## How to approach a system design interview question

> 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](#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](solutions/system_design/pastebin/README.md), discuss:

* Generating and storing a hash of the full url
    * [MD5](solutions/system_design/pastebin/README.md) and [Base62](solutions/system_design/pastebin/README.md)
    * 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](#index-of-system-design-topics).

### Back-of-the-envelope calculations

You might be asked to do some estimates by hand.  Refer to the [Appendix](#appendix) for the following resources:

* [Use back of the envelope calculations](http://highscalability.com/blog/2011/1/26/google-pro-tip-use-back-of-the-envelope-calculations-to-choo.html)
* [Powers of two table](#powers-of-two-table)
* [Latency numbers every programmer should know](#latency-numbers-every-programmer-should-know)

### Source(s) and further reading

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

* [How to ace a systems design interview](https://www.palantir.com/2011/10/how-to-rock-a-systems-design-interview/)
* [The system design interview](http://www.hiredintech.com/system-design)
* [Intro to Architecture and Systems Design Interviews](https://www.youtube.com/watch?v=ZgdS0EUmn70)
* [System design template](https://leetcode.com/discuss/career/229177/My-System-Design-Template)