Scaling
| Pattern | What it does |
|---|---|
| Load balancer | Distributes traffic across replicas (round-robin, least-conn, consistent-hash) |
| Horizontal scaling | Add more machines |
| Vertical scaling | Bigger machine |
| Sharding | Partition data by key across shards |
| Replication | Keep copies for read scale / failover |
| Read replicas | Scale reads; writes go to primary |
| CDN | Cache static assets geographically near users |
| Service mesh | Sidecar proxies for routing, retries, mTLS |
Caching
| Pattern | Notes |
|---|---|
| Cache-aside | App reads cache first, falls back to DB |
| Write-through | Write to cache and DB together |
| Write-behind | Write to cache now, flush async to DB |
| Read-through | Cache transparently fetches on miss |
| TTL / stale-while-revalidate | Serve stale briefly while refreshing |
| Cache invalidation | Hardest problem — prefer event-driven eviction |
Async / messaging
| Pattern | Notes |
|---|---|
| Message queue | Buffer producer/consumer; retry on failure |
| Pub/sub | Fan-out to multiple subscribers |
| Event sourcing | Store event log; replay to reconstruct state |
| CQRS | Separate read model from write model |
| Saga | Long-running multi-service transaction with compensations |
| Outbox pattern | Atomically write to DB + outbox; relay publishes |
Resilience
| Pattern | Notes |
|---|---|
| Circuit breaker | Stop calling a failing dependency to let it recover |
| Bulkhead | Isolate resources — one failure doesn't drown others |
| Timeout + retry + backoff | Never retry without limits; use exponential backoff with jitter |
| Rate limiting | Protect from overload (token bucket, leaky bucket) |
| Graceful degradation | Serve reduced functionality when dependencies fail |
| Idempotency keys | Allow safe retries without double-effects |
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