System Design
Learn System Design
Introduction to System Design
How to Learn System Design?
Key Characteristics of Distributed Systems
Scalability
Availability
Latency and Performance
Concurrency and Coordination
Monitoring and Observability
Resilience and Error Handling
Fault Tolerance vs. High Availability
Flashcards Review
Chapter Assessment
Network Essentials
HTTP vs. HTTPS
TCP vs. UDP
HTTP: 1.0 vs. 1.1 vs 2.0 vs. 3.0
URL vs. URI vs. URN
What Happens When You Type a URL into the Browser
Flashcards Review
Chapter Assessment
Long-Polling vs. WebSockets vs. Server-Sent Events
Introduction to Real-Time Communication
What is Long-Polling?
What is WebSocket?
What are Server-Sent Events?
Difference Between Long-Polling, WebSockets, and Server-Sent Events
Flashcards Review
Chapter Assessment
Domain Name System (DNS)
Introduction to DNS
DNS Resolution Process
DNS Load Balancing and High Availability
Flashcards Review
Chapter Assessment
Proxies
What is a Proxy Server?
Uses of Proxies
VPN vs. Proxy Server
Flashcards Review
Chapter Assessment
Load Balancing
Introduction to Load Balancing
Load Balancing Algorithms
Uses of Load Balancing
Load Balancer Types
Stateless vs. Stateful Load Balancing
High Availability and Fault Tolerance
Scalability and Performance
Challenges of Load Balancers
Flashcards Review
Chapter Assessment
API Gateway
Introduction to API Gateway
Usage of API gateway
Advantages and disadvantages of using API gateway
Flashcards Review
Chapter Assessment
API Design
What Is an API?
What Are REST APIs?
Resources, Not Actions
HTTP Methods and Their Semantics
URL Design
Request and Response Shapes
Status Codes and Error Design
Pagination from the Consumer's View
Idempotency Keys
Versioning and Backward Compatibility
Concurrency and Conditional Requests
REST vs gRPC vs GraphQL
Flashcards Review
Chapter Assessment
Rate Limiting and Throttling
What Is Rate Limiting
Rate Limiting Algorithms
Distributed Rate Limiting
Rate Limiting in Practice
Flashcards Review
Chapter Assessment
Caching
Introduction to Caching
Why is Caching Important?
Types of Caching
Cache Replacement Policies
Cache Invalidation
Cache Read Strategies
Cache Coherence and Consistency Models
Caching Challenges
Cache Performance Metrics
Flashcards Review
Chapter Assessment
CDN
What is CDN?
Origin Server vs. Edge Server
CDN Architecture
Push CDN vs. Pull CDN
Flashcards Review
Chapter Assessment
Data Partitioning
Introduction to Data Partitioning
Partitioning Methods
Data Sharding Techniques
Benefits of Data Partitioning
Common Problems Associated with Data Partitioning
Flashcards Review
Chapter Assessment
Redundancy and Replication
What is Redundancy?
What is Replication?
Replication Methods
Data Backup vs. Disaster Recovery
Flashcards Review
Chapter Assessment
CAP & PACELC Theorems
Introduction to CAP Theorem
Components of CAP Theorem
Trade-offs in CAP Theorem
Examples of CAP Theorem in Practice
Beyond CAP Theorem
System Design Trade-offs in Interviews
Flashcards Review
Chapter Assessment
Databases (SQL vs. NoSQL)
Introduction to Databases
SQL Databases
NoSQL Databases
SQL vs. NoSQL
ACID vs BASE Properties
Real-World Examples and Case Studies
SQL Normalization and Denormalization
In-Memory Database vs. On-Disk Database
Data Replication vs. Data Mirroring
Database Federation
Flashcards Review
Chapter Assessment
Indexes
What are Indexes?
How a B-Tree Index Works
Types of Indexes
B-Tree vs. LSM Tree
Indexes in Distributed Systems
Flashcards Review
Chapter Assessment
Bloom Filters
Introduction to Bloom Filters
Benefits & Limitations of Bloom Filters
Variants and Extensions of Bloom Filters
Applications of Bloom Filters
Flashcards Review
Chapter Assessment
Quorum
Why Quorum?
What is Quorum?
Flashcards Review
Chapter Assessment
Leader and Follower
What is Leader and Follower Pattern?
Flashcards Review
Chapter Assessment
Heartbeat
What is Heartbeat?
