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1. Requirements Clarifications

• Ask about the exact scope of the problem
  • For a twitter like system design
    • will users of our service be able to post tweets and follow others?
    • Create and display user’s timeline?
    • Tweets contain photos and videos?
    • Front end design too?
    • Searchable tweets?
    • Display hot trending topics?
    • Push notification for new tweets?

2. Back of the envelope estimation

• estimate the scale of the system we are going to design

  • could help later when we will be focusing on scaling, partitioning, load balancing and caching

    • what scale is expected

      • number of new tweets
      • number of tweet views
      • number of timeline generations per sec

    • how much storage will we need

      • videos and photos will play important role on our estimation

    • what network bandwidth usage are we expecting

      • crucial in deciding how we will manage traffic and balance load between servers

3. System Interface Definition

Establish exact contract expected from the system

postTweet(user_id, tweet_data, tweet_location, user_location, timestamp, …)  

generateTimeline(user_id, current_time, user_location, …)  

markTweetFavorite(user_id, tweet_id, timestamp, …)  

4. Define Data Model

Define the data model, to clarify how data will flow between different components of the system. Later, will guide for data partitioning and management.

User: UserID, Name, Email, DoB, CreationData, LastLogin, etc.
Tweet: TweetID, Content, TweetLocation, NumberOfLikes, TimeStamp, etc.
UserFollow: UserID1, UserID2
FavoriteTweets: UserID, TweetID, TimeStamp

5. High level design

• Draw a block diagram with 5 - 6 boxes representing the core components of our system. We need to identify enough components that are needed to solve the actual problem from end2end.
• For twitter
  • Multiple application servers to serve all the read/ write requests with load balancers in front of them for traffic distributions
  • we could have separate servers for handling specific traffic (like much more read than write)
  • Efficient database that can store all the tweets and can support a huge number of reads
  • Also need a distributed file storage system for storing photos and videos

6. Detailed Design

• Dig deeper to 2 or 3 components
  • present different approaches
  • pros and cons
  • explain why we prefer one on the other
  • consider tradeoffs between different options while keeping system contraints in mind

• Question Examples
  • Since we will be storing a massive amount of data, how should we partition our data to distribute it to multiple databases? Should we try to store all the data of a user on the same database? What issue could it cause?
  • How will we handle hot users who tweet a lot or follow lots of people?
  • Since users’ timeline will contain the most recent (and relevant) tweets, should we try to store our data in such a way that is optimized for scanning the latest tweets?
  • How much and at which layer should we introduce cache to speed things up?
  • What components need better load balancing?

7 Identifying and resolving bottlenecks

• Discuss as many bottlenecks as possible and different approaches to mitigate them
  • Any single point of failure in our system?
  • Do we have enough replicas of the data so that if we lose a few servers, we can still serve our users?
  • Do we have enough copies of different services running such that a fewe failures will not cause a total system shutdown?
  • How to monitor performance of our service?

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