System Design Interview: An Insider’s Guide

1 System Design Interview: An Insider s GuideAll rights reserved. This book or any portion thereof may not be reproduced or used in anymanner whatsoever without the express written permission of the publisher except for the useof brief quotations in a book the author:Alex Xu is an experienced software engineer and entrepreneur. Previously, he worked atTwitter, Apple, Zynga and Oracle. He received his from Carnegie Mellon has a passion for designing and implementing complex subscribe to our email list if you want to be notified when new chapters are available: more information, contact Paul SolomonTable of ContentsSystem Design Interview: An Insider s GuideFORWARDCHAPTER 1: SCALE FROM ZERO TO MILLIONS OF USERSCHAPTER 2: BACK-OF-THE-ENVELOPE ESTIMATIONCHAPTER 3: A FRAMEWORK FOR System Design INTERVIEWSCHAPTER 4: Design A RATE LIMITERCHAPTER 5: Design CONSISTENT HASHINGCHAPTER 6: Design A KEY-VALUE STORECHAPTER 7: Design A UNIQUE ID GENERATOR IN DISTRIBUTED SYSTEMSCHAPTER 8: Design A URL SHORTENERCHAPTER 9: Design A WEB CRAWLERCHAPTER 10: Design A NOTIFICATION SYSTEMCHAPTER 11: Design A NEWS FEED SYSTEMCHAPTER 12: Design A CHAT SYSTEMCHAPTER 13: Design A SEARCH AUTOCOMPLETE SYSTEMCHAPTER 14: Design YOUTUBECHAPTER 15: Design GOOGLE DRIVECHAPTER 16.

2 THE LEARNING CONTINUESAFTERWORDFORWARDWe are delighted that you have decided to join us in learning the System Design Design interview questions are the most difficult to tackle among all the technicalinterviews. The questions require the interviewees to Design an architecture for a softwaresystem, which could be a news feed, Google search, chat System , etc. These questions areintimidating, and there is no certain pattern to follow. The questions are usually very bigscoped and vague. The processes are open-ended and unclear without a standard or widely adopt System Design interviews because the communication and problem-solving skills tested in these interviews are similar to those required by a software engineer sdaily work. An interviewee is evaluated based on how she analyzes a vague problem and howshe solves the problem step by step. The abilities tested also involve how she explains theidea, discusses with others, and evaluates and optimizes the System .

3 In English, using she flows better than he or she or jumping between the two. To make reading easier, we use thefeminine pronoun throughout this book. No disrespect is intended for male System Design questions are open-ended. Just like in the real world, there are manydifferences and variations in the System . The desired outcome is to come up with anarchitecture to achieve System Design goals. The discussions could go in different waysdepending on the interviewer. Some interviewers may choose high-level architecture to coverall aspects; whereas some might choose one or more areas to focus on. Typically, systemrequirements, constraints and bottlenecks should be well understood to shape the direction ofboth the interviewer and objective of this book is to provide a reliable strategy to approach the System designquestions. The right strategy and knowledge are vital to the success of an book provides solid knowledge in building a scalable System .

4 The more knowledgegained from reading this book, the better you are equipped in solving the System book also provides a step by step framework on how to tackle a System Design provides many examples to illustrate the systematic approach with detailed steps that youcan follow. With constant practice, you will be well-equipped to tackle System designinterview 1: SCALE FROM ZERO TO MILLIONS OFUSERSD esigning a System that supports millions of users is challenging, and it is a journey thatrequires continuous refinement and endless improvement. In this chapter, we build a systemthat supports a single user and gradually scale it up to serve millions of users. After readingthis chapter, you will master a handful of techniques that will help you to crack the systemdesign interview server setupA journey of a thousand miles begins with a single step, and building a complex System is nodifferent. To start with something simple, everything is running on a single server.

5 Figure 1-1shows the illustration of a single server setup where everything is running on one server: webapp, database, cache, understand this setup, it is helpful to investigate the request flow and traffic source. Let usfirst look at the request flow (Figure 1-2).1. Users access websites through domain names, such as Usually, theDomain Name System (DNS) is a paid service provided by 3rd parties and not hosted byour Internet Protocol (IP) address is returned to the browser or mobile app. In the example,IP address is Once the IP address is obtained, Hypertext Transfer Protocol (HTTP) [1] requests aresent directly to your web The web server returns HTML pages or JSON response for , let us examine the traffic source. The traffic to your web server comes from twosources: web application and mobile application. Web application: it uses a combination of server-side languages (Java, Python, etc.) tohandle business logic, storage, etc.

