HTTP Conditional Request Interview Questions: ETags, 304s, and Lost ...

Practice HTTP conditional request interviews with ETags, 304 responses, If-Match concurrency control, weak validators, and a worked lost-update recovery case.

HTTP Conditional Requests explained

Cache validation, concurrency control, and resumable downloads all depend on conditional requests, HTTP requests whose outcome hinges on comparing a validator against the current state of the target resource. The server evaluates the condition before processing and returns a short response (typically 304 or 412) when the condition fails. Two primary use cases drive conditional requests: cache ...

HTTP Conditional Requests: ETags, Last-Modified & 304

How HTTP conditional requests avoid retransmitting unchanged content — ETags for byte-accurate validation, Last-Modified for time-based checks, If-None-Match and If-Modified-Since headers, 304 Not Modified responses, and the interplay with CDN caching. Step-by-step animated simulation you can run in your browser, free.

Conditional requests (ETag, Last-Modified, 304) · Website Spec

Send a validator — ETag or Last-Modified — on every cacheable response, and honour If-None-Match / If-Modified-Since so unchanged resources return an empty 304 instead of the full body.

Conditional Requests & Validation | Developer Guide

How to use ETag, Last-Modified, and precondition headers to enable 304 responses and prevent lost updates in HTTP APIs.

Conditional requests - Varnish Software Documentation

The crux of conditional requests is to send an HTTP 304 Not Modified as early as possible. In order to take advantage of this in your web application, you need your application framework to validate the Etag as quickly as possible and exit as early as possible.

HTTP 304 Caching, ETags, Conditional Requests | Code Reference

Deep dive into HTTP 304 Not Modified - ETag vs Last-Modified, Cache-Control interaction, browser cache. Instant results online, free and browser-based.

Understanding HTTP 304 Not Modified: Conditional Requests, ETags, and ...

What 304 Not Modified really means, how conditional GET with ETag and If-Modified-Since works, why 304 isn't an error, and how to return it correctly from Express and NGINX.

Understanding HTTP Conditional Caching with ETag and Last-Modified

HTTP Conditional Caching: How to Check Validity When the request is made again, the client sends additional headers along with the GET request, which I evaluate on the server side. If the resource has the same ETag If the content matches the cached version (If-None-Match), I return a 304 Not Modified response without a body.

Cache-Control, ETag, and Conditional Requests Guide 2026

Quick answer: Cache-Control, ETag, and Conditional Requests work together to optimize HTTP delivery: Cache-Control defines freshness lifetime, ETag provides a unique content fingerprint, and Conditional Requests (If-None-Match) allow servers to return 304 Not Modified instead of full payloads when content hasn't changed, saving bandwidth and CPU.