How to Build Serverless APIs with Azure Functions in Azure Portal

Azure Functions is a serverless compute service that allows us to run event-driven code without having to manage infrastructure. It enables developers to focus on business logic while Azure handles scaling, maintenance, and security. This article walks you through creating and deploying a serverless API using Azure Functions in the Azure Portal.

Why Use Azure Functions for APIs?

Scalability: Automatically scales based on demand.
Cost-Effective: Pay only for execution time.
Easy Integration: It works seamlessly with other Azure services.
Event-Driven: Trigger functions from HTTP requests, messages, or database changes.

Prerequisites

Before you begin, ensure you have,

An Azure Subscription
Azure Portal access to create and manage resources.
Basic knowledge of C# or JavaScript (optional but helpful).

Step 1. Create an Azure Function App.

Sign in to the Azure Portal.
Click Create a resource and search for Function App.
Click Create and fill in the required details.
- Subscription: Select your Azure subscription.
- Resource Group: Create a new one or use an existing one.
- Function App name: Enter a unique name.
- Runtime Stack: Choose .NET, Node.js, Python, or Java.
- Region: Select a region close to your users.
Click Review + Create, then Create.

Step 2. Create a Serverless API with HTTP Trigger.

Navigate to your Function App in the Azure Portal.
Click Functions in the left menu and then + Create.
Select HTTP trigger as the function template.
Provide a function name and choose the Authorization level:
- Anonymous: No authentication required.
- Function: Requires a function key.
- Admin: Requires an admin key.
Click Create.

Step 3. Write and Test the Function.

Open your newly created function and click Code + Test.

Modify the default code (C# example)

using System.IO;
using Microsoft.AspNetCore.Mvc;
using Microsoft.Azure.WebJobs;
using Microsoft.Azure.WebJobs.Extensions.Http;
using Microsoft.AspNetCore.Http;
using Newtonsoft.Json;
using System.Threading.Tasks;

public static class MyFunction
{
    [FunctionName("MyFunction")]
    public static async Task<IActionResult> Run(
        [HttpTrigger(AuthorizationLevel.Function, "get", "post", Route = null)] HttpRequest req)
    {
        string name = req.Query["name"];

        string requestBody = await new StreamReader(req.Body).ReadToEndAsync();
        dynamic data = JsonConvert.DeserializeObject(requestBody);
        name = name ?? data?.name;

        return name != null
            ? new OkObjectResult($"Hello, {name}!")
            : new BadRequestObjectResult("Please pass a name in the query string or in the request body.");
    }
}

Click Test/Run, enter a query parameter (?name=Azure), and check the response.

Step 4. Deploy and Expose the API.

Click Get Function URL and copy the generated URL.
Test the API using Postman or a browser.
https://<function-app-name>.azurewebsites.net/api/MyFunction?name=Azure
(Optional) Secure the API using Azure API Management or Azure Active Directory (AAD).

Step 5. Monitor and Scale.

Navigate to Monitor in the function app to check logs and analytics.
Enable Application Insights for detailed telemetry.
Set scaling options based on usage patterns.

Conclusion

Azure Functions provide a fast, scalable, and cost-efficient way to build APIs without managing servers. By following this guide, you have successfully created and deployed a serverless API using Azure Functions. Explore additional triggers like Azure Storage, Service Bus, and Event Grid to expand your API capabilities.