Understand why after warm-up a first call to web API / Azure Function is a bit slower than following calls

Dima G 1

Summary: I have warmed-up and (Always On) Azure Functions app with http trigger. Then I call the http api from client console app (.net core) 4 times in a row.
Every time the console app runs, the first call takes longer than the other 3.

Please see the full details on Stack Overflow:
https://stackoverflow.com/questions/64228179/azure-function-slow-response-on-first-https-call-with-always-on-same-with-asp

I am looking to understand this behavior...
Thanks!

JayaC-MSFT 5,531 Reputation points

2020-11-05T05:06:59.3+00:00

@Dima G I see that your azure function is running on Consumption Plan. Since Azure Functions recycles resources, the first invocation to a function app after a long period of inactivity can take much longer to finish. This is known as Cold Start - to learn more, please visit Understanding Serverless Cold Start.

There are a few things you can do to mitigate cold start, which are checked for below. To eliminate cold start entirely, consider changing your hosting plan - either upgrade to the Premium plan, or switch to the App Service (Dedicated) plan.

Please let me know if this helps.
Dima G 1 Reputation point

2020-11-05T07:56:13.743+00:00

I am well aware of cold start and thats why I wrote "after warm-up" in the title. Also, it is mentioned in the Stack Overflow, and also, I reverted it to consumption plan after I've tried a dedicated plan with always on.
Cold start is NOT the reason for this behavior. I suspect it must be something in combination with client side. Even when the server is warm for sure - each time I start the client console app, there is first call overhead. And this what my question is about. Any other ideas?

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Understand why after warm-up a first call to web API / Azure Function is a bit slower than following calls

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