Do Azure Functions triggered by storage queue take single message or all messages?

Question

If I create an Azure Function that is triggered by storage queue messages... will the system launch multiple parallel functions to reach each message from the queue or will a single function get called that reads in all available messages?

In short, are queued messages handled individually or in batches?

Answer 1

API-wise your function will be called once per each individual message in the queue.

But Azure Functions runtime will retrieve and process messages in batches, calling several instances of your function in parallel.

Answer 2

First, as Mikhail said, Azure Functions runtime retrieve and process queue messages in batches. And the default batchSize is 16 and the maximum batchSize is 32

Besides, we can do configuration for 'queue' triggers and specify/modify batchSize in host.json file.

Configuration settings for 'queue' triggers

"queues": {
  "maxPollingInterval": 2000,
  "visibilityTimeout" : "00:00:10",
  "batchSize": 16,
  "maxDequeueCount": 5,
  "newBatchThreshold": 8
}

Answer 3

It doesn't handle all the messages in one go but it supports message batching. In order to enable batching you need to make the function's input an array of the type rather than the type itself. (e.g. EventData[] rather than EventData) then the batching applies. You can set the batch size up to 32 as @Fei mentioned. Checkout the following link talks about it briefly:

https://docs.microsoft.com/en-us/azure/azure-functions/functions-best-practices