System Design Questions

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As a technologist I love system designs. I like to tell interview candidates you don’t need to practice them, it’s what you do every day. In fact, it’s one of the favorite parts of my technical role. I never liked crossword puzzles, but I could totally do system design problems instead. Here’s a great list ..

Most of these are copied from the system design interview books/tutorials.

Theory

Why do these?

For All System Design Qs

Users and Use Cases

High Level Design

APIs

Data

Connections

What to call this?

Polling - Wake up every ‘k’ seconds and ask

Push - Usually better (always?), only down side is harder to implement, more server load?

Long Poll - Open a http request, leave it open for a while then answer on the server. - Weird hack - What if client dies

Bi-directional - E.g. WebSocket (ideal) - Ideal - Setup over HTTP - Traverses Fire walls

TCP vs UDP

WebSocket vs HTTP/2 + Server Side Events (SSE)

Throughput/Availability

  • Load balancing/bottlenecks/fault points
  • Caching, purging, replication, consistency
  • Serving data from edge CDN

Capacity/Constraints

  • Capacity
  • Scale and usage estimates (include memory, data transmission)
  • There are 10e5 seconds in a day
  • Other

Regionalization I18N

  • Per locale laws
  • Languages
  • Data practices
  • Geo-distributed
  • Edge/CDN

Non Functional requriemetns

  • Latency
  • Scalability
  • Availability
  • Reliability
  • Observability
    • Test ability
    • Monitoring
    • Metrics
    • Logging
    • Alarming
  • Deployment
  • Supportability
  • Non Functional Requirements
    • Security, authentication, authorization

Capacity Estimation

Money: Monetization + COGS

Specific Tech

Geo Approaches

  • As the crow flies - lat/long search - 2d union is expensive
  • Geo Hash - Use each digit to divide space into quadrants
  • Quad Tree - Geo Divide but so leafs are ‘equal sized’

Design and Architecture

Loading (/design)

Cloud first applications

Loading (/td/cloud-first-applications)

Data Systems

Loading (/td/data-systems)

Security

Loading (/td/better-security-design)

SQL vs No-SQL

Todo Link to Data Systems

the table

Type TPS Notes
Key Value (Dynamo) 40K Upto 5 Indexes
SQL (Aurora) ??  

Use cases

Proximity Service

Google Maps

Distributed Message Queue

Metrics Monitoring and Alerting System

Ad Click Event Aggregation

Hotel Reservation System

Distributed Email Service

S3-like Object Storage

Real-time Gaming Leaderboard

Payment System

Digital Wallet

Stock Exchange

Rate limiter

Consistent hashing

Key-value store

Unique id generator

Url shortener

Web crawler

Notification system

News feed system

Chat system

  • 1:1 vs Group
  • Online vs Offlie
  • External Notification System
  • Presence
    • How to detect user is no longer online
    • How to propagate presence to other users (similar to propagate messages)

Search Auto Complete

Youtube

Google Drive

Nearby Friends

begin

sequenceDiagram
    participant U as Montoring User
    participant U2 as Moving User
    participant W_PS as WebPubSub
    participant FL_PS as FriendLocationPubSub
    participant F_DB as FriendsDB
    par Initialize Connection
        U ->> W_PS: RegisterFriendLocationChange()
        W_PS ->> F_DB: GetFriends()
        W_PS ->> FL_PS: RegisterFriendLocationChange (friends)
    end
    par A friend changes their location, initial sub generates location
        U2 ->> W_PS: NewLocation(location)
        W_PS ->> FL_PS: NewLocation(U2,location)
        FL_PS ->>  W_PS: OnFriendLocationChange
        NOTE over W_PS: Check Close Enough <br> Coalse and Debounce
        W_PS ->>  U: OnFriendLocationChange
    end

end

classDiagram

class FriendsDB {
User:UserId
Friend:UserId
CanSee:bool
}
class UserLocationHistory {
ID: AutoIncrement
User:UserId
Location:LocData
Time: TimeStamp
}

Resources

System design resources

Online: - https://github.com/donnemartin/system-design-primer

Add Links to Books * System Design Interviews 1 and 2