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System Design

A practical framework for designing distributed systems and architecting scalable services. This skill covers the core building blocks - load balancers, databases, caches, queues, and CDNs - plus the trade-off reasoning required to use them well. It is built around interview scenarios because they compress the full design process into a repeatable structure you can also apply in real-world architecture decisions. Agents can use this skill to work through any system design problem from capacity estimation through detailed component design.

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When to use this skill

Trigger this skill when the user:

• Asks "how would you design X?" where X is a product or service
• Needs to choose between SQL and NoSQL databases
• Is evaluating load balancing, sharding, or replication strategies
• Asks about the CAP theorem or consistency vs availability trade-offs
• Is designing a caching strategy (what to cache, where, how to invalidate)
• Needs to estimate traffic, storage, or bandwidth for a system
• Is preparing for a system design interview
• Asks about rate limiting, API gateways, or CDN placement

Do NOT trigger this skill for:

• Line-level code review or specific algorithm implementations (use a coding skill)
• DevOps/infrastructure provisioning details like Terraform or Kubernetes manifests

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Key principles

1. Start simple and justify complexity - Design the simplest system that satisfies the requirements. Introduce each new component (queue, cache, shard) only when you can name the specific constraint it solves. Complexity is a cost, not a feature.

2. Network partitions will happen - choose C or A - CAP theorem says distributed systems must sacrifice either consistency or availability during a partition. You cannot avoid partitions (P is not a choice). Pick CP for financial and inventory data; pick AP for feeds, caches, and preferences.

3. Scale horizontally, partition vertically - Stateless services scale out behind a load balancer. Data scales by separating hot from cold paths: read replicas before sharding, sharding before multi-region. Vertical scaling buys time; horizontal scaling buys headroom.

4. Design for failure at every layer - Every service will go down. Every disk will fill. Design fallback behavior before the happy path. Timeouts, retries with backoff, circuit breakers, and bulkheads are not optional refinements - they are table stakes.

5. Single responsibility for components - A component that does two things will be bad at both. Load balancers balance load. Caches serve reads. Queues decouple producers from consumers. Mixing responsibilities creates invisible coupling that makes the system fragile under load.

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Core concepts

System design assembles six core building blocks. Each solves a specific problem.

Load balancers distribute requests across backend instances. L4 balancers route by TCP/IP; L7 balancers route by HTTP path, headers, and cookies. Use L7 for HTTP services. Algorithms: round-robin (default), least-connections (when request latency varies), consistent hashing (when you need sticky routing, e.g., cache affinity).

Caches reduce read latency and database load. Sit in front of the database. Patterns: cache-aside (default), write-through (strong consistency), write-behind (high write throughput, tolerate loss). Key concerns: TTL, invalidation strategy, and stampede prevention. Redis is the default; Memcached only when pure key-value at massive scale.

Databases are the source of truth. SQL for structured data with ACID transactions; NoSQL for scale, flexible schemas, or specific access patterns. Read replicas for read-heavy workloads. Sharding for write-heavy workloads that exceed one node.

Message queues decouple producers from consumers and absorb traffic spikes. Use for async work, fan-out events, and unreliable downstream dependencies. Always configure a dead-letter queue. SQS for AWS-native work; Kafka for high-throughput event streaming or replay.

CDNs cache static assets and edge-terminate TLS close to users. Reduces origin load and cuts latency for geographically distributed users. Use for images, JS/CSS, and any content with high read-to-write ratio.

API gateways enforce cross-cutting concerns - auth, rate limiting, request logging, TLS termination - at a single entry point. Never build a custom gateway; use Kong, Envoy, or a managed provider.

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Common tasks

Design a URL shortener

Clarifying questions: Read-heavy or write-heavy? Need analytics? Custom slugs? Global or single-region?

Components:

1. API service (stateless, horizontally scaled) behind L7 load balancer
2. Key generation service - pre-generate Base62 short codes in batches and store in a pool; avoids hot write path
3. Database - a relational DB works at moderate scale; switch to Cassandra for multi-region or >100k writes/sec
4. Cache (Redis) - store short_code -> long_url mappings; TTL 24 hours; cache-aside

Redirect flow: Client hits CDN -> cache hit returns 301/302 -> cache miss reads DB -> populates cache -> returns redirect.

Scale signal: 100M URLs stored, 10B reads/day -> cache hit rate must be >99% to protect the DB.

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Design a rate limiter

Algorithm choices:

• Token bucket (default) - allows bursts up to bucket capacity; fills at a constant rate. Best for user-facing APIs.
• Fixed window - simple counter per time window. Prone to burst at window edge.
• Sliding window log - exact, but memory-intensive.
• Sliding window counter - approximation using two fixed windows. Good balance.

Storage: Redis with atomic INCR and EXPIRE. Single Redis node is enough up to ~50k RPS per rule; use Redis Cluster for more.

Placement: In the API gateway (preferred) or as middleware. Always return X-RateLimit-Remaining and Retry-After headers with 429 responses.

Distributed concern: With multiple gateway nodes, the counter must be centralized (Redis) - local counters undercount.