6 , and client-side languages (HTML and JavaScript) forpresentation. Mobile application: HTTP protocol is the communication protocol between the mobileapp and the web server. JavaScript Object Notation (JSON) is commonly used APIresponse format to transfer data due to its simplicity. An example of the API response inJSON format is shown below:GET /users/12 Retrieve user object for id = 12 DatabaseWith the growth of the user base, one server is not enough, and we need multiple servers: onefor web/mobile traffic, the other for the database (Figure 1-3). Separating web/mobile traffic(web tier) and database (data tier) servers allows them to be scaled databases to use?You can choose between a traditional relational database and a non-relational database. Letus examine their databases are also called a relational database management System (RDBMS) orSQL database. The most popular ones are MySQL, Oracle database, PostgreSQL, databases represent and store data in tables and rows.

7 You can perform joinoperations using SQL across different database databases are also called NoSQL databases. Popular ones are CouchDB,Neo4j, Cassandra, HBase, Amazon DynamoDB, etc. [2]. These databases are grouped intofour categories: key-value stores, graph stores, column stores, and document stores. Joinoperations are generally not supported in non-relational most developers, relational databases are the best option because they have been aroundfor over 40 years and historically, they have worked well. However, if relational databasesare not suitable for your specific use cases, it is critical to explore beyond relationaldatabases. Non-relational databases might be the right choice if: Your application requires super-low latency. Your data are unstructured, or you do not have any relational data. You only need to serialize and deserialize data (JSON, XML, YAML, etc.). You need to store a massive amount of scaling vs horizontal scalingVertical scaling, referred to as scale up , means the process of adding more power (CPU,RAM, etc.)

8 To your servers. Horizontal scaling, referred to as scale-out , allows you to scaleby adding more servers into your pool of traffic is low, vertical scaling is a great option, and the simplicity of vertical scaling isits main advantage. Unfortunately, it comes with serious limitations. Vertical scaling has a hard limit. It is impossible to add unlimited CPU and memory to asingle server. Vertical scaling does not have failover and redundancy. If one server goes down, thewebsite/app goes down with it scaling is more desirable for large scale applications due to the limitations ofvertical the previous Design , users are connected to the web server directly. Users will unable toaccess the website if the web server is offline. In another scenario, if many users access theweb server simultaneously and it reaches the web server s load limit, users generallyexperience slower response or fail to connect to the server. A load balancer is the besttechnique to address these balancerA load balancer evenly distributes incoming traffic among web servers that are defined in aload-balanced set.

9 Figure 1-4 shows how a load balancer shown in Figure 1-4, users connect to the public IP of the load balancer directly. With thissetup, web servers are unreachable directly by clients anymore. For better security, privateIPs are used for communication between servers. A private IP is an IP address reachable onlybetween servers in the same network; however, it is unreachable over the internet. The loadbalancer communicates with web servers through private Figure 1-4, after a load balancer and a second web server are added, we successfullysolved no failover issue and improved the availability of the web tier. Details are explainedbelow: If server 1 goes offline, all the traffic will be routed to server 2. This prevents the websitefrom going offline. We will also add a new healthy web server to the server pool tobalance the load. If the website traffic grows rapidly, and two servers are not enough to handle the traffic,the load balancer can handle this problem gracefully.

10 You only need to add more serversto the web server pool, and the load balancer automatically starts to send requests to the web tier looks good, what about the data tier? The current Design has one database,so it does not support failover and redundancy. Database replication is a common techniqueto address those problems. Let us take a replicationQuoted from Wikipedia: Database replication can be used in many database managementsystems, usually with a master/slave relationship between the original (master) and the copies(slaves) [3].A master database generally only supports write operations. A slave database gets copies ofthe data from the master database and only supports read operations. All the data-modifyingcommands like insert, delete, or update must be sent to the master database. Mostapplications require a much higher ratio of reads to writes; thus, the number of slavedatabases in a System is usually larger than the number of master databases.