How to build an HTTP API with Azure Functions

Azure Functions is Microsoft's serverless compute service. Functions run in response to triggers (HTTP, queues, timers) and scale automatically. On the consumption plan you pay only for execution, with a generous free grant each month.

This tutorial builds an HTTP-triggered API and deploys it to a consumption plan.

Prerequisites

• An active Azure subscription.
• Azure CLI (az) signed in via az login.
• Azure Functions Core Tools and a runtime (.NET 8 or Node.js 20).

Steps

1. Install Core Tools

npm i -g azure-functions-core-tools@4 --unsafe-perm true

2. Scaffold the project

func init helloapi --worker-runtime dotnet-isolated
cd helloapi
func new --name Hello --template "HTTP trigger" --authlevel function

The function auth level requires a key, keeping the endpoint private by default.

3. Write the HTTP trigger

Edit the generated Hello function to return JSON:

[Function("Hello")]
public HttpResponseData Run(
  [HttpTrigger(AuthorizationLevel.Function, "get")] HttpRequestData req)
{
  var res = req.CreateResponse(HttpStatusCode.OK);
  res.WriteString("{\"message\":\"hello\"}");
  return res;
}

4. Run locally

func start

Call http://localhost:7071/api/Hello to confirm the function responds.

5. Create cloud resources

az group create --name rg-hello --location eastus
az storage account create --name sthello$RANDOM --resource-group rg-hello --sku Standard_LRS
az functionapp create --resource-group rg-hello --consumption-plan-location eastus \
  --runtime dotnet-isolated --functions-version 4 --name fa-hello-$RANDOM \
  --storage-account <storage-name>

A Function App requires a backing storage account for state and logs.

6. Deploy and test

func azure functionapp publish <function-app-name>

The CLI prints the invoke URL and key. Call it:

curl "https://<app>.azurewebsites.net/api/Hello?code=<function-key>"

Verification

Open the Function App in the Azure Portal and check the Monitor blade or Application Insights for invocation traces. Confirm the function scales by sending concurrent requests and watching the instance count.

Next Steps

Move the function key into Azure Key Vault references, add a custom domain, and consider the Flex Consumption plan for faster cold starts and VNet integration.