Flashcards Review
Chapter Assessment
Checksum
What is Checksum?
Uses of Checksum
Flashcards Review
Chapter Assessment
Distributed Messaging System
Introduction to Messaging System
Introduction to Kafka
Messaging patterns
Popular Messaging Queue Systems
RabbitMQ vs. Kafka vs. ActiveMQ
Scalability and Performance
Flashcards Review
Chapter Assessment
Distributed File Systems
What is a Distributed File System?
Architecture of a Distributed File System
Key Components of a DFS
Flashcards Review
Chapter Assessment
Security
What is Security and Privacy?
What is Authentication?
What is Authorization?
Authentication vs. Authorization
OAuth vs. JWT for Authentication
What is Encryption?
What are DDoS Attacks?
Flashcards Review
Chapter Assessment
Misc Concepts
Batch Processing vs. Stream Processing
XML vs. JSON
Synchronous vs. Asynchronous Communication
Push vs. Pull Notification Systems
Microservices vs. Serverless Architecture
Message Queues vs. Service Bus
Stateful vs. Stateless Architecture
Event-Driven vs. Polling Architecture
Flashcards Review
Chapter Assessment
Quiz - System Design Fundamentals
Quiz
How to Approach a System Design Interview
System Design Interviews - A step by step guide
Functional vs. Non-functional Requirements
What are Back-of-the-Envelope Estimations?
Things to Avoid During System Design Interview
System Design Master Template
System Design Master Template
Quiz
Designing a URL Shortening Service like TinyURL
Designing a URL Shortening Service like TinyURL
Quiz - Designing URL Shortner
Designing Pastebin
Designing Pastebin
Quiz - Designing Pastebin
Designing Instagram
Designing Instagram
Quiz - Designing Instagram
Designing Dropbox
Designing Dropbox
Quiz - Designing Dropbox
Designing Facebook Messenger
Designing Facebook Messenger
Quiz - Designing Facebook Messenger
Designing Twitter
Designing Twitter
Quiz - Designing Twitter
Designing Youtube or Netflix
Designing Youtube or Netflix
Quiz - Designing Youtube
Designing Typeahead Suggestion
Designing Typeahead Suggestion
Quiz - Designing Typeahead Suggestion
Designing an API Rate Limiter
Designing an API Rate Limiter
Quiz - Designing an API Rate Limiter
Designing Twitter Search
Designing Twitter Search
Quiz - Designing Twitter Search
Designing a Web Crawler
Designing a Web Crawler
Quiz - Designing a Web Crawler
Designing Facebook’s Newsfeed
Designing Facebook’s Newsfeed
Quiz - Designing Facebook’s Newsfeed
Designing Yelp or Nearby Friends
Designing Yelp or Nearby Friends
Quiz - Designing Yelp or Nearby Friends
Designing Uber backend
Designing Uber backend
Quiz - Designing Uber backend
Designing Ticketmaster
Designing Ticketmaster
Quiz - Designing Ticketmaster
Dynamo: How to design a key value store?
Dynamo: Introduction
High-Level Architecture
Data Partitioning
Replication
Vector Clocks and Conflicting Data
The Life of Dynamo’s put() & get() Operations
Anti-entropy Through Merkle Trees
Gossip Protocol
Dynamo Characteristics and Criticism
Summary: Dynamo
Quiz: Dynamo
Mock Interview: Dynamo
Designing YouTube Likes Counter (medium)
YouTube Likes Counter
Quiz
Cassandra: How to Design a Wide-column NoSQL Database?
Cassandra: Introduction
High-level Architecture
Replication
Cassandra Consistency Levels
Gossiper
Anatomy of Cassandra's Write Operation
Anatomy of Cassandra's Read Operation
Compaction
Tombstones
Summary: Cassandra
Quiz: Cassandra
Mock Interview: Cassandra
Kafka: How to Design a Distributed Messaging System?
Messaging Systems: Introduction
Kafka: Introduction
High-level Architecture
Kafka: Deep Dive
Consumer Groups
Kafka Workflow
Role of ZooKeeper
Controller Broker
Kafka Delivery Semantics
Kafka Characteristics
Summary: Kafka
Quiz: Kafka
Mock Interview: Kafka
Chubby: How to Design a Distributed Locking Service?
