system design.md

🧠 System Design Fundamentals — Engineer Gist

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1️⃣ Scalability

Meaning: Ability of system to handle growth (users, data, traffic).

Types

• Vertical → Bigger machine
• Horizontal → More machines (preferred)

flowchart LR
User --> LB
LB --> S1
LB --> S2
LB --> S3

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2️⃣ Availability

Meaning: System is accessible when needed.

Example: 99.99% uptime = ~52 min downtime/year

flowchart LR
User --> Primary
Primary --> Backup

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3️⃣ Reliability

Meaning: System works correctly over time (no data loss, correct results).

flowchart LR
User --> Service --> DB
DB --> BackupDB

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4️⃣ Latency vs Throughput vs Bandwidth

Term	Meaning
Latency	Time for one request
Throughput	Requests per second
Bandwidth	Data transfer capacity

flowchart LR
User -->|"Latency"| Server
Server -->|"Throughput"| ManyUsers

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5️⃣ Client-Server Architecture

flowchart LR
Client --> Server
Server --> Database

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6️⃣ Databases

Persistent storage for data.

Types:

• Relational
• Key-value
• Document
• Graph

flowchart LR
App --> DB[(Database)]

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7️⃣ SQL vs NoSQL

SQL	NoSQL
Structured	Flexible
ACID	BASE
Joins	Denormalized

flowchart LR
App --> SQLDB[(SQL DB)]
App --> NoSQLDB[(NoSQL DB)]

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8️⃣ Load Balancing

Distributes traffic across servers.

flowchart LR
User --> LB
LB --> S1
LB --> S2

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9️⃣ Load Balancing Algorithms

• Round Robin
• Least Connections
• IP Hash

flowchart LR
LB --> S1
LB --> S2
LB --> S3

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🔟 Caching

Store frequently accessed data in fast storage.

flowchart LR
User --> Cache
Cache --> DB

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1️⃣1️⃣ Cache Invalidation

Hardest problem 😄

Strategies:

• TTL
• Write-through
• Cache-aside

flowchart LR
App --> Cache
Cache --> DB

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1️⃣2️⃣ CDN (Content Delivery Network)

flowchart LR
User --> CDN
CDN --> OriginServer

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1️⃣3️⃣ DNS

Domain → IP resolution.

flowchart LR
Browser --> DNS
DNS --> ServerIP

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1️⃣4️⃣ API Design

Principles:

• Versioning
• Idempotency
• Pagination
• Error codes

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1️⃣5️⃣ REST APIs

flowchart LR
Client -->|GET/POST| REST_API
REST_API --> DB

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1️⃣6️⃣ Authentication vs Authorization

AuthN	AuthZ
Who are you	What can you access

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1️⃣7️⃣ Session vs Token Auth

flowchart LR
User --> Server
Server --> SessionStore

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flowchart LR
User -->|JWT| Server

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1️⃣8️⃣ OAuth / OAuth2 / OpenID Connect

• OAuth → Authorization delegation
• OAuth2 → Modern standard
• OIDC → Identity layer on OAuth2

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1️⃣9️⃣ JWT

flowchart LR
User --> AuthServer
AuthServer --> JWT
JWT --> API

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2️⃣0️⃣ Rate Limiting

Protects system from abuse.

flowchart LR
User --> RateLimiter --> API

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2️⃣1️⃣ Single Point of Failure (SPOF)

flowchart LR
User --> OnlyServer

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2️⃣2️⃣ High Availability vs Fault Tolerance

HA	FT
Minimal downtime	Zero interruption

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2️⃣3️⃣ CAP Theorem

Pick 2:

• Consistency
• Availability
• Partition Tolerance

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2️⃣4️⃣ Consistency Models

• Strong
• Eventual
• Causal

flowchart LR
Node1 --> Node2
Node2 --> Node3

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2️⃣5️⃣ Data Replication

flowchart LR
Primary --> Replica1
Primary --> Replica2

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2️⃣6️⃣ Read Replicas

flowchart LR
App --> PrimaryDB
App --> ReadReplica

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2️⃣7️⃣ Sharding

Split data across DBs.

flowchart LR
Router --> Shard1
Router --> Shard2
Router --> Shard3

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2️⃣8️⃣ Data Partitioning

Horizontal → rows Vertical → columns

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2️⃣9️⃣ Consistent Hashing

flowchart LR
Key --> HashRing
HashRing --> Node

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3️⃣0️⃣ Denormalization

Store duplicate data for faster reads.

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3️⃣1️⃣ Indexing

flowchart LR
Query --> Index --> Data

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3️⃣2️⃣ Microservices Architecture

flowchart LR
API --> UserService
API --> OrderService
API --> PaymentService

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3️⃣3️⃣ Monolithic Architecture

flowchart LR
Client --> Monolith --> DB

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3️⃣4️⃣ Serverless Architecture

flowchart LR
User --> API_Gateway --> Lambda --> DB

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3️⃣5️⃣ Event-Driven Architecture

flowchart LR
Producer --> EventBus --> Consumer

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3️⃣6️⃣ Message Queues

flowchart LR
Producer --> Queue --> Worker

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3️⃣7️⃣ Pub/Sub

flowchart LR
Publisher --> Topic
Topic --> Sub1
Topic --> Sub2

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3️⃣8️⃣ Sync vs Async Communication

Sync

sequenceDiagram
Client->>Server: Request
Server-->>Client: Response

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Async

sequenceDiagram
Client->>Queue: Job
Worker->>Queue: Pull Job
Worker-->>Client: Done later

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⭐ Mental Model (If You Remember Only This)

Traffic → DNS → CDN → Load Balancer → App → Cache → DB → Replicas → Backup