Absolutely. Think of a CDN as a geographically distributed cache in front of your origin server.
CDN architecture flow
┌──────────────────────┐
│ Origin Server │
│ (AWS S3 / Web App) │
└──────────┬───────────┘
│
Content fetched on MISS
│
▼
┌─────────────────────────────┐
│ CDN Network │
│ │
│ ┌─────────┐ ┌─────────┐ │
│ │ Edge │ │ Edge │ │
│ │ India │ │ USA │ │
│ └────┬────┘ └────┬────┘ │
│ │ │ │
│ ┌────┴────┐ ┌────┴────┐ │
│ │ Cache │ │ Cache │ │
│ └─────────┘ └─────────┘ │
└──────┬──────────────┬───────┘
│ │
Internet Internet
│ │
┌──────▼──────┐ ┌────▼───────┐
│ User India │ │ User USA │
│ Browser │ │ Browser │
└─────────────┘ └────────────┘Step-by-step: User in India requests an image
Suppose your application has:
https://cdn.myapp.com/images/photo123.jpgThe flow is:
1. User Browser
│
│ GET /images/photo123.jpg
▼
2. CDN
┌───────────────┐
│ Indian Edge │
│ Server │
└───────┬───────┘
│
│ Is image in cache?
│
┌───┴────┐
│ │
YES NO
│ │
│ ▼
│ 3. Origin
│ S3 / Server
│ │
│ │ Returns image
│ ▼
│ 4. CDN Edge
│ caches image
│ │
└────────┤
▼
5. User Browser
receives imageThe important concept: Cache HIT vs Cache MISS
First user:
User → CDN Edge → Cache MISS → S3 Origin
↓
Get image
↓
Store in CDN cache
↓
User gets imageSecond user:
User → CDN Edge → Cache HIT
↓
Image already there
↓
User gets imageSo the second request does not need to travel all the way to S3.
Where does S3 fit?
For an architecture like your earlier Web App + S3 + CDN design:
┌──────────────────┐
│ Web Browser │
└────────┬─────────┘
│
API / HTML request
│
▼
┌──────────────────┐
│ Web App │
│ Backend Server │
└────────┬─────────┘
│
│ Returns image URL
▼
https://cdn.myapp.com/photo.jpg
│
▼
┌──────────────────┐
│ CDN │
│ Edge Server │
└────────┬─────────┘
│
Cache MISS
│
▼
┌──────────────────┐
│ Amazon S3 │
│ Origin Storage │
└──────────────────┘One very important point
S3 does not normally give the CDN URL to the client.
Your application/CDN configuration determines the public CDN URL.
For example:
S3:
bucket = my-photo-bucket
object = photos/123.jpg
CDN:
https://cdn.myapp.com/photos/123.jpgYour backend/API can return:
{
"photoId": "123",
"photoUrl": "https://cdn.myapp.com/photos/123.jpg"
}Then the browser simply does:
<img src="https://cdn.myapp.com/photos/123.jpg">The browser talks to the CDN, not directly to S3.
Complete picture
┌─────────────────────┐
│ Application │
│ Backend │
└──────────┬──────────┘
│
Generate/return
CDN URL
│
▼
┌─────────────┐
│ Browser │
└──────┬──────┘
│
Request CDN URL
│
▼
┌───────────────────┐
│ CDN │
│ Edge Location │
└─────────┬─────────┘
│
┌─────┴─────┐
│ │
CACHE HIT CACHE MISS
│ │
│ ▼
│ ┌─────────┐
│ │ S3 │
│ │ Origin │
│ └────┬────┘
│ │
│ Store in CDN
│ │
└─────┬─────┘
▼
┌─────────┐
│ Browser │
│ Image │
└─────────┘Interview one-liner:
CDN is a geographically distributed caching layer between the client and the origin (such as S3). On a cache hit, the CDN serves content directly; on a cache miss, it fetches the content from the origin, caches it, and then serves it to the client.
--------
Yes. A typical Java/Spring Boot implementation has two separate responsibilities:
Upload the image to S3
Construct/return the corresponding CDN URL
The CDN URL is not generated by S3. Your application knows the CDN domain configured for your bucket.
Spring Boot + AWS S3 example
@Service
public class ImageService {
private final S3Client s3Client;
// Your S3 bucket
private final String bucketName = "my-photo-bucket";
// Your CDN domain
private final String cdnBaseUrl = "https://cdn.myapp.com";
public ImageService(S3Client s3Client) {
this.s3Client = s3Client;
}
public String uploadImage(MultipartFile file) throws IOException {
// 1. Generate unique object key
String fileName = UUID.randomUUID() + "-" + file.getOriginalFilename();
String objectKey = "photos/" + fileName;
// 2. Upload image to S3
PutObjectRequest putObjectRequest = PutObjectRequest.builder()
.bucket(bucketName)
.key(objectKey)
.contentType(file.getContentType())
.build();
s3Client.putObject(
putObjectRequest,
RequestBody.fromBytes(file.getBytes())
);
// 3. Construct CDN URL
String cdnUrl = cdnBaseUrl + "/" + objectKey;
// 4. Return CDN URL
return cdnUrl;
}
}Controller
@RestController
@RequestMapping("/api/images")
public class ImageController {
private final ImageService imageService;
public ImageController(ImageService imageService) {
this.imageService = imageService;
}
@PostMapping("/upload")
public ResponseEntity<String> uploadImage(
@RequestParam("file") MultipartFile file) throws IOException {
String cdnUrl = imageService.uploadImage(file);
return ResponseEntity.ok(cdnUrl);
}
}Complete flow
POST /api/images/upload
│
│ image.jpg
▼
┌─────────────────────┐
│ Spring Boot API │
│ │
│ ImageService │
└──────────┬──────────┘
│
│ putObject()
▼
┌─────────────────────┐
│ AWS S3 │
│ │
│ photos/abc123.jpg │
└─────────────────────┘
Application constructs:
https://cdn.myapp.com/
photos/abc123.jpg
│
▼
Return to ClientThe API response could be:
{
"photoId": "123",
"photoUrl": "https://cdn.myapp.com/photos/abc123.jpg"
}Then the frontend uses:
<img src="https://cdn.myapp.com/photos/abc123.jpg">What happens when the browser loads it?
Browser
│
│ GET https://cdn.myapp.com/photos/abc123.jpg
▼
CDN Edge
│
├── Cache HIT ──────► Return image
│
└── Cache MISS
│
▼
AWS S3
│
▼
CDN caches image
│
▼
BrowserOne important production consideration
If the S3 bucket is private—which is generally preferable—you don't make the S3 object publicly accessible just because you're returning a CDN URL.
Instead:
Client
│
▼
CDN
│
│ authenticated origin request
▼
Private S3With AWS CloudFront, for example, you can use Origin Access Control (OAC) so that users can access the image through CloudFront while direct public access to S3 is blocked.
So in an interview, remember this simple statement:
Application uploads the object to S3 → application constructs the CDN URL → application returns the CDN URL to the client → client downloads the object through CDN → CDN fetches from S3 only on cache miss.
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