A Complete Guide to Amazon CloudFront Functions: Pricing, Use Cases, and Implementation

CloudFront Functions enable low-latency request processing at AWS edge locations. This guide covers how they work, pricing, use cases, and best practices to optimize performance and costs.

In this article, you will learn:

• What are Amazon CloudFront Functions?
• Why Use Amazon CloudFront Functions?
• Key Use Cases of Amazon CloudFront Functions
• Examples of CloudFront Functions in Action
• Amazon CloudFront Functions Pricing: Cost-Efficiency at Scale
• Limitations of CloudFront Functions
• Best Practices for Optimization
• Conclusion

Speed and efficiency are critical in content delivery, but optimizing how requests and responses are processed at the edge is just as important. Businesses running applications on AWS need solutions that minimize latency, enhance security, and control traffic without adding infrastructure overhead.

Traditionally, these tasks required server-side logic or third-party integrations, adding complexity and cost. AWS enables developers to process lightweight functions directly at CloudFront edge locations, helping them modify requests in real time without extra server load.

But how does this work in practice? What kinds of tasks can be handled at the edge, and how does AWS structure pricing for these functions? To answer these questions, this guide covers how Amazon CloudFront Functions work, when to use them, how they are priced, and best practices for implementation. Read on!

What are Amazon CloudFront Functions?

Amazon CloudFront Functions is a serverless feature that enables developers to execute lightweight JavaScript code at AWS's global edge locations. This allows for high-scale, latency-sensitive customizations of content delivery. By running code closer to end-users, CloudFront Functions facilitate real-time HTTP request and response manipulation, enhancing performance and reducing the need for origin serverprocessing.

CloudFront Functions are designed for rapid execution, with sub-millisecond startup times, and can scale immediately to handle millions of requests per second. They operate within the CloudFront content delivery network (CDN) and can be triggered during two specific events:

• Viewer Request: Executed when CloudFront receives a request from a viewer, allowing for modifications before the request reaches the origin server.
• Viewer Response: Executed before CloudFront returns the response to the viewer, enabling adjustments to the response coming from the origin server.

This design ensures high performance and security for operations such as header manipulations, URL rewrites, and access authorization.

Why Use Amazon CloudFront Functions?

Modern applications demand low-latency request processing and efficient content delivery without overwhelming origin servers. Traditional server-side logic often introduces unnecessary delays and infrastructure costs, making real-time request handling a challenge.

CloudFront Functions solves this by executing lightweight logic at AWS edge locations, eliminating the need to route every request back to the origin. This is crucial for modifying requests before caching decisions such as handling authentication, rewriting URLs, or enforcing security policies; where origin approval isn't possible.

With global deployment and straightforward implementation, CloudFront Functions offer an easy way to enforce custom logic at scale, ensuring fast, optimized content delivery while reducing backend load.

Key Use Cases of Amazon CloudFront Functions

• Security Header Enforcement – Add or modify security headers like Strict-Transport-Security (HSTS) and Content-Security-Policy (CSP) to prevent attacks and enforce HTTPS.

• Cookie-Based Personalization – Read and modify cookies in incoming requests to enable customized content delivery based on user preferences.

• Geolocation-Based Access Control – Restrict or redirect users based on their geographic location, which is useful for compliance, licensing, or content restrictions.

• Referrer-Based Content Restrictions – Restrict content access based on the referring website to prevent hotlinking. Enforce content distribution policies by validating referrer headers.

• Origin Modification – For example, you can use origin modification to determine the geographic location of a viewer and then forward the request, on cache misses, to the closest AWS Region running your application.

• Leverage CloudFront Key-Value-Store – KeyValueStore enables low-latency, edge-accessible key-value storage, facilitating use cases like URL rewrites, A/B testing, and access authorization.

By running directly at the edge, Amazon CloudFront Functions provide real-time processing at scale while keeping costs low and offloading unnecessary work from origin servers.

Examples of CloudFront Functions in Action

URL Rewrite for Single Page Applications

When building a single-page application, you often need all URL routes to point to your main HTML file (usually index.html). This trick allows your SPA to work correctly even when a user refreshes the page or enters a URL directly into the browser.

With CloudFront Functions, you can easily implement this directly on the edge, which is super fast and saves load on your origin server.

