Azure Functions

Code that runs when something happens

Azure Functions runs small pieces of code in response to a trigger: an HTTP request, a timer, a message on a queue, a new blob, a Cosmos DB change and more. Bindings connect inputs and outputs declaratively, so writing to a queue can be a return value rather than SDK code. Hosting plans change behaviour: the Flex Consumption plan (the newer serverless option) and the older Consumption plan scale to zero and bill per execution, at the cost of cold starts; the Premium plan keeps pre-warmed instances and supports VNet integration; a Dedicated plan runs on an App Service plan. Functions should be short, stateless and idempotent, because triggers can retry. For long-running, stateful workflows, Durable Functions adds orchestrations that checkpoint their progress.

Trigger, function, binding

An event arrives, your code runs, and bindings move data in and out.

An HTTP function and a queue-triggered function (Python v2 model)

Decorators declare the trigger and output binding; AzureWebJobsStorage is the connection setting name.

import azure.functions as func
import logging

app = func.FunctionApp()

@app.route(route="orders", methods=["POST"], auth_level=func.AuthLevel.FUNCTION)
@app.queue_output(arg_name="msg", queue_name="orders", connection="AzureWebJobsStorage")
def create_order(req: func.HttpRequest, msg: func.Out[str]) -> func.HttpResponse:
    msg.set(req.get_body().decode())
    return func.HttpResponse("accepted", status_code=202)

@app.queue_trigger(arg_name="item", queue_name="orders", connection="AzureWebJobsStorage")
def process_order(item: func.QueueMessage) -> None:
    logging.info("processing %s", item.get_body().decode())

Make handlers idempotent

Queue triggers deliver at least once. If a function crashes after charging a card but before completing, the message comes back. Store an order ID and skip work that is already done.

Quick check: Which plan avoids cold starts with pre-warmed instances?

• Consumption
• Free App Service
• Premium
• Azure Container Instances