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Redirects (Often Confused)

• 307 vs 308: both preserve method; 307 temporary, 308 permanent—use these for POST/PUT redirects
• 301/302 may change POST to GET (browser behavior)—don't use for API redirects with body
• Include Location header with absolute URL—relative may fail in older clients
• Redirect loops: limit to 5-10 follows; infinite loops crash clients

Caching Combinations

• Cache-Control: no-store for sensitive data—never written to disk
• no-cache still caches but revalidates every time—not "don't cache"
• private, max-age=0, must-revalidate for user-specific, always-fresh content
• public, max-age=31536000, immutable for versioned static assets
• Vary: Accept-Encoding, Authorization when response depends on these headers—forgetting Vary breaks caching

Conditional Requests

• ETag + If-None-Match: prefer for APIs—content hash based
• Strong vs weak ETags: "abc" vs W/"abc"—weak allows semantically equivalent responses
• If-Match for optimistic locking: fail update if resource changed since read
• 412 Precondition Failed when If-Match fails—not 409 Conflict

CORS Preflight Triggers

• Custom headers (anything not Accept, Accept-Language, Content-Language, Content-Type simple values)
• Content-Type other than: application/x-www-form-urlencoded, multipart/form-data, text/plain
• PUT, DELETE, PATCH methods—even to same origin if other conditions met
• ReadableStream body—triggers preflight
• Preflight cached per Access-Control-Max-Age—set to 86400 to reduce OPTIONS spam

Security Headers (Always Set)

• Strict-Transport-Security: max-age=31536000; includeSubDomains—HSTS, once set can't easily undo
• X-Content-Type-Options: nosniff—prevents MIME sniffing attacks
• X-Frame-Options: DENY or SAMEORIGIN—prevents clickjacking
• Content-Security-Policy—complex but essential; start with report-only mode

Range Requests

• Accept-Ranges: bytes signals support—clients can request partial content
• Range: bytes=0-1023 requests first 1024 bytes; bytes=-500 requests last 500
• Return 206 Partial Content with Content-Range: bytes 0-1023/5000
• 416 Range Not Satisfiable if range invalid—include Content-Range: bytes */5000

Error Response Best Practices

• Structured JSON errors: {"error": {"code": "VALIDATION_FAILED", "message": "...", "details": [...]}}
• Include request ID in error response—enables log correlation
• Don't leak stack traces in production—log server-side, return generic message
• 409 Conflict for business rule violations (duplicate email, insufficient funds)—not just 400

Retry Patterns

• Retry only idempotent methods by default—GET, PUT, DELETE, HEAD
• POST retry needs idempotency key—Idempotency-Key: <client-generated-uuid>
• Exponential backoff: 1s, 2s, 4s, 8s... with jitter—prevents thundering herd
• Respect Retry-After header—can be seconds or HTTP date
• Set reasonable timeout (30s typical)—don't wait forever

Headers Often Forgotten

• Vary: must include headers that affect response—CORS without Vary: Origin breaks
• Content-Disposition: attachment; filename="report.pdf" for downloads
• X-Request-ID: generate if not present, propagate to downstream services
• Accept-Language for localized responses—respect with graceful fallback

Connection Behavior

• HTTP/1.1 without Content-Length or chunked = connection close after response
• Transfer-Encoding: chunked for streaming—can't set Content-Length
• HTTP/2 is binary, multiplexed—no head-of-line blocking at HTTP level
• WebSocket upgrade: GET with Connection: Upgrade, Upgrade: websocket