Azure Function trigger from Service Bus not fired

Question

Azure Function trigger from Service Bus not fired

Jan 0

Hi there,

we have an Azure Function App with a trigger from Service Bus. The issue is that all messages end up in the Dead Letter Queue rather than being delivered.

Function App

written in JavaScript using v4 Programming Model
uses connection string stored in environment variables
no errors in App insights
no invocations in Function App -> [App] -> Overview -> [function]

// File process-queue.js

app.serviceBusQueue('queueProcessJob', {
  connection: 'SERVICE_BUS_CONNECTION_STRING',
  autoComplete: true,
  queueName: process.env.SERVICE_BUS_QUEUE_NAME,
  handler: async (message, context) => {
    context.info('[Process Job] Message received', message);

    // We are using zod to validate the message
    const parsedMessage = jobMessageSchema.safeParse(typeof message === 'string' ? JSON.parse(message) : message);

    if (!parsedMessage.success) {
      context.error('Service Bus Handler Function received invalid message', JSON.stringify(parsedMessage.error.issues, null, 2));
      throw new Error('Invalid queue job message');
    }

    // We process the job here -> we are not calling CompleteMessageAsync or returning anything
  }
});

SERVICE_BUS_CONNECTION_STRING contains connection string in format Endpoint=sb://[resource-name].servicebus.windows.net/;SharedAccessKeyName=RootManageSharedAccessKey;SharedAccessKey=[key] taken from Service Bus -> Settings -> Shared Access Policies -> RootManageSharedAccessKey.
Key has Manage, Send, and Listen claims
Queue name is stored in SERVICE_BUS_QUEUE_NAME and the value is correct.

When I invoke the function directly, it processes the data without any error, which leads me to believe that the "processing" is not an issue, but the trigger is not fired at all.

Also there are no exceptions or traces in App Insights, I tried running following queries:

traces | where message contains "queueProcessJob"

and

exceptions

Service Bus

created in the same "subscription" plan
messages appear in Queue and then are moved to Dead-letter queue with reason MaxDeliveryCountExceeded
time to live = 86400000
delivery count = 10

Questions

Is there a way to debug the trigger? Why is it not firing and immediately goes to Dead-letter?

Rakesh Mishra 165 Reputation points Microsoft External Staff Moderator

2025-08-07T13:09:15.3166667+00:00
Hi @Jan ,

Could you please confirm you’ve bundled the Service Bus extension. Your Function App must load the Service Bus binding at runtime. In the root of your project (and in your deployed files) you need a host.json file like:

{ "version": "2.0", "extensionBundle": { "id": "Microsoft.Azure.Functions.ExtensionBundle", "version": "[4.0.0, 5.0.0)" } }

If you omit extensionBundle, no Service Bus listener is registered, and every delivery silently goes to DLQ after reaching maxDeliveryCount.

Also, no function invocations and logs in Application Insights suggest malformed or missing host.json file. Please verify if Application Insight Environment variable is correctly set. And you are getting some logs from function app.

You can check in Azure Portal > Function App > Advanced Tools (Kudu) > Debug console, browse to wwwroot and confirm you see host.json and your code files.

In your package.json ensure you’ve installed the JS v4 Functions library. Without this, the host can’t load app.serviceBusQueue(…).

{ "dependencies": { "@azure/functions": "^4.0.0" } }

Reference:

Azure Functions extension bundles

Azure Service Bus trigger for Azure Functions
Rakesh Mishra 165 Reputation points Microsoft External Staff Moderator

2025-08-08T13:00:21.75+00:00

@Jan Malčák , following up to see if you had a chance to check my previous response and if it was helpful. Do let me know if you're still facing the issue.

1 answer

Your answer

Rakesh Mishra 165 Reputation points Microsoft External Staff Moderator

2025-08-07T13:09:15.3166667+00:00

Hi @Jan ,

Could you please confirm you’ve bundled the Service Bus extension. Your Function App must load the Service Bus binding at runtime. In the root of your project (and in your deployed files) you need a host.json file like:

{ "version": "2.0", "extensionBundle": { "id": "Microsoft.Azure.Functions.ExtensionBundle", "version": "[4.0.0, 5.0.0)" } }

If you omit extensionBundle, no Service Bus listener is registered, and every delivery silently goes to DLQ after reaching maxDeliveryCount.

Also, no function invocations and logs in Application Insights suggest malformed or missing host.json file. Please verify if Application Insight Environment variable is correctly set. And you are getting some logs from function app.

