Core Distributed Systems Patterns
Load Balancing
Problem: single server can't handle all traffic.
Approaches: Round Robin (simple, ignores load) | Weighted Round Robin (accounts for capacity) | Least Connections (fewest active) | IP Hash (session affinity) | Layer 4/transport (IP/port, fast) | Layer 7/application (HTTP-aware, smarter)
Placement: client-to-web | web-to-app | app-to-database
Trade-offs: LB itself is SPOF -- use active-passive pair | session affinity complicates horizontal scaling -- prefer stateless servers | health checks critical
Caching
Problem: repeated DB reads are slow.
Strategies: Cache-aside/lazy loading (app checks cache, fills on miss -- most common) | Write-through (write cache+DB simultaneously) | Write-behind (cache only, async to DB) | Read-through (cache fronts DB transparently)
Cache-aside pattern: Read: cache.get(key) -> hit? return : db.read -> cache.set -> return | Write: db.write -> cache.delete(key)
Eviction: LRU (most common) | LFU (skewed access) | TTL (time-based)
Problems: thundering herd (many misses simultaneously -- use locking/coalescing) | cache penetration (non-existent keys -- Bloom filter or cache null) | cache avalanche (mass expiration -- jittered TTLs) | size cache based on working set, not total data
Database Replication
Master-Slave: all writes to master, reads to replicas | replication lag = eventual consistency | master fails: promote replica
Multi-Master: writes to any node, conflict resolution required | better write availability, much more complex | suitable for multi-region
Trade-offs: sync replication = consistency but higher write latency | async = lower latency but data loss risk on failure
Database Sharding
Problem: single DB can't handle write volume or data size.
Strategies: Hash-based (hash(key) % N -- even but resharding painful) | Range-based (ranges, can have hotspots) | Directory-based (lookup table, flexible but SPOF)
Partition key: must distribute data AND queries evenly | must be in most queries | common: user_id, tenant_id, region
Challenges: resharding (consistent hashing helps) | celebrity/hotspot problem | cross-shard joins (expensive -- denormalize) | referential integrity (enforce in app) | schema changes across all shards
Consistent Hashing
Problem: traditional hash(key) % N remaps almost all keys when N changes.
How: hash output space as ring (0 to 2^32-1) | servers at positions on ring | keys walk clockwise to first server | adding/removing server affects only adjacent keys
Virtual nodes: each physical server gets 100-200 positions | ensures even distribution | handles heterogeneous capacities
Used in: DynamoDB, Cassandra, Discord, Akamai CDN
Message Queues
Problem: tight coupling; spikes overwhelm downstream.
Properties: decoupling | buffering (absorbs spikes) | async processing | guaranteed delivery
Patterns: Point-to-point (one consumer per message) | Pub/Sub (all subscribers get message) | Dead letter queue (failed messages for debugging)
When: email/notification sending | image/video processing | analytics ingestion | cross-service communication | any op where user doesn't need immediate result
Technologies: Kafka (high throughput, log-based, event streaming) | RabbitMQ (flexible routing, task queues) | SQS (managed, AWS) | Redis Streams (lightweight)
Rate Limiting
Problem: protect services from abuse and cascading overload.
| Algorithm | Mechanism | Pros | Cons | |-----------|-----------|------|------| | Token Bucket | tokens refill at fixed rate | allows bursts, simple | memory per user | | Leaking Bucket | queue with fixed processing rate | smooth output | no burst flexibility | | Fixed Window | count per time window | simple | burst at edges | | Sliding Window Log | track each request timestamp | precise | memory-intensive | | Sliding Window Counter | hybrid fixed + weighted | good balance | approximate |
Token Bucket is industry standard (AWS, Stripe, GitHub). Implementation: API gateway or per-service | Redis counters with TTL | return 429 with Retry-After and X-RateLimit headers
CDN
Problem: static content from origin adds latency for distant users.
How: assets cached at edge servers worldwide | DNS routes to nearest edge | cache miss fetches from origin
Push vs Pull: Push (upload to CDN, infrequent changes) | Pull (CDN fetches on first request, simpler)
Invalidation: URL versioning (preferred) | CDN API purge | TTL expiration
Bloom Filters
Problem: quickly check "is X in set?" without storing full set.
How: bit array + k hash functions | insert sets k bits | query checks k bits | false positives possible, false negatives impossible
Used for: cache penetration prevention | duplicate URL detection (crawlers) | spam filtering
Config: 10 bits per element ~ 1% false positive rate | cannot delete (use Counting Bloom Filter)
Unique ID Generation
| Approach | Sortable | Size | Coordination | Throughput | |----------|----------|------|-------------|------------| | UUID v4 | No | 128b | None | Unlimited | | DB auto-inc | Yes | 64b | High | Limited | | Ticket server | Yes | 64b | Medium | Limited | | Snowflake | Yes | 64b | Minimal | Very high |
Snowflake: [1 unused | 41 timestamp | 5 datacenter | 5 machine | 12 sequence] -- ~4M IDs/sec/DC | clock sync via NTP is Achilles heel
Fan-out Strategies
Fan-out on write (push): post immediately written to all followers' feeds | read is instant | expensive for celebrities
Fan-out on read (pull): feed computed at read time | write is fast | read is slow
Hybrid (production): push for normal users | pull for celebrities (>10K followers)
Real-time Communication
Long Polling: server holds request open until data or timeout | simple, resource-intensive
WebSocket: full-duplex persistent | low latency | stateful (complicates LB -- need sticky sessions)
SSE: server pushes over HTTP | unidirectional | auto-reconnect | simpler for notification/feed
Geohashing and Spatial Indexing
Geohash: encodes lat/lon to string, nearby share prefix | precision by length (4=39km, 6=1.2km, 8=38m) | boundary problem: query target + 8 neighbors
Quadtree: recursive subdivision into 4 quadrants | adaptive to density | in-memory, 200M items ~1.7GB
Geohash vs Quadtree: geohash simpler (string prefix), DB-friendly | quadtree adaptive to density, in-memory only
Data Replication Strategies
Single-leader: one primary writes, replicas read | simple but SPOF
Multi-leader: multiple write nodes, conflict resolution | better for multi-DC
Leaderless (Dynamo): any node reads/writes | quorum W+R>N | W=1,R=N fast writes | W=N,R=1 fast reads | W=R=N/2+1 balanced | anti-entropy + read repair
Conflict Resolution
LWW: timestamp-based, simple but lossy | Vector clocks: detect conflicts, app resolves | CRDTs: auto-merge data types | Application-level: present to user (like Git)
CAP Theorem
CP (consistency): reject writes during partition | HBase, MongoDB, Redis Cluster | financial transactions
AP (availability): accept writes, resolve later | Cassandra, DynamoDB, CouchDB | social feeds, shopping carts
Real question: "what happens during network partition?" | most systems need availability for reads, consistency for certain writes | tunable consistency (Cassandra) gives flexibility
Write-Ahead Log (WAL)
Before applying mutation, write to append-only log | acknowledge to client | periodically apply to data structure | on crash: replay from last checkpoint. Used in PostgreSQL, MySQL, Cassandra, Kafka.
Gossip Protocol
Each node maintains member list with heartbeat counters | periodically exchanges state with random peer | propagates in O(log N) rounds. Used for membership, failure detection, config propagation.