Design Facebook - a social network

Facebook is an online social networking service where users can connect with other users to post and read messages. Users access Facebook through their website interface or mobile apps.

System Requirements

We will focus on the following set of requirements while designing Facebook:

Each member should be able to add information about their basic profile, work experience, education, etc.
Any user of our system should be able to search other members, groups or pages by their name.
Members should be able to send and accept/reject friend requests from other members.
Members should be able to follow other members without becoming their friend.
Members should be able to create groups and pages, as well as join already created groups, and follow pages.
Members should be able to create new posts to share with their friends.
Members should be able to add comments to posts, as well as like or share a post or comment.
Members should be able to create privacy lists containing their friends. Members can link any post with a privacy list to make the post visible only to the members of that list.
Any member should be able to send messages to other members.
Any member should be able to add a recommendation for any page.
The system should send a notification to a member whenever there is a new message or friend request or comment on their post.
Members should be able to search through posts for a word.

Extended Requirement: Write a function to find a connection suggestion for a member.

Use case diagram

We have three main Actors in our system:

Member: All members can search for other members, groups, pages, or posts, as well as send friend requests, create posts, etc.
Admin: Mainly responsible for admin functions like blocking and unblocking a member, etc.
System: Mainly responsible for sending notifications for new messages, friend requests, etc.

Here are the top use cases of our system:

Add/update profile: Any member should be able to create their profile to reflect their work experiences, education, etc.
Search: Members can search for other members, groups or pages. Members can send a friend request to other members.
Follow or Unfollow a member or a page: Any member can follow or unfollow any other member or page.
Send message Any member can send a message to any of their friends.
Create post Any member can create a post to share with their friends, as well as like or add comments to any post visible to them.
Send notification The system will be able to send notifications for new messages, friend requests, etc.

Class diagram

Here are the main classes of the Facebook system:

Member: This will be the main component of our system. Each member will have a profile which includes their Work Experiences, Education, etc. Members will be connected to other members and they can follow other members and pages. Members will also have suggestions to send friend requests to other members.
Search: Our system will support searching for other members, groups and pages by their names, and through posts for any word.
Message: Members can send messages to other members with text, photos, and videos.
Post: Members can create posts containing text and media, as well as like and share a post.
Comment: Members can add comments to posts as well as like any comment.
Group: Members can create and join groups.
PrivacyList: Members can create privacy lists containing their friends. Members can link any post with a privacy list, to make the post visible only to the members of that list.
Page: Members can create pages that other members can follow, and share messages there.
Notification: This class will take care of sending notifications to members. The system will be able to send a push notification or an email.

Class diagram

Activity diagrams

Add work experience to profile: Any Facebook member can perform this activity. Here are the steps to add work experience to a member’s profile:

Create a new post: Any Member can perform this activity. Here are the steps for creating a post:

Code

Here is the high-level definition for the classes described above.

Enums, data types, and constants: Here are the required enums, data types, and constants:

Python

Java

Python

class ConnectionInvitationStatus(Enum):
  PENDING, ACCEPTED, REJECTED, CANCELED = 1, 2, 3, 4


class AccountStatus(Enum):
  ACTIVE, CLOSED, CANCELED, BLACKLISTED, DISABLED = 1, 2, 3, 4, 5


class Address:
  def __init__(self, street, city, state, zip_code, country):
    self.__street_address = street
    self.__city = city
    self.__state = state
    self.__zip_code = zip_code
    self.__country = country

Account, Person, Member, and Admin: These classes represent the different people that interact with our system:

Python

Java

Python

# For simplicity, we are not defining getter and setter functions. The reader can
# assume that all class attributes are private and accessed through their respective
# public getter methods and modified only through their public methods function.

class Account:
  def __init__(self, id, password, status=AccountStatus.Active):
    self.__id = id
    self.__password = password
    self.__status = status

  def reset_password(self):
    None


# from abc import ABC, abstractmethod
class Person(ABC):
  def __init__(self, name, address, email, phone, account):
    self.__name = name
    self.__address = address
    self.__email = email
    self.__phone = phone
    self.__account = account


class Member(Person):
  def __init__(self, id, date_of_membership, name):
    self.__member_id = id
    self.__date_of_membership = date_of_membership
    self.__name = name

    self.__profile = Profile()
    self.__member_follows = []
    self.__member_connections = []
    self.__page_follows = []
    self.__member_suggestions = []
    self.__connection_invitations = []
    self.__group_follows = []

