Explain ETag vs Last-Modified.

ETag and Last-Modified are HTTP cache validators: ETag is a unique identifier for a resource’s version, while Last-Modified indicates the timestamp of its most recent update.

When to Use

Use ETag when content changes frequently or with high precision (e.g., dynamic APIs). Use Last-Modified for static files where second-level accuracy is sufficient.

Example

A chat app serving fast-changing messages should use ETags, while a static blog image can rely on Last-Modified headers.

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Why Is It Important

Validators prevent unnecessary data transfers by letting servers respond with 304 Not Modified, saving bandwidth and improving response times.

Interview Tips

Define both clearly: ETag = content version ID, Last-Modified = timestamp. Mention granularity differences and illustrate with a short example (dynamic API vs static file). This demonstrates both technical and practical understanding.

Trade-offs

ETag: Highly accurate but requires server computation and consistency across clusters. Last-Modified: Lightweight but limited to 1-second resolution and may refresh unnecessarily if timestamps change without actual content updates.

Pitfalls

Misconfigured ETags in distributed systems can break caching if different servers generate different tags. Last-Modified may give false positives when file metadata changes but the content remains the same.

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System Design Interview

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Design Gurus Team