Caching S3 Signed URL Images in the Browser Using the Cache API

You have images served via signed URLs from Amazon S3, and you want to reduce network costs by caching these images in the browser using the Cache API, specifically with methods like Cache.add() or Cache.put(). Signed URLs provide temporary access to private S3 objects and typically include query parameters like signatures and expiration times, which may change even for the same image. This tutorial will guide you through implementing a caching solution that accounts for these challenges, ensuring efficient caching and cost savings.

The key idea is to cache images based on their content rather than their potentially changing URLs. We'll use a service worker to intercept image requests, generate a consistent cache key using the image's S3 object path and ETag (a unique identifier for the content), and store the responses in the browser's Cache Storage. We'll also modify how signed URLs are generated on the server to include the ETag, enabling the client to identify the image content reliably.

Below is a detailed, step-by-step tutorial to achieve this.

Prerequisites

• Basic knowledge of JavaScript, HTML, and web development.
• An S3 bucket with images and the ability to generate signed URLs (e.g., via AWS SDK).
• A web application where images are loaded via tags or similar.
• Node.js and AWS SDK installed on your server (if you're generating signed URLs server-side).

Step 1: Understanding the Problem

Signed URLs from S3 typically look like this:

https://your-bucket.s3.amazonaws.com/path/to/image.jpg?AWSAccessKeyId=...&Signature=...&Expires=...

• Challenge: The query parameters (e.g., Signature, Expires) may change each time a signed URL is generated, even for the same image. The Cache API keys entries by the full URL, so a new URL means a cache miss, leading to repeated S3 fetches and increased costs.
• Goal: Cache the image based on its content, not its transient URL, so that subsequent requests—regardless of URL changes—use the cached version unless the image itself is updated.
• Solution: Use the S3 object’s ETag (a hash of the content) as part of a custom cache key, combined with the URL path. The ETag changes only when the image content changes, making it ideal for content-based caching.

Step 2: Modify Server-Side Signed URL Generation

To enable content-based caching, include the image’s ETag in the signed URL as a query parameter. The ETag is a unique identifier provided by S3 for each object version. Here’s how to do it using Node.js and the AWS SDK v3.

Server-Side Code (Node.js)

Create a function to generate a signed URL with the ETag appended:

const { S3Client, HeadObjectCommand, GetObjectCommand } = require("@aws-sdk/client-s3");
const { getSignedUrl } = require("@aws-sdk/s3-request-presigner");

const s3Client = new S3Client({ region: "your-region" }); // Replace with your AWS region

async function getSignedUrlWithEtag(bucket, key) {
  try {
    // Step 1: Get the ETag of the object
    const headCommand = new HeadObjectCommand({ Bucket: bucket, Key: key });
    const headResponse = await s3Client.send(headCommand);
    const etag = headResponse.ETag.replace(/"/g, ""); // Remove quotes from ETag (e.g., "abc123" -> abc123)

    // Step 2: Generate the signed URL
    const getCommand = new GetObjectCommand({ Bucket: bucket, Key: key });
    const signedUrl = await getSignedUrl(s3Client, getCommand, {
      expiresIn: 3600, // URL valid for 1 hour
    });

    // Step 3: Append the ETag as a query parameter
    const signedUrlWithEtag = signedUrl + "&etag=" + encodeURIComponent(etag);
    return signedUrlWithEtag;
  } catch (error) {
    console.error("Error generating signed URL:", error);
    throw error;
  }
}

// Example usage
(async () => {
  const url = await getSignedUrlWithEtag("your-bucket", "path/to/image.jpg");
  console.log("Signed URL with ETag:", url);
})();

Explanation

• HeadObjectCommand: Retrieves metadata (including the ETag) for the S3 object without downloading it.
• getSignedUrl: Generates a standard signed URL with an expiration time (e.g., 1 hour).
• Appending ETag: The ETag is added as &etag= after the signed URL is generated. S3 ignores extra query parameters for GET requests, so this doesn’t affect URL validity.
• Output: A URL like https://your-bucket.s3.amazonaws.com/path/to/image.jpg?AWSAccessKeyId=...&Signature=...&Expires=...&etag=abc123.

Use this URL in your web application (e.g., in tags) to load images.

Step 3: Set Up the Client-Side Web Application

In your web app, register a service worker to manage caching. The service worker will intercept image requests and use the Cache API.