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Design a notification system

Components:

1. Notification API - accepts events from internal services
2. Router service - reads user preferences and determines channels (push, email, SMS)
3. Channel-specific workers (separate services) - dequeued from per-channel queues
4. Template service - renders notification copy
5. Delivery tracking - records sent/delivered/failed per notification

Queue design: One queue per channel (push-queue, email-queue, sms-queue). Isolates failure - SMS provider outage does not back up email delivery.

Critical path vs non-critical path:

• OTP and security alerts: synchronous, priority queue
• Marketing and social notifications: async, best-effort, can be batched

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Design a chat system

Protocol: WebSockets for real-time bidirectional messaging. Long-polling as fallback for restrictive networks.

Storage split:

• Message history: Cassandra, keyed by (channel_id, timestamp). Append-only, high write throughput, easy time-range queries.
• User presence and metadata: Redis (in-memory, fast reads).
• User and channel info: PostgreSQL (relational, ACID).

Fanout: When a user sends a message, the server writes to the DB and then publishes to a pub/sub channel (Redis Pub/Sub or Kafka). Each recipient's connection server subscribes to relevant channels and pushes to the WebSocket.

Scale concern: Connection servers are stateful (WebSockets). Route users to the same connection server with consistent hashing. Use a service mesh for connection server discovery.

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Choose between SQL vs NoSQL

Use this decision table:

Need	Choose
ACID transactions across multiple entities	SQL
Complex joins and ad-hoc queries	SQL
Strict schema with referential integrity	SQL
Horizontal write scaling beyond single node	NoSQL (Cassandra, DynamoDB)
Flexible or evolving schema	NoSQL (MongoDB, DynamoDB)
Graph traversals	Graph DB (Neo4j)
Time-series data at high ingestion rate	TimescaleDB or InfluxDB
Key-value at very high throughput	Redis or DynamoDB

Default: Start with PostgreSQL. It handles far more scale than most teams expect and its JSONB column covers flexible-schema needs up to moderate scale. Migrate to specialized stores when you have a measured bottleneck.

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Estimate system capacity

Use the following rough constants in back-of-envelope estimates:

Metric	Value
Seconds per day	~86,400 (~100k rounded)
Bytes per ASCII character	1
Average tweet/post size	~300 bytes
Average image (compressed)	~300 KB
Average video (1 min, 720p)	~50 MB
QPS from 1M DAU, 10 actions/day	~115 QPS

Process:

1. Clarify scale (DAU, requests per user per day)
2. Derive QPS: (DAU * requests_per_day) / 86400
3. Derive peak QPS: average QPS * 2-3x
4. Derive storage: writes_per_day * record_size * retention_days
5. Derive bandwidth: peak QPS * average_response_size

State assumptions explicitly. Interviewers care about your reasoning, not the exact number.

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Design caching strategy

Step 1 - Identify what to cache:

• Expensive reads that change infrequently (user profiles, product catalog)
• Computed aggregations (dashboard stats, leaderboards)
• Session tokens and auth lookups

Do NOT cache: frequently mutated data, financial balances, anything requiring strong consistency.

Step 2 - Choose pattern:

• Default: cache-aside with TTL
• Strong read-after-write: write-through
• High write throughput, loss acceptable: write-behind

Step 3 - Define invalidation:

• TTL expiry for most cases
• Explicit DELETE on write for cache-aside
• Never try to update a cached value in-place; DELETE then let the next read repopulate

Step 4 - Prevent stampede:

• Use a distributed lock (Redis SETNX) for high-traffic keys
• Add jitter to TTLs (base TTL +/- 10-20%) to spread expiry

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Anti-patterns / common mistakes

Mistake	Why it's wrong	What to do instead
Designing without clarifying requirements	You optimize for the wrong bottleneck and miss key constraints	Always spend 5 minutes on scope: scale, consistency needs, latency SLAs
Sharding before replication	Sharding is complex and expensive; replication + caching handles most read bottlenecks	Add read replicas and caching first; only shard when writes are the bottleneck
Shared database between services	Creates hidden coupling; one service's slow query can kill another	One database per service; expose data through APIs or events
Cache without invalidation plan	Stale reads cause data inconsistency; cache-DB drift grows silently	Define TTL and invalidation triggers before adding any cache
Ignoring the tail: all QPS estimates as average	p99 latency matters more than p50; a 2x peak multiplier is the minimum	Always model peak QPS (2-3x average) and design capacity for it
Single point of failure at every layer	Load balancer with no standby, single queue broker, one region	Identify SPOFs explicitly; add redundancy for any component whose failure kills the system

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References

For detailed frameworks and opinionated defaults, read the relevant file from the references/ folder:

• references/interview-framework.md - step-by-step interview process (RESHADED), time allocation, common follow-up questions, and how to communicate trade-offs

Only load the references file when the task requires it - it is long and will consume context.

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Related skills

When this skill is activated, check if the following companion skills are installed. For any that are missing, mention them to the user and offer to install before proceeding with the task. Example: "I notice you don't have [skill] installed yet - it pairs well with this skill. Want me to install it?"

• clean-architecture - Designing, reviewing, or refactoring software architecture following Robert C.
• microservices - Designing microservice architectures, decomposing monoliths, implementing inter-service...
• database-engineering - Designing database schemas, optimizing queries, creating indexes, planning migrations, or...
• performance-engineering - Profiling application performance, debugging memory leaks, optimizing latency,...

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