ensure ordering is preserved. However, in practice most messages will be consistently ordered across multiple clients/servers.

When you have a single server (and all clients are connected to that server), ordering should be preserved, since SendAsync completes when the message enters the in-memory queues for each client, and the next SendAsync will thus insert the message after the previous one.

When multiple servers are involved, connected via a backplane such as Redis or the Azure SignalR Service, it depends on how the backplane behaves. If two servers send messages to all clients (messages A and B), each server will send that message to the backplane to be distributed to all servers in the cluster. As long as the backplane preserves ordering, they should remain ordered. However, if the backplane doesn’t preserve ordering, some clients may see A B and other may see B A.

The official backplanes (Redis and the Azure SignalR Service) should preserve ordering, since the underlying technologies used are ordered.

However, as I said above, since SignalR doesn’t enforce this ordering itself, we avoid making any guarantees as to ordering.

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