How to Determine Azure ServiceBus PrefetchCount and ReceiveBatch Size

Question

I have a queue processor that is retrieving all messages from a ServiceBus Queue. I am wondering how I should determine the MessageReceiver PrefetchCount and the ReceiveBatch messageCount to optimize performance. I am currently setting these to the arbitrary number 500, as seen below:

var receiverFactory = MessagingFactory.CreateFromConnectionString("ConnectionString");
var receiver = await receiverFactory.CreateMessageReceiverAsync("QueueName", ReceiveMode.PeekLock);
receiver.PrefetchCount = 500;

bool loopBatch = true;
while (loopBatch) 
{
    var tempMessages = await receiver.ReceiveBatchAsync(500, TimeSpan.FromSeconds(1));
    // Do some message processing...
    loopBatch = tempMessages.Any();
}

When running, I see that my batches often take time to "warm up," retrieving counts such as "1, 1, 1, 1, 1, 1, 1, 1, 125, 125, 125, 125..." where the batch retrieval number suddenly jumps much higher.

From the Prefetching optimization docs:

When using the default lock expiration of 60 seconds, a good value for SubscriptionClient.PrefetchCount is 20 times the maximum processing rates of all receivers of the factory. For example, a factory creates 3 receivers, and each receiver can process up to 10 messages per second. The prefetch count should not exceed 20 X 3 X 10 = 600. By default, QueueClient.PrefetchCount is set to 0, which means that no additional messages are fetched from the service.

I don't really understand how to determine the receiver's "messages per second" when the batch retrieval seems to retrieve widely-varying numbers of messages at a time. Any assistance would be greatly appreciated.

Answer 1

I don't really understand how to determine the receiver's "messages per second" when the batch retrieval seems to retrieve widely-varying numbers of messages at a time.

Prefetch makes more sense in the scenario when OnMessage API is used. In that scenario a callback is registered that takes a single message for processing and you can estimate an average processing time of that message. OnMessage API allows to define how many concurrent callback will be running. It would be extremely innefficient to retrieve messages one by one knowing there is a constant flow of incoming messages. Hence, PrefetchCount is used to specify how many mesasges should be retrieved in a "batch" by clients in the background to save the roundtrips back to the server.