hld-design

High-Level Design (HLD) Framework

Structured methodology for designing production systems. Works for interview prep and real architecture decisions. Each phase builds on the last — skip none.

Context

System to design: $ARGUMENTS

If no system is provided, ask: "What system are you designing? What's the scale and key constraints?"

---

Phase 1: Clarify Requirements (5 min)

Never design in a vacuum. Extract:

Functional requirements — what the system must do:

• Core user-facing features (list 3-5 specific capabilities)
• What does success look like for the user?

Non-functional requirements — how the system must perform:

• Scale: DAU, QPS (reads vs writes), data volume
• Latency: p50/p99 targets (e.g., "search results < 200ms p99")
• Availability: 99.9% (8.7h downtime/year) vs 99.99% (52min/year)
• Consistency: strong vs eventual — where does it matter?
• Durability: what data cannot be lost?

Constraints and assumptions:

• Read-heavy or write-heavy? (affects caching and replication strategy)
• Global or regional? (affects latency, data residency, CDN needs)
• Expected growth: 2x in 6 months? 10x in 2 years?

Out of scope — explicitly state what you're NOT designing.

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Phase 2: Capacity Estimation (5 min)

Back-of-envelope numbers that drive architecture decisions. Quantify before designing.

Traffic:

DAU = X million
Reads per user per day = Y
Writes per user per day = Z

Read QPS  = DAU × Y / 86,400
Write QPS = DAU × Z / 86,400
Peak QPS  = avg × 3-5x (account for traffic spikes)

Storage:

Object size = X KB (e.g., tweet = ~300 bytes, image = ~200 KB)
Daily writes = Write QPS × 86,400
3-year storage = Daily writes × 365 × 3

Bandwidth:

Inbound  = Write QPS × avg object size
Outbound = Read QPS × avg response size

Cache:

Hot data = 20% of reads hit 80% of data (Pareto)
Cache size = hot data × avg object size

State your assumptions explicitly. Numbers don't need to be perfect — they need to inform decisions.

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Phase 3: High-Level Components (10 min)

Draw the system as boxes and arrows. For each component, state its role and why it exists.

Standard components to consider:

[Clients] -> [CDN] -> [Load Balancer] -> [API Gateway]
                                              |
                          +---------+---------+---------+
                          |         |                   |
                     [Service A] [Service B]       [Service C]
                          |         |                   |
                     [Cache]   [Message Queue]    [Search Index]
                          |         |                   |
                     [Primary DB] [Worker]         [Object Store]
                          |
                     [Read Replica]

For each service/component, explain:

• What it does (single responsibility)
• Why it's separate (coupling, scaling, failure isolation)
• Technology choice with brief rationale

Common patterns:

• Stateless API servers behind a load balancer (horizontal scaling)
• CDN for static assets and geographic distribution
• Message queue to decouple async work from request path
• Read replicas to offload read traffic from primary
• Cache layer (Redis) for hot reads

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Phase 4: Data Model (5 min)

Define the core entities and relationships before writing APIs.

For each entity:

• Fields (name, type, constraints)
• Primary key strategy (UUID, auto-increment, composite)
• Indexes needed for query patterns
• Embed vs reference decision (if document store)

Access pattern analysis:

Query: "Get user by email" -> Index on email (unique)
Query: "Get posts by user, sorted by time" -> Composite index (user_id, created_at DESC)
Query: "Get all comments for a post" -> Index on post_id

State the dominant query patterns first, then design the schema to serve them.

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Phase 5: API Design (5 min)

Define the external contract. Use REST unless there's a specific reason for GraphQL or gRPC.

For each endpoint:

POST /api/v1/users
Authorization: Bearer <token>
Request:  { email, password, displayName }
Response: { id, email, displayName, createdAt }
Errors:   400 (validation), 409 (email taken), 500 (server error)

Include:

• Versioning strategy (URL path /v1/ is simplest)
• Auth mechanism (JWT Bearer, API key, OAuth2)
• Pagination (cursor-based for large/real-time sets, offset for admin views)
• Rate limiting (429 Too Many Requests, include Retry-After header)
• Idempotency keys for write operations that must not double-execute

---

Phase 6: Deep Dives (10 min)

Pick 2-3 hardest sub-problems and solve them in detail. Common picks:

Feed generation:

• Fan-out on write (push) vs fan-out on read (pull)
• Hybrid: push for normal users, pull for celebrities with 10M+ followers

Search:

• Inverted index (Elasticsearch) vs full-text in PostgreSQL (pg_trgm)
• Tokenization, stemming, ranking (BM25 vs vector embeddings)

Notifications (real-time):

• WebSockets vs Server-Sent Events vs long polling
• Fanout: who gets notified and when

Media upload:

• Pre-signed S3 URLs (client uploads directly, bypasses your servers)
• Async transcoding via message queue after upload

Distributed rate limiting:

• Token bucket in Redis with atomic Lua script
• Sliding window log vs fixed window counter tradeoffs

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Phase 7: Failure Modes (5 min)

Enumerate what can fail and how the system handles it.

Component	Failure	Detection	Recovery
Primary DB	Crash	Health check + replica lag monitor	Promote replica, update DNS (< 30s)
Cache (Redis)	Eviction / miss	Cache hit rate metric	Serve from DB, warm cache async
Message queue	Consumer lag	Queue depth metric > threshold	Scale consumers, alert
External API	Timeout / 5xx	Circuit breaker (half-open after 30s)	Fallback response or queue for retry
API server	Memory leak	RSS growth + OOM kill	Horizontal scaling + auto-restart

For each failure: detection -> recovery -> prevention

Also address:

• Data consistency on partial writes (idempotency, sagas)
• Split-brain scenarios (for distributed state)
• Thundering herd on cache miss (probabilistic early expiration, mutex lock)

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Output Format

## System Design: [System Name]

### Requirements
**Functional:** ...
**Non-functional:** ...
**Out of scope:** ...

### Capacity Estimates
| Metric | Calculation | Result |
|--------|-------------|--------|

### Architecture Diagram
[ASCII or Mermaid diagram]

### Component Breakdown
| Component | Role | Technology | Rationale |

### Data Model
[Schema with fields, indexes, relationships]

### API Contract
[Key endpoints with request/response]

### Deep Dives
[2-3 detailed sub-problems solved]

### Failure Modes
[Table: component, failure, detection, recovery]

### Tradeoffs
[3-5 explicit tradeoffs made in this design]