How can I cache ManagedIdentityCredential in an Azure Function with a ServiceBusTrigger to reduce token delays?

Below are the explanations and some code suggestions to address both questions:

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1. Concurrency Model in Azure Functions (Python)

Azure Functions does not create a new OS process for every incoming message. Instead, an instance of the Functions host (which is its own process) handles many messages concurrently. In the Azure Functions Python worker, this concurrency is typically achieved via asynchronous tasks (or threads internally managed by the worker/runtime). When you set MaxConcurrentCalls to 200, you’re telling the runtime to process up to 200 messages concurrently within the same instance (or across multiple instances if scaled out). In summary:

• Within an instance: Multiple messages are processed concurrently using asynchronous tasks or threads.
• Across instances: The Functions scale-out model creates additional instances (separate processes) if necessary, but each instance processes messages concurrently within itself.

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2. Sharing/Caching the Credential

The observed delay occurs because each execution is initializing a new DefaultAzureCredential instance, which then makes its own call to the Managed Identity (MSI) endpoint. The good news is that:

• The DefaultAzureCredential object is designed to cache tokens internally after retrieving them.
• To take advantage of this caching across invocations, you need to instantiate the DefaultAzureCredential (and, by extension, any clients that depend on it) only once per host instance rather than on every message.

Recommended approach:
Move the credential (and the corresponding client initializations) to module-level (global) scope so that when the function execution environment is “warmed,” every function invocation within the same instance reuses the same credential object. This avoids the overhead of hitting the MSI endpoint for every single message.

For example, you can write your code as follows:

PYTHONCopied
import logging
import json
import azure.functions as func
from azure.identity import DefaultAzureCredential
from azure.servicebus import ServiceBusClient
from azure.data.tables import TableServiceClient

# Initialize global credentials and clients at module scope
credential = DefaultAzureCredential()

# Initialize global Service Bus client (create this once per instance)
_azure_servicebus_client = ServiceBusClient(
    fully_qualified_namespace="your_servicebus_namespace_fq",
    credential=credential
)

# Initialize global Table Service Client
# Here, using a connection string if available or falling back to a credential-based client.
_table_connection_string = "your_connection_string"  # if available
if _table_connection_string:
    _azure_table_service_client = TableServiceClient.from_connection_string(conn_str=_table_connection_string)
else:
    _azure_table_service_client = TableServiceClient(
        endpoint="https://your_storage_account.table.core.windows.net",
        credential=credential
    )

app = func.FunctionApp()

@app.function_name(name="ServiceBusQueueTrigger1")
@app.service_bus_queue_trigger(arg_name="msg",
                               queue_name="<QUEUE_NAME>",
                               connection="<CONNECTION_SETTING>")
def test_function(msg: func.ServiceBusMessage):
    logging.info("ServiceBus queue trigger processed message: %s", msg.get_body().decode("utf-8"))
    
    # Because the clients and credential are already initialized in module scope,
    # there is no need to reinitialize or refresh the credential for each invocation.
    
    # Process the message
    message_body = msg.get_body().decode("utf-8")
    message_json = json.loads(message_body)
    result = process_message(message_json)
    # Return or log the result as needed.

Key Points:

• Global Initialization: By moving credential initialization out of the function (i.e., at the module level), you ensure that the DefaultAzureCredential object is created only once per instance. Subsequent calls can reuse the cached token.
• Internal Caching: The DefaultAzureCredential caches tokens per instance as part of its design. Hence, reusing the same instance minimizes the number of calls to the /msi/token endpoint.

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Following this pattern helps reduce the overhead caused by repeatedly acquiring tokens on each function invocation. Make sure that any shared objects are thread-safe or that their usage does not lead to unintended side effects in a concurrent execution environment.