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Event Scalers in Azure Container Apps

Event scalers are the primary fit for worker-style apps where pending work exists outside the HTTP request path. Azure Container Apps implements these through KEDA-backed custom scale rules.

Event-driven rule pattern

The common rule shape is:

template:
  scale:
    minReplicas: 0
    maxReplicas: 30
    rules:
      - name: worker-rule
        custom:
          type: <keda-scaler-type>
          metadata:
            <key>: <value>
          auth:
            - secretRef: <secret-name>
              triggerParameter: <parameter-name>

flowchart TD
    A[Queue or event backlog grows] --> B[KEDA custom scaler polls source]
    B --> C[Desired replica count rises]
    C --> D[Worker revision scales out]
    D --> E[Backlog drains]
    E --> F[Scale in toward minReplicas]

Supported event-scaler families

Event source Scaler type Learn-confirmed metadata/auth details Notes
Azure Service Bus azure-servicebus queueName, namespace, messageCount, auth mapping for connection Learn provides explicit examples
Azure Queue Storage azure-queue accountName, queueName, queueLength, secret or managed identity examples Covers Azure Storage Queue workloads
Azure Event Hubs Event Hubs via KEDA custom rule Source family is documented, but current Learn page does not include a full ACA metadata example Validate exact metadata before production
Apache Kafka Kafka via KEDA custom rule Source family is documented, but current Learn page does not include a full ACA metadata example Validate exact metadata before production

Service Bus example

az containerapp update \
  --name "$APP_NAME" \
  --resource-group "$RG" \
  --min-replicas 0 \
  --max-replicas 30 \
  --scale-rule-name "servicebus-orders" \
  --scale-rule-type azure-servicebus \
  --scale-rule-metadata "queueName=orders" "namespace=$SERVICEBUS_NAMESPACE" "messageCount=25" \
  --scale-rule-auth "connection=servicebus-connection"

Azure Queue Storage example

az containerapp update \
  --name "$APP_NAME" \
  --resource-group "$RG" \
  --min-replicas 0 \
  --max-replicas 40 \
  --scale-rule-name "storagequeue-ingest" \
  --scale-rule-type azure-queue \
  --scale-rule-metadata "queueName=ingest" "queueLength=50" "accountName=$STORAGE_ACCOUNT" \
  --scale-rule-auth "connection=storage-connection"

Identity options

Microsoft Learn documents two common patterns:

  • Secret-based auth using secretRef mappings for trigger parameters such as connection
  • Managed identity using the identity field for supported scalers

Example managed identity shape from Learn:

{
  "type": "azure-queue",
  "metadata": {
    "accountName": "<storage-account>",
    "queueName": "<queue-name>",
    "queueLength": "1"
  },
  "identity": "system"
}

Conservative guidance for Event Hubs and Kafka

Full ACA-specific metadata examples for Event Hubs and Kafka are not currently enumerated in Microsoft Learn

Microsoft Learn confirms these event-source families are supported through KEDA-backed scaling, but the current page does not provide the complete metadata key sets in the way it does for Service Bus and Azure Queue. Validate exact metadata against the current scaler contract before production rollout.

See Also

Sources