Chubby: Introduction
High-level Architecture
Design Rationale
How Chubby Works
File, Directories, and Handles
Locks, Sequencers, and Lock-delays
Sessions and Events
Master Election and Chubby Events
Caching
Database
Scaling Chubby
Summary: Chubby
Quiz: Chubby
Mock Interview: Chubby
HDFS: How to Design File Storage System?
Hadoop Distributed File System: Introduction
High-level Architecture
Deep Dive
Anatomy of a Read Operation
Anatomy of a Write Operation
Data Integrity & Caching
Fault Tolerance
HDFS High Availability (HA)
HDFS Characteristics
Summary: HDFS
Quiz: HDFS
Mock Interview: HDFS
GFS: How to Design a Distributed File System Storage?
Google File System: Introduction
High-level Architecture
Single Master and Large Chunk Size
Metadata
Master Operations
Anatomy of a Read Operation
Anatomy of a Write Operation
Anatomy of an Append Operation
GFS Consistency Model and Snapshotting
Fault Tolerance, High Availability, and Data Integrity
Garbage Collection
Criticism on GFS
Summary: GFS
Quiz: GFS
Mock Interview: GFS
BigTable: How to Design a Wide Column Storage System?
BigTable: Introduction
BigTable Data Model
System APIs
Partitioning and High-level Architecture
SSTable
GFS and Chubby
Bigtable Components
Working with Tablets
The Life of BigTable's Read & Write Operations
Fault Tolerance and Compaction
BigTable Refinements
BigTable Characteristics
Summary: BigTable
Quiz: BigTable
Mock Interview: BigTable
Designing Reddit (medium)
Design Reddit
Quiz
Designing Notification Service (medium)
Designing a Notification System
Quiz
Design Google Calendar (medium)
Design Google calendar (Medium)
Quiz
Design a Recommendation System (medium)
Design a Recommendation System for Netflix
Quiz
Designing Gmail (medium)
Design Gmail
Quiz
Designing Google News (medium)
Design Google News, a Global News Aggregator System (Medium)
Quiz
Designing Unique ID Generator (medium)
Design Unique ID Generator (Easy)
Quiz
Designing Code Judging System (medium)
Design Code Judging System like LeetCode (Medium)
Quiz
Designing Payment System (hard)
Design Payment System
Quiz
Designing Flash Sale System (hard)
Design a Flash Sale for an E-commerce Site (Hard)
Quiz
Designing Reminder Alert System (hard)
Design a Reminder Alert System
Quiz
System Design Patterns
Introduction: System Design Patterns
1. Bloom Filters
2. Consistent Hashing
3. Quorum
4. Leader and Follower
5. Write-ahead Log
6. Segmented Log
7. High-Water Mark
8. Lease
9. Heartbeat
10. Gossip Protocol
11. Phi Accrual Failure Detection
12. Split Brain
13. Fencing
14. Checksum
15. Vector Clocks
16. CAP Theorem
17. PACELC Theorem
18. Hinted Handoff
19. Read Repair
20. Merkle Trees
Quiz
Importance of Discussing Trade-offs
caching
consistency models
availability
databases
+1
Two candidates reach the same point in a design and say almost the same thing.
The first says: "I'll put a cache in front of the database."
The second says: "I'll put a cache in front of the database. That takes most reads off the database and cuts read latency. It costs us a window where a user can see stale data. I'm accepting that because the requirement says the feed can be a few seconds behind. If this were account balances, I would not."
Both named the same component. Only one of them made a decision. That difference is what this chapter is about.
What a Trade-off Is
A trade-off is what you give up in order to get something else.
It is not a drawback you failed to avoid, and it is not a mistake to apologize for. If one option were better in every way, there would be no decision to make. Nobody would ask you about it. A design question is interesting exactly because every answer costs something.
So when an interviewer asks you to justify a choice, they are not looking for the perfect component. They are checking two things. Do you know what your choice costs? And did you pick it on purpose?
A Sentence You Can Reuse
Most candidates know the trade-offs. What they lack is a habit for saying them out loud during the interview. This pattern gives you one:
I'll use X. That gives us A. It costs us B. I'm accepting B because of requirement R. If R changed, I'd use Y instead.
Five parts: the choice, the gain, the cost, the reason, and the alternative.
The last two matter most. Anyone can list a pro and a con. Tying the decision to a requirement shows you are designing this system, not reciting a comparison table. Naming the alternative shows you considered one.