You can check in Azure Portal > Function App > Advanced Tools (Kudu) > Debug console, browse to wwwroot and confirm you see host.json and your code files.

In your package.json ensure you’ve installed the JS v4 Functions library. Without this, the host can’t load app.serviceBusQueue(…).

{ "dependencies": { "@azure/functions": "^4.0.0" } }

Reference:

Azure Functions extension bundles

Azure Service Bus trigger for Azure Functions
Rakesh Mishra 165 Reputation points Microsoft External Staff Moderator

2025-08-08T13:00:21.75+00:00

@Jan Malčák , following up to see if you had a chance to check my previous response and if it was helpful. Do let me know if you're still facing the issue.

Answer 1

Jan 0

Hi Rakesh, I think I found a solution. It was not something from your suggestions.

We were getting traces/exceptions for other functions. Just nothing relevant for this function triggered by Service Bus. Sorry for bad wording in the initial question.

In the end what helped was adding “function:{functionName}” (where {functionName} is the name of function triggered by Service Bus) to the Scaling configuration of our Consumption plan. This ensures the function never goes to sleep and there is always at least one instance ready.

Is it possible that the service bus trigger cannot wake up the function?

Rakesh Mishra 165 Reputation points Microsoft External Staff Moderator

2025-08-12T05:14:44.62+00:00
Hi @Jan Malčák , Glad to hear that issue is fixed.

There can be different scenarios where a Service Bus trigger appears to not wake up a function. Below is the list of common root causes, what to check, and quick fixes.

Wrong connection string / binding name

Symptom: Function never sees messages; no incoming metrics on function.

Check: AzureWebJobsServiceBus (or whatever setting name you use) in App Settings; queue/topic/subscription name in the trigger attribute/binding.

Fix: Correct the setting name/value or binding configuration.

Authorization / SAS / RBAC issues

Symptom: Errors in logs like auth/401/403, or retries then failures.

Check: Shared Access Policy keys, expiry, or Role-Based Access Control permissions for Managed Identity.

Fix: Recreate/renew SAS key or grant correct RBAC role to the function identity.

Network / firewall / Private Endpoint

Symptom: Function cannot reach Service Bus -> network timeouts in logs.

Check: Service Bus firewall rules, Private Endpoint configuration, and Function outbound network (VNet integration, NAT).

Fix: Allow function outbound to Service Bus, or set Service Bus to allow the function's outbound IPs / use private endpoints correctly.

Host startup or binding extension failure

Symptom: Host fails to start, or Service Bus binding extension not loaded; binding errors in streaming logs.

Check: Function host logs (Application Insights or streaming logs) for extension/binding exceptions. For isolated worker model, ensure the Service Bus extension is registered.

Fix: Fix startup exceptions, install/register correct Service Bus extension package, adjust extension bundle.

Message state: scheduled / deferred / dead-letter / expired

Symptom: Messages appear in queue but not delivered (active count low).

Check: Service Bus metrics and explore queue via Service Bus Explorer — look for scheduled, deferred, or dead-lettered messages; check TTL/expiry/time-to-live.

Fix: Re-schedule or move messages back from DLQ; ensure message TTL is correct.

Session-enabled queue/topic but function not session-aware

Symptom: Messages sit in queue; function not picking them up.

Check: Is the entity session-enabled? If yes, confirm function uses session handler (SessionId or SessionEnabled=true).

Fix: Enable session support in the trigger or use a non-session entity.

Another consumer has locked the session/message

Symptom: Messages locked by another consumer; no delivery to your function.

Check: Who is conne2cting to the entity; check lock status and sessions.

Fix: Stop competing consumer or ensure only expected consumers are connected.

Scale controller / cold start / host scaled to zero

Symptom: Slight delay before processing; rarely causes complete no-delivery.

Check: Consumption plan scaling logs; look for host start events.

Fix: Increase plan, pre-warm, or use Premium/Dedicated plan if low-latency is required.

Throttling / quotas on Service Bus

Symptom: Throttling errors (503) or slow delivery.

Check: Service Bus throttling metrics and error logs.

Fix: Increase throughput unit (if using SKU that supports it), or retry/backoff.

Extension/SDK mismatches (Functions runtime vs Service Bus extension)

Symptom: Binding attribute not recognized or runtime errors after upgrade.

Check: Function runtime version, extensions bundle, and NuGet package versions (for compiled functions).

Fix: Align extension versions and bundles, follow migration docs for new SDKs.

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Azure Function trigger from Service Bus not fired

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