Here's an example that does the job:

async function handler(event) {
    var request = event.request;
    var uri = request.uri;
    // Check whether the URI is missing a file extension.
    else if (!uri.includes('.')) {
        request.uri += '/index.html';
    }
    // Check whether the URI is missing a file name.
    if (uri.endsWith('/')) {
        request.uri += 'index.html';
    }
    return request;
}

This example simply checks if the URL contains a dot for files (like .js, .css, .png, etc.) or slash for directories. If there's no dot, or there is a slash at the end, we assume it's a route in our SPA and rewrite the URI to "/index.html".

Language-based Redirection

Another commonly used case for CloudFront Functions is language-based redirection. If you have a web application or e-commerce site that supports multiple languages, you can use CloudFront Functions to automatically redirect users to the correct language version of your website based on their browser's language settings. This ensures a better user experience without the need for manual language selection.

Let's take a look at an example of how to implement this behavior:

async function handler(event) {
    var request = event.request;
    var headers = request.headers;
    var host = request.headers.host.value;
    var country = 'DE' // Choose a country code
    var newurl = https://${host}/de/index.html; // Change the redirect URL to your choice 
  
    if (headers['cloudfront-viewer-country']) {
        var countryCode = headers['cloudfront-viewer-country'].value;
        if (countryCode === country) {
            var response = {
                statusCode: 302,
                statusDescription: 'Found',
                headers:
                    { "location": { "value": newurl } }
                }

            return response;
        }
    }
    return request;
}

This is just a simple example. In your implementation, you can utilize CloudFront KeyValueStore, which is a secure, global, low-latency key-value datastore. You can store locations for multiple supported languages in the KeyValueStore and modify them as needed without changing your function code. This approach allows you to update language mappings dynamically, simplifying management and making it easy to add new languages without deploying code changes.

Amazon CloudFront Functions Pricing: Cost-Efficiency at Scale

Amazon CloudFront Functions follow a simple, pay-as-you-go pricing model based solely on the number of invocations. This makes them highly predictable and cost-effective, especially for workloads requiring millisecond-level execution at scale.

Pricing Breakdown

• $0.10 per 1 million invocations (fixed rate, no additional compute or memory charges).

Why It’s Cost-Effective

• No additional compute or memory costs – Pricing is fixed per invocation, regardless of function complexity.
• Massive scalability – Supports millions of requests per second without increasing operational costs.
• Lower cost than Lambda@Edge – At $0.60 per million requests plus compute time, Lambda@Edge is significantly more expensive for lightweight tasks.

By charging only for invocations, CloudFront Function costs a minimal amount while providing high-performance edge processing, making them ideal for workloads with frequent, low-latency request handling.

Limitations of CloudFront Functions

While CloudFront Functions are highly efficient, they come with certain constraints due to their design for ultra-fast execution at the edge:

• Execution Time Limit – Functions must complete within 1 millisecond, making them unsuitable for complex logic.
• Memory Restriction – Limited to 2MB of memory, preventing large-scale computations.
• No Network Calls – Cannot interact with external APIs, databases, or AWS services.
• Limited Language Support – Only JavaScript (ECMAScript 5.1) is supported.
• Request and Response Payload Limits – Cannot modify the request or response body, only headers and metadata.

Best Practices for Optimization

To maximize efficiency and ensure seamless performance, follow these best practices:

• Keep Functions Small and Optimized – Write minimal, efficient code that executes within the strict 1-millisecond limit. Remove unnecessary loops, large dependencies, and excessive logic that could slow execution. Keep functions focused on simple tasks like header modifications, redirects, or access control.

• Use Environment-Specific Configurations – Deploy different function versions for staging and production environments. This helps prevent unintended changes in live applications and allows testing in isolated settings before rolling out updates. AWS provides testing tools to simulate function execution before deployment.

• Monitor Logs and Performance Metrics – Use CloudFront real-time logs and AWS monitoring tools to track execution behavior, identify errors, and detect performance bottlenecks. Regular monitoring ensures functions remain efficient and aligned with traffic patterns.

• Minimize String Operations and Data Processing – String parsing and manipulation can increase processing time. Use efficient coding practices, such as avoiding excessive string concatenation or large JSON object parsing, to ensure execution stays within time limits.

By adhering to these best practices, businesses can ensure optimal performance, reliability, and cost-efficiency when using CloudFront Functions.

Conclusion

CloudFront Functions are a powerful tool for lightweight request modifications at the edge, but they work best when used for simple, stateless tasks. If your use case requires external API calls, complex logic, or data persistence, Lambda@Edge may be a better fit.

Before implementing CloudFront Functions, evaluate your workload’s needs and then make an informative decision. As you continue to develop and optimize your cloud strategy, follow StormIT’s blog for more valuable content on cloud technologies and best practices.