Topic
Conditional Requests
Use HTTP validators and preconditions to avoid unnecessary transfers, prevent conflicting writes, and serve cached responses safely.
- HTTP Status Codes
412 Precondition Failed: Meaning and How to Fix It
412 means an If-Match or If-Unmodified-Since condition was false, usually because the resource changed after you read it. How to fix it and retry safely.
- HTTP Status Codes
428 Precondition Required
The server requires the request to be conditional. Learn when to use 428 Precondition Required to prevent lost updates and race conditions.
- Guides
HTTP Headers and Caching: A Practical Guide
Master HTTP caching with Cache-Control, ETag, Last-Modified, and conditional request headers. Learn how to optimize performance with proper cache strategies.
- HTTP Headers
If-Match Header
Learn how the If-Match header makes requests conditional based on ETag matching. Prevent conflicts and lost updates in concurrent editing scenarios.
- HTTP Headers
If-Modified-Since Header
Learn how the If-Modified-Since header requests resources only if modified since a specific date. Reduce bandwidth with efficient conditional caching.
- HTTP Headers
If-None-Match Header
Learn how the If-None-Match header makes conditional requests using ETags. Avoid downloading unchanged resources and reduce bandwidth with cache validation.
- HTTP Headers
If-Range Header
Learn how the If-Range header requests partial content only if the resource is unchanged. Efficiently resume downloads without re-fetching entire files.
- HTTP Headers
If-Unmodified-Since Header
Learn how the If-Unmodified-Since header makes requests conditional on resources not being modified. Prevent conflicts in concurrent update scenarios.
More topics
- 63 pages
Security
Study HTTP security controls for transport, cookies, browser isolation, authentication, content policy, and response hardening.
- 24 pages
Performance
Improve HTTP delivery with caching, compression, connection reuse, timing signals, and response metadata.
- 22 pages
Caching
Learn how browsers and intermediaries store, validate, and reuse HTTP responses with cache directives, validators, and conditional requests.
- 22 pages
Cookies
Understand cookie storage, scope, transport, expiration, and security across HTTP headers, attributes, and session flows.