Requirements of Quora's Design

Requirements

Let’s understand the functional and non-functional requirements below:

Functional requirements

A user should be able to perform the following functionalities:

• Questions and answers: Users can ask questions and give answers. Questions and answers can include images and videos.

• Upvote/downvote and comment: It is possible for users to upvote, downvote, and comment on answers.

• Search: Users should have a search feature to find questions already asked on the platform by other users.

• Recommendation system: A user can view their feed, which includes topics they’re interested in. The feed can also include questions that need answers or answers that interest the reader. The system should facilitate user discovery with a recommender system.

• Ranking answers: We enhance user experience by ranking answers according to their usefulness. The most helpful answer will be ranked highest and listed at the top.

Non-functional requirements

• Scalability: The system should scale well as the number of features and users grow with time. It means that the performance and usability should not be impacted by an increasing number of users.

• Consistency: The design should ensure that different users’ views of the same content should be consistent. In particular, critical content like questions and answers should be the same for any collection of viewers. However, it is not necessary that all users of Quora see a newly posted question, answer, or comment right away.

• Availability: The system should have high availability. This applies to cases where servers receive a large number of concurrent requests.

• Performance: The system should provide a smooth experience to the user without a noticeable delay.

Resource estimation

In this section, we’ll make an estimate about the resource requirements for Quora service. We’ll make assumptions to get a practical and tractable estimate. We’ll estimate the number of servers, the storage, and the bandwidth required to facilitate a large number of users.

Assumptions: It is important to base our estimation on some underlying assumptions. We, therefore, assume the following:

• There are a total of 1 billion users, out of which 300 million are daily active users.

• Assume 15% of questions have an image, and 5% of questions have a video embedded in them. A question cannot have both at the same time.

• We’ll assume an image is estimated to be 250 KBs, and a video is considered 5 MBs.

Number of servers estimation

Let’s estimate our requests per second (RPS) for our design. If there are an average of 300 million daily active users and each user can generate 20 requests per day, then the total number of requests in a day will be:

Estimating RPS

Daily active users	300	million
Requests per day per user	20
Requests Per Second (RPS)	f69444

We already established in the back-of-the-envelope calculations chapter that we’ll use the following formula to estimate a pragmatic number of servers:

Therefore, the total number of servers required to facilitate 300 million users generating an average of 69,500 requests per second will be 37,500.

Storage estimation

Let’s keep in mind our assumption that 15% of questions have images and 5% have videos. So, we’ll make the following assumptions to estimate the storage requirements for our design:

• Each of the 300 million active users posts 1 question in a day, and each question has 2 responses on average, 10 upvotes, and 5 comments in total.

• The collective storage required for the textual content (including the question, answer(s), and comment(s) text) of one question equals 100 ��100 KB.

Storage Requirements Estimation Calculator

Questions per user	1	per day
Total questions per day	f300	millions
Size of textual content per question	100	KB
Image size	250	KB
Video size	5	MB
Questions containing images	15	percent
Questions containing videos	5	percent
Storage for textual content	f30	TB
Storage for image content	f11.25	TB
Storage for video content	f75	TB

See Detailed Calculations

Bandwidth estimation

The bandwidth estimate requires the calculation of incoming and outgoing data through the network.

Bandwidth Requirements Estimation Calculator

Total storage required per day	116.25	TB
Incoming traffic bandwidth	f11	Gbps
Questions viewed per user	20	per day
Total questions viewed	f69444	per second
Bandwidth for text of all questions	f55.56	Gbps
Bandwidth for 15% of image content	f20.83	Gbps
Bandwidth for 5% of video content	f138.89	Gbps
Outgoing traffic bandwidth	f215.3	Gbps

Detailed Calculations

• Load balancers will be used to divide the traffic load among the service hosts.

• Databases are essential for storing all sorts of data, such as user questions and answers, comments, and likes and dislikes. Also, user data will be stored in the databases. We may use different types of databases to store different data.

• A distributed caching system will be used to store frequently accessed data. We can also use caching to store our view counters for different questions.

• The blob store will keep images and video files.