Azure Functions
An Azure service that provides an event-driven serverless compute platform.
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What does "event source length" mean?
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Gallatin 21V
206
Reputation points
Hi Team，
https://docs.azure.cn/en-us/azure-functions/functions-target-based-scaling?tabs=v5%2Ccsharp
According to the document, the expansion of the number of function instances in the EP pricing layer is carried out according to the above formula.
The trigger type of the function is eventhub trigger. Then, does the event source length is the number of incoming messages in the eventhub?According to my calculation, it seems that it is not "incoming messages" in eventhub.
Azure Functions
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Shireesha Eeraboina 3,435 Reputation points Microsoft External Staff Moderator2025-06-25T15:21:58.5866667+00:00 Hi Gallatin 21V,
Thanks for your question.
You're correct that the function instance scaling formula in the Elastic Premium (EP) plan is based on the event source length. However, for Event Hub triggers, the term “event source length” does not directly refer to the total number of incoming messages in the Event Hub.
Instead, it refers to the number of unprocessed events that are available in the Event Hub partition(s) that the function is monitoring. This value is influenced by:
- The current lag (messages not yet checkpointed),
- The number of partitions, and
- The function’s processing throughput.
So if your function is fast enough to process incoming messages, the "event source length" may remain low even if the Event Hub is receiving a high volume of messages.
If you're seeing differences between the actual number of incoming messages and what triggers scaling, it's likely because the unprocessed event backlog (i.e., the source length) is not growing significantly — meaning the function is keeping up well with the incoming load. To help you better understand, kindly refer to the documentation below.
I hope this is helpful! Feel free to reach out if you have any more questions or need further assistance.
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Shireesha Eeraboina 3,435 Reputation points Microsoft External Staff Moderator
2025-06-26T14:34:03.9466667+00:00 Hello Gallatin 21V,
I hope you had a chance to review the response above. If there are any updates or if you need further assistance, please feel free to update here.
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Gallatin 21V 206 Reputation points
2025-06-27T08:07:21.4633333+00:00 Thank you for your reply. However, how can we calculate the number of unprocessed messages that are accumulated in the EventHub? I understand it should be "incoming messages" minus "outgoing messages".Am I correct in my understanding?
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