Object vs Block vs File

• Object storage (S3, R2, GCS) for immutable blobs: images, videos, backups, logs — cheap, scales infinitely, but no partial updates
• Block storage (EBS, Persistent Disks) for databases and apps needing filesystem semantics — faster, but tied to single instance
• Network file systems (NFS, EFS) when multiple instances need shared filesystem access — convenient but latency and cost add up
• Default to object storage for user uploads — block storage for database files only

When SQL vs NoSQL

• SQL when you need joins, transactions, or complex queries — fighting against NoSQL for relational data wastes months
• Document stores (MongoDB, Firestore) for nested/variable schemas where you always fetch the whole document
• Key-value (Redis, DynamoDB) for simple lookups by ID at massive scale — not for complex queries
• Time-series databases (InfluxDB, TimescaleDB) for metrics with timestamp-based queries — regular SQL struggles with retention policies
• Start with PostgreSQL unless you have a specific reason not to — it handles JSON, full-text search, and scales further than most assume

Local vs Cloud Storage

• Local disk for ephemeral data: temp files, build artifacts, caches — assume it disappears on restart
• Cloud storage for anything that must survive instance termination — never store user data only on local disk
• Local SSD for databases in production — network-attached storage adds latency to every query
• Hybrid: local cache in front of cloud storage for frequently accessed files

CDN Patterns

• Put CDN in front of static assets always — origin requests are slower and more expensive
• Set long cache TTLs with versioned URLs (style.abc123.css) — cache invalidation is slow and unreliable
• CDN for dynamic content only if latency matters more than freshness — adds complexity for marginal gains
• Edge caching for API responses works but cache keys get tricky — start simple, add only when needed

Upload Handling

• Never accept uploads directly to app server disk in production — use presigned URLs to cloud storage
• Set file size limits at load balancer level, not just application — prevents memory exhaustion attacks
• Generate unique keys for uploads (UUIDs) — user-provided filenames cause collisions and path traversal risks
• Validate file types by content (magic bytes), not extension — extensions are trivially spoofed

Data Locality

• Keep compute and storage in same region — cross-region data transfer adds latency and cost
• Replicate data to regions where users are, not where developers are
• Multi-region storage adds complexity — single region with backups elsewhere usually sufficient
• Database read replicas in user regions for read-heavy workloads

Retention and Lifecycle

• Define retention policy before storing data — "keep everything" becomes expensive and legally risky
• Automate deletion of temporary data — manual cleanup never happens consistently
• Tiered storage for aging data: hot → warm → cold → archive — but check retrieval costs before archiving
• Separate storage for logs vs business data — different retention, different compliance requirements

Cost Traps

• Egress fees dominate cloud storage costs — calculate before choosing provider
• Many small files cost more than few large files — batch small writes when possible
• Minimum storage duration on cold tiers — early deletion still charges full period
• API request costs matter at scale — millions of LIST operations add up

Backup Strategy

• 3-2-1 rule: 3 copies, 2 different media types, 1 offsite — cloud counts as one location
• Test restores regularly — untested backups are not backups
• Point-in-time recovery for databases — daily snapshots lose a day of data
• Version important files — deletion or corruption often discovered late