Three Worked Examples
Choosing a database. "I'll use a key-value store for the session data. All lookups are by session id, so reads take single-digit milliseconds, and the data splits across machines easily. It costs us the ability to query sessions by anything except the id. I'm accepting that because no requirement asks for other queries. If we later need 'all active sessions in this region', I'd add a relational store or a separate index."
Choosing how services communicate. "The order service will publish an event, and the email service will consume it, instead of a direct call. A slow or failed email service then cannot fail an order. It costs us a short window where the order exists and the email has not been sent. I'm accepting that because a late email is fine and a lost order is not. If the caller needed the result before answering the user, I'd make it a direct call."
Choosing a replication setup. "Writes go to a primary and reads go to replicas. That scales reads, and this workload is about ninety percent reads. It costs us replication lag, so a user can write something and not see it on the next read. I'm accepting that in general. But I'd send a user's own reads to the primary right after they write, so everyone sees their own changes."
None of these takes longer than a few sentences, and none needs unusual knowledge. They are ordinary decisions stated completely.
Four Ways to Lose Points
Naming a technology without its cost. "I'll use Kafka" is not a decision. The interviewer cannot tell whether you chose it or only recognized it. Say what it gives you and what it costs.
Comparing without deciding. A balanced comparison that ends without a choice looks like avoiding the decision. Interviewers grade judgment, and you cannot show judgment without committing. Pick one, say why, and move on.
Re-deciding a settled question. If the requirements already say the system must stay available during a regional outage, the consistency question is answered. Do not spend three minutes weighing it again. Say that the requirement settles it, and continue. Time spent on settled questions is taken from the open ones.
Conceding a cost nobody challenged. Some candidates repeat every weakness of their design until they abandon a reasonable choice. State the cost once, calmly, then keep the decision. Raising a concern once shows confidence. Repeating it shows doubt.
When the Interviewer Asks "Why Not the Other One?"
The most common mistake here is to assume you were wrong and switch.
Usually the interviewer is testing whether you know why you chose. Sometimes they are adding new information. You can tell the difference by checking one thing: did a requirement change?
If no requirement changed, keep your choice. Explain the reasoning again, starting from the requirement that drove it. If a requirement did change, say so and update the design: "This now has to work across regions, so the trade-off changes. I'd accept eventual consistency here." Changing your mind for a stated reason is a strength. Changing it only because you were questioned is not.
How This Chapter Is Organized
The rest of the chapter covers the trade-offs that appear most often. They are grouped by where in a system the decision is made, the same way the System Design Master Template is grouped. Read the chapter in order to prepare generally, or go straight to the group you need.
Foundations. The two measurements every design is judged against, and the basic scaling choice: Latency vs Throughput and Horizontal vs Vertical Scaling.
How the request reaches you. Decisions made at the edge, before your code runs: Proxy vs Reverse Proxy, CDN Usage vs Direct Server Serving, and Load Balancer vs API Gateway. Then API Gateway vs Direct Service Exposure and Token Bucket vs Leaky Bucket.
Where the work happens. How the services are shaped: Stateful vs Stateless Architecture and Serverless vs Traditional Server-based. And how they communicate: REST vs RPC, Synchronous vs Asynchronous Communication, and Push vs Pull Architecture. Finally, Polling vs Long-Polling vs WebSockets vs Webhooks and Batch Processing vs Stream Processing.
Where the data is stored. The group that usually decides whether a design works at the stated scale: SQL vs NoSQL, Normalization vs Denormalization, and ACID vs BASE. Then Strong vs Eventual Consistency, Primary-Replica vs Peer-to-Peer Replication, and Read Heavy vs Write Heavy System.
How you make it fast. Where a cache sits, and how it behaves once it is there: Server-Side Caching vs Client-Side Caching and Cache-Aside vs Read-Through, Write-Through vs Write-Back.
Start with Latency vs Throughput, because it gives you the vocabulary the rest of the chapter is written in.
💡 In the interview: you do not need to discuss every trade-off, and trying to do so will cost time you need elsewhere. Give the most detail to the two or three decisions that shape the architecture. Those are usually the data store, the consistency model, and how services communicate. For everything else, one clause is enough: "load balancer here, standard round robin, nothing interesting about it."
Key takeaway: a trade-off is what you give up to get something else, and every real design decision has one. State it in a fixed shape: choice, gain, cost, the requirement that makes the cost acceptable, and the alternative if that requirement changed. Tie each decision to a requirement, commit to it, and do not re-open what the question already settled.
Discussion
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