The scaling documentation for Azure Functions is a bit light on details for how Azure Functions decide when to add more instances of an app.
Say for example I have a function that is triggered by a Github webhook. 10,000 people simultaneously commit to the Github repo (with no merge conflicts ;) ), and Github calls my function 10,000 times in a very short period of time.
What can I expect to happen? Specifically,
- Will Azure Functions throttle the webhook calls? i.e., will Azure Functions reject certain function calls if my function app is under high load?
- Does Azure Functions queue the requests somehow? If so, where/how?
- How many instances of my function app will Azure Functions create in this scenario? One for each request (i.e., 10,000), and each will run in parallel?
- If my app function was scaled down to zero instances, because there was no load on it, could I expect to see some "warm-up time" before the first function is executed? Roughly how long?
- Azure Functions won't reject a webhook call, but in the case of sudden, extreme load, some requests may timeout. For web apis, please include retry on the client, as a best practice.
- They aren't queued in any persistent place. They are (implementation detail) managed by IIS.
- (Implementation detail) Number of instances isn't a hard set thing. We have certain, unpublished protections in place, but we're designed to scale quite far. Your requests will be handled by multiple instances.
- Yes. Right now, it's pretty hefty (seconds), but we'll be working to improve it. For perf sensitive situations, a canary or a timer trigger to keep it awake is recommended.
I'm from the Azure Functions team. The things I marked as implementation details aren't promises and will likely also change as we evolve our service; just an attempt at transparency.
- tested today. it took more than seconds :(
ACTUAL PERFORMANCE
--------------
ClientConnected: 13:58:41.589
ClientBeginRequest: 13:58:41.592
GotRequestHeaders: 13:58:41.592
ClientDoneRequest: 13:58:41.592
Determine Gateway: 0ms
DNS Lookup: 65ms
TCP/IP Connect: 40ms
HTTPS Handshake: 114ms
ServerConnected: 13:58:41.703
FiddlerBeginRequest: 13:58:41.816
ServerGotRequest: 13:58:41.817
ServerBeginResponse: 14:00:36.790
GotResponseHeaders: 14:00:36.790
ServerDoneResponse: 14:00:36.790
ClientBeginResponse: 14:00:36.790
ClientDoneResponse: 14:00:36.790
Overall Elapsed: **0:01:55.198**
