CODEWITHSUNDEEP
pythonazureazure-functionsazureservicebusazure-python-sdk
Niladri
Niladri

Reputation: 5962

Python - Azure function service bus trigger batch processing

I am using Azure function service bus trigger in Python to receive messages in batch from a service bus queue. Even though this process is not well documented in Python, but I managed to enable the batch processing by following the below Github PR.

https://github.com/Azure/azure-functions-python-library/pull/73

Here is the sample code I am using -

function.json

{
  "scriptFile": "__init__.py",
  "bindings": [
    {
      "name": "msg",
      "type": "serviceBusTrigger",
      "direction": "in",
      "cardinality": "many",
      "queueName": "<some queue name>",
      "dataType": "binary",
      "connection": "SERVICE_BUS_CONNECTION"
    }
  ]
}

__init__.py

import logging

import azure.functions as func
from typing import List

def main(msg: List[func.ServiceBusMessage]):
    message_length = len(msg)
    if message_length > 1:
        logging.warn('Handling multiple requests')

    for m in msg:
       #some call to external web api

host.json

"version": "2.0",
  "extensionBundle": {
    "id": "Microsoft.Azure.Functions.ExtensionBundle",
    "version": "[3.3.0, 4.0.0)"
  },
  "extensions": {
    "serviceBus": {
      "prefetchCount": 100,
      "messageHandlerOptions": {
        "autoComplete": true,
        "maxConcurrentCalls": 32,
        "maxAutoRenewDuration": "00:05:00"
      },
      "batchOptions": {
        "maxMessageCount": 100,
        "operationTimeout": "00:01:00",
        "autoComplete": true
      }
    }
  }
}

After using this code , I can see that service bus trigger is picking up messages in a batch of 100 (or sometimes < 100) based on the maxMessageCount but I have also observed that most of the messages are ending up in the dead letter queue with the MaxDeliveryCountExceeded reason code. I have tried with different values of MaxDeliveryCount from 10-20 but I had the same result. So my question is do we need to adjust/optimize the MaxDeliveryCount in case of batch processing of service bus messages ? How both of them are related ? What kind of change can be done in the configuration to avoid this dead letter issue ?

Upvotes: 3

Views: 1788

Answers (1)

Alex
Alex

Reputation: 18526

From what we discussed in the comments, this is what you encounter:

  • Your function app is fetching 100 messages from ServiceBus (prefetchCount) and locking them for a maximum of maxAutoRenewDuration
  • Your function code is processing messages one at a time at a slow rate because of the API you call.
  • By the time you finish a batch of messages (maxMessageCount), the lock already expired which is why you have exceptions and the message gets redelivered again. This eventually causes MaxDeliveryCountExceeded errors.

What can you do to improve this?

  • Reduce maxMessageCount and prefetchCount
  • Increase maxAutoRenewDuration
  • Increase the performance of your API (how to do that would be a different question)
  • Your current code would be much better off by using a "normal" single message trigger instead of the batch trigger

PS: Beware that your function app may scale horizontally if you are running in a consumption plan, further increasing the load on your struggling API.

Upvotes: 1

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