system-design（系统设计）

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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.

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

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.

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.

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.

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.

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

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.

Choose between SQL vs NoSQL

Use this decision table:

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.

Estimate system capacity

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

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.

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

Anti-patterns / common mistakes

Gotchas

1. CAP theorem is about partitions, not a free choice - You cannot "choose" to sacrifice partition tolerance. P is always present in distributed systems. The real choice is between C and A when a partition occurs. Framing it as a three-way trade-off is wrong.
2. Caching invalidation is the hard part, not caching itself - Most designs add Redis without defining when data becomes stale. The moment a cache-aside entry is written, define the exact condition that invalidates it. "We'll figure that out later" causes stale reads in production.
3. Read replicas have replication lag - Writes go to the primary; reads from replicas may be 10-100ms stale. If you route reads to replicas immediately after writes (e.g., "create, then fetch profile"), users will see the old version. Use read-after-write consistency or route critical reads to primary.
4. Consistent hashing does not eliminate hotspots - If one key receives dramatically more requests than others (celebrity user, viral post), consistent hashing still routes all requests for that key to the same shard. Solve with key-based sharding variants like adding a suffix, or cache at a higher layer.
5. Message queues do not guarantee exactly-once delivery - SQS standard queues deliver at-least-once; consumers must be idempotent. Kafka can deliver exactly-once within a single cluster but not across network boundaries. Design consumers to handle duplicate messages before relying on queue semantics.

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.

Companion check

On first activation of this skill in a conversation: check which companion skills are installed by running ls ~/.claude/skills/ ~/.agent/skills/ ~/.agents/skills/.claude/skills/.agent/skills/.agents/skills/ 2>/dev/null. Compare the results against the recommended_skills field in this file's frontmatter. For any that are missing, mention them once and offer to install: `` npx skills add AbsolutelySkilled/AbsolutelySkilled --skill <name> ` Skip entirely if recommended_skills` is empty or all companions are already installed.