AzureServiceBus - Publish messages in a guaranteed order

Question

When publishing messages to a service bus topic, if I loop over 3 messages:

{ A, B, C }

And await the SendAsync() each time, I'd expect them to be published to the topic in the order:

{ A, B, C }

public async Task PublishMessage(string topic, string json, string sessionId)
{
       var topicClient = new TopicClient(_connectionString, topic);
       var busMessage = new Message(Encoding.UTF8.GetBytes(json));
       busMessage.SessionId = sessionId;
       await topicClient.SendAsync(busMessage);
}

A number of employees have suggested this isn't guaranteed to be the case, and that in certain scenarios (i.e large messages), this publishing order isn't guaranteed. I've never encountered a scenario of this myself, does ASB not guarantee publish ordering even when the sending of messages is awaited like the above?

This article https://devblogs.microsoft.com/premier-developer/ordering-messages-in-azure-service-bus/ uses this quote:

"While Azure Service Bus allows for a FIFO approach (First-In-First-Out), we cannot guarantee that messages are entered in the order we want them to be processed"

This all seems quite baffling to me, as I'd have assumed SendAsync() would only return a successful result once the message has been added into the topic. Do we really need to write layers of complexity around this to manage it?

Please note this only relates to the publishing of messages, we use SessionIds to handle consumption.

Answer 1

Even if you wait for one message to be sent and then only send the next one, the FIFO is not guaranteed. This is due to too many probable causes. In order to ensure you get guaranteed ordering, you need to use session enabled queues or subscriptions.