System Design

A high level break down of key components when doing a system design problem.

Requirements	Estimations	Network	Application	Database
Functional	Calculations	WAN	Load Balancer	SQL
Non-Functional	QPS	Payload	Service Discovery	NoSQL
Architecture	Max QPS	Gateway	Consensus	CAP PACELC
Key Performance Indicators	Storage Bytes	VPC	Compute	Partition
-	Retention Bytes	-	Message Queue	Index
-	In E Gress Bytes	-	Cache	ACID

Requirements

Get requirements and estimations to scope your system design.

Functional

Identity what you are trying to build.

Features	System
1 on 1 chat	Mobile/Web
News feed	Notifications

Non Functional

Identify how your system will behave.

User Experience	System
Full end to end encryption	High Availability
See new post fast	Reliability

Architecture

• High Availability
• Failover

• Reliability
• RPO/RTO

Key Performance Indicators

• Observability
• Metrics

Estimations

Value	Number	Byte
Hundred	-	B
Thousand	K	KB
Million	M	MB
Billion	B	GB
Trillion	T	TB
Quadrillion	Q	PB
Quintillion	Qu	EB
Sextillion	S	ZB
Septillion	Se	YB

Estimate the scope of your users and data points.

Users (DAU)	File Type (File Bytes)	Storage (Retention Time)
100 B Message/day	1 KB (avg)	10 years
10 B Post/day	10 KB (max)	10% new data per year

Calculations

Using your estimations do some back of envelope.

QPS	Max QPS	Storage-Bytes	Retention Bytes	In/E Gress Bytes
DAU / ~100K	2 * QPS	DAU * FileBytes	StorageBytes * Retention Time	(QPS * FileBytes) / 8 Bit

QPS

• Queries Per Second
• Daily Active Users
• Time = Round up (86,400 seconds in day)

Max QPS

• Double Queries Per Second
• Calculate (Read % : Write %) Ratio
  • 1 Read : 10 Write
  • Read % = 1 * Queries Per Second
  • Write % = 10 * Queries Per Second

Storage Bytes

• Storage Bytes
• Daily Active Users
• File Bytes

Retention Bytes

• Retention Bytes
• Storage Bytes
• Retention Time
  • 365 days in year

In E Gress Bytes

• In-gress
• E-gress
• 1 Byte / 8 Bits
• Queries Per Second
• File Bytes

Network

Wide Area Network

• Clients
  • Mobile
  • Browser
  • CLI
• DNS
  • DNSSec (Signed Registries)
  • DNS over HTTPS (Privacy)
• CDN
• APN

Payload

• API
• Request Lines
• Authentication/Authorization
• Protocols

Public Gateway

• Firewall
• Rate Limiter
• Authentication/Authorization
• Reverse Proxy
• Forward Proxy

Virtual Private Cloud

• Access Control List
• Security Group
• Route Table
• Classless Inter-Domain Routing
• Gateway
  • Internet
  • Network Adress Translation
  • VPC Peering

Application

Load Balancer

• Layer 3/4/7 OSI
• Strategy
• Use cases

Service Discovery

• Auto Scale
  • DevOps
• Protocols
• Zeekeeper/etcd

Consensus

• Raft Consensus
• Replicated State Machine

Compute

• Session(less)
• State(full)/(less)
• Server(less)

Message Queue

• Producer
• Consumer

Cache

• Eviction Strategy

Database

SQL

• Relational Data
• Transactions

NoSQL

• Key Value
• Document
• Wide Column
• Graph
• TimeSeries
• Spatial

CAP PACELC

• CAP
  • Consistency
  • Availability
  • Partition
• PACELC
  • Partition == true
    • IF
  • Availability
    • or
  • Consistency
    • ELSE
  • Latency
    • or
  • Consistency

Partition

• Consistent Hashing
• Range Partition
• Vertical Partition

Index

• B Tree
• Skip List
• Hash
• Inverted

ACID

Atomicity

• Transactions
• Rollbacks

Consistency

SQL Consistency

• Schema
• Constraint
• Fan Trap

NoSQL Consistency

• Eventual Consistency
• Sequential Consistency
• Strong Consistency

• Strong Eventual Consistency
• Quorum Replication
• (Dotted) Vector Clocks
• Chain Replication

Isolation

• Race Conditions
• Lock
• Concurrency

Durability

• High Watermark
• Checkpoint
• Write Ahead Log