  def send_message(self, message):
    None

  def create_post(self, post):
    None

  def send_connection_invitation(self, invitation):
    None

  def search_member_suggestions(self):
    None


class Admin(Person):
  def block_user(self, customer):
    None

  def unblock_user(self, customer):
    None

  def enable_page(self, page):
    None

  def disable_page(self, page):
    None


class ConnectionInvitation:
  def __init__(self, member_invited, name, status=ConnectionInvitationStatus.PENDING):
    self.__member_invited = member_invited
    self.__status = status
    self.__date_created = datetime.date.today()
    self.__date_updated = datetime.date.today()

  def accept_connection(self):
    None

  def reject_connection(self):
    None

Profile and Work: A member’s profile will have their work experiences, educations, places, etc:

Python

Java

Python

class Profile:
  def __init__(self, profile_picture, cover_photo, gender):
    self.__profile_picture = profile_picture
    self.__cover_photo = cover_photo
    self.__gender = gender

    self.__work_experiences = []
    self.__educations = []
    self.__places = []
    self.__stats = []

  def add_work_experience(self, work):
    None

  def add_education(self, education):
    None

  def add_place(self, place):
    None


class Work:
  def __init__(self, title, company, location, date_from, date_to, description):
    self.__title = title
    self.__company = company
    self.__location = location
    self.__from = date_from
    self.__to = date_to
    self.__description = description

Page and Recommendation: Each page can have multiple recommendations, and members will follow/like pages:

Python

Java

Python

class Page:
  def __init__(self, id, name, description, type, total_members):
    self.__page_id = id
    self.__name = name
    self.__description = description
    self.__type = type
    self.__total_members = total_members
    self.__recommendation = []

  def get_recommendation(self):
    return self.__recommendation


class Recommendation:
  def __init__(self, id, rating, description):
    self.__recommendation_id = id
    self.__rating = rating
    self.__description = description
    self.__created_at = datetime.date.today()

Group, Post, Message, and Comment: Members can create posts, comment on posts, send messages and join groups:

Python

Java

Python

class Group:
  def __init__(self, id, name, description, total_members):
    self.__group_id = id
    self.__name = name
    self.__description = description
    self.__total_members = total_members
    self.__members = []

  def add_member(self, member):
    None

  def update_description(self, description):
    None


class Post:
  def __init__(self, id, text, total_likes, total_shares, owner):
    self.__post_id = id
    self.__text = text
    self.__total_likes = total_likes
    self.__total_shares = total_shares
    self.__owner = owner


class Message:
  def __init__(self, id, sent_to, body, media):
    self.__message_id = id
    self.__sent_to = sent_to
    self.__message_body = body
    self.__media = media

  def add_member(self, member):
    None


class Comment:
  def __init__(self, id, text, total_likes, owner):
    self.__comment_id = id
    self.__text = text
    self.__total_likes = total_likes
    self.__owner = owner

Search interface and SearchIndex: SearchIndex will implement Search to facilitate searching of members, groups, pages, and posts:

Python

Java

Python

from abc import ABC, abstractmethod

class Search(ABC):
  def search_member(self, name):
    None

  def search_group(self, name):
    None

  def search_page(self, name):
    None

  def search_post(self, word):
    None


class SearchIndex(Search):
  def __init__(self):
    self.__member_names = {}
    self.__group_names = {}
    self.__page_titles = {}
    self.__posts = {}

  def add_member(self, member):
    if member.get_name() in self.__member_names:
      self.__member_names.get(member.get_name()).add(member)
    else:
      self.__member_names[member.get_name()] = member

  def add_group(self, group):
    None

  def add_page(self, page):
    None

  def add_post(self, post):
    None

  def search_member(self, name):
    return self.__member_names.get(name)

  def search_group(self, name):
    return self.__group_names.get(name)

  def search_page(self, name):
    return self.__page_titles.get(name)

  def search_post(self, word):
    return self.__posts.get(word)

Extended requirement

Here is the code for finding connection suggestions for a member.

There can be many strategies to search for connection suggestions; we will do a two-level deep breadth-first search to find people who have the most connections with each other. These people could be good candidates for a connection suggestion, here is the sample Java code:

import java.util.HashSet;
import java.util.LinkedHashMap;
import java.util.Map;
import java.util.stream.Collectors;
import static java.util.Collections.reverseOrder;

public class Member extends Person {
  private Integer memberId;
  private Date dateOfMