Azure Function batchSize

Question

I am wondering about the parallel principle in Azure Function. If I have a batchSize of 32 and a threshold of 16. If the queue grow to large the Scale controller spins up a new function to withstand the pressure. I understand this bit. What I don't understand is: does a single instance work on the batch? That is, do I only have one function running pr batch, or does the runtime scale out and run multiple threads with the function?

Could I risk having two instances running, each with a 32 messages, and concurrently 32 threads running 32 functions pr once?

Imaging I have a function calling a webapi. This means that the api will get 64 calls at once which I don't want.

What I want is 2 functions working on 32 messages each making 1 call pr message pr function.

I hope you guys understand.

PramodValavala · Accepted Answer · 2019-07-11 17:08:45Z

1

Yes. That is indeed how scaling works. The same is explained with a bit more details in the docs as well.

According to that, your function (one instance) could run up to 48 messages at a time (32 from a new batch + 16 from the existing batch) and could potentially scale to multiple instances depending on the queue length.

To achieve the scenario you've mentioned, you would have to

Set the batchSize to 1 to avoid parallel processing per instance
Set the WEBSITE_MAX_DYNAMIC_APPLICATION_SCALE_OUT app setting to 2 to limit scale out to a max of 2 instances

Note that all 32 messages won't be loaded by either instance but will work through the queue nonetheless.

answered Jul 11, 2019 at 17:08

PramodValavala

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