System Design Primer
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Approaching an interview question

A four-step framework for leading an open-ended design conversation.

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  1. Step 1: Outline use cases, constraints, and assumptions
  2. Step 2: Create a high level design
  3. Step 3: Design core components
  4. Step 4: Scale the design
  5. Back-of-the-envelope calculations

How to tackle a system design interview question.

The system design interview is an open-ended conversation. You are expected to lead it.

You can use the following steps to guide the discussion. To help solidify this process, work through the System design interview questions with solutions section using the following steps.

Step 1: Outline use cases, constraints, and assumptions#

Gather requirements and scope the problem. Ask questions to clarify use cases and constraints. Discuss assumptions.

  • Who is going to use it?
  • How are they going to use it?
  • How many users are there?
  • What does the system do?
  • What are the inputs and outputs of the system?
  • How much data do we expect to handle?
  • How many requests per second do we expect?
  • What is the expected read to write ratio?

Step 2: Create a high level design#

Outline a high level design with all important components.

  • Sketch the main components and connections
  • Justify your ideas

Step 3: Design core components#

Dive into details for each core component. For example, if you were asked to design a url shortening service, discuss:

  • Generating and storing a hash of the full url
    • MD5 and Base62
    • Hash collisions
    • SQL or NoSQL
    • Database schema
  • Translating a hashed url to the full url
    • Database lookup
  • API and object-oriented design

Step 4: Scale the design#

Identify and address bottlenecks, given the constraints. For example, do you need the following to address scalability issues?

  • Load balancer
  • Horizontal scaling
  • Caching
  • Database sharding

Discuss potential solutions and trade-offs. Everything is a trade-off. Address bottlenecks using principles of scalable system design.

Back-of-the-envelope calculations#

You might be asked to do some estimates by hand. Refer to the Appendix for the following resources:

Sources and further reading4 links

Check out the following links to get a better idea of what to expect:

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