Main JavaScript File (e.g., app.js)

Add this code to your main JavaScript file to register the service worker:

if ("serviceWorker" in navigator) {
  window.addEventListener("load", () => {
    navigator.serviceWorker
      .register("/service-worker.js")
      .then((registration) => {
        console.log("Service Worker registered with scope:", registration.scope);
      })
      .catch((error) => {
        console.error("Service Worker registration failed:", error);
      });
  });
}

HTML Example

Ensure your images use the signed URLs with ETags:

 lang="en">

   charset="UTF-8">
  


   src="https://your-bucket.s3.amazonaws.com/path/to/image.jpg?AWSAccessKeyId=...&Signature=...&etag=abc123" alt="Cached Image">
  

Step 4: Implement the Service Worker

Create a file named service-worker.js in your project’s root directory. This script will intercept image requests, create a custom cache key, and manage caching.

Service Worker Code (service-worker.js)

self.addEventListener("fetch", (event) => {
  // Check if the request is for an image
  if (event.request.destination === "image") {
    const originalUrl = new URL(event.request.url);
    const etag = originalUrl.searchParams.get("etag");

    if (etag) {
      // Create a custom cache key: origin + pathname + ?etag=
      const cacheKeyUrl = originalUrl.origin + originalUrl.pathname + "?etag=" + etag;
      const cacheKeyRequest = new Request(cacheKeyUrl);

      event.respondWith(
        caches.match(cacheKeyRequest).then((cachedResponse) => {
          if (cachedResponse) {
            // Serve from cache if found
            return cachedResponse;
          }
          // Fetch from network, cache it, then serve
          return fetch(event.request).then((response) => {
            return caches.open("image-cache").then((cache) => {
              cache.put(cacheKeyRequest, response.clone());
              return response;
            });
          });
        })
      );
    } else {
      // If no ETag, fetch normally without caching
      event.respondWith(fetch(event.request));
    }
  }
});

Explanation

• Event Listener: Listens for fetch events, which occur for every resource request.
• Image Check: event.request.destination === "image" ensures we only process image requests (e.g., from tags).
• ETag Extraction: Parses the request URL to get the etag query parameter.
• Cache Key: Constructs a custom key (origin + pathname + ?etag=) that identifies the image by its path and content (ETag), ignoring signature parameters.
• caches.match: Checks if the cache has an entry for this key.
  • Hit: Returns the cached response.
  • Miss: Fetches the image using the original signed URL, caches it with cache.put(), and returns the response.
• Fallback: If no ETag is present, fetches the image without caching.

Step 5: Test the Implementation

1. Deploy Your App: Serve your web app via a local server (e.g., npx serve) or a production server with HTTPS (required for service workers outside localhost).
2. Generate a Signed URL: Use the server-side function to get a URL with an ETag and load it in an tag.
3. Inspect Cache:
   • Open your browser’s DevTools (e.g., Chrome).
   • Go to the “Application” tab > “Cache Storage” > “image-cache”.
   • Verify that cached entries use URLs like https://your-bucket.s3.amazonaws.com/path/to/image.jpg?etag=abc123.
4. Simulate Requests:
   • Reload the page within the signed URL’s expiration time. Check the “Network” tab; the image should load from the service worker cache.
   • Update the image on S3, generate a new signed URL with a different ETag, and reload. The new image should be fetched and cached.

How It Saves Network Costs

• Initial Fetch: The first request fetches the image from S3 and caches it locally using the custom key (path + ETag).
• Subsequent Requests: If the signed URL changes (e.g., new signature) but the ETag remains the same, the service worker serves the cached image, avoiding an S3 fetch.
• Content Updates: When the image is updated on S3, the ETag changes, triggering a new fetch and cache update, ensuring freshness.
• Result: Fewer S3 data transfer requests, reducing costs, especially for frequently accessed images.

Security and Considerations

• Expiration: Cached images persist beyond the signed URL’s expiration. This is acceptable if users retaining local copies aligns with your security policy (since they already had access). For sensitive images, consider shorter cache durations or additional controls.
• Cache Management: The cache grows with each unique image. Implement cache eviction (e.g., delete old entries) if storage becomes a concern.
• Browser Support: The Cache API and Service Workers are widely supported in modern browsers (Chrome, Firefox, Safari, Edge).

Optional Enhancements

• Cache Expiration: Add a max-age policy by checking timestamps or adding a custom header during cache.put().
• Pre-Caching: Use the service worker’s install event to pre-cache known images.
• Error Handling: Add try-catch blocks in the service worker to handle fetch failures gracefully.

This tutorial provides a robust solution to cache S3 signed URL images in the browser, minimizing network costs while handling the complexities of changing URLs and content updates. Implement it, test thoroughly, and adjust based on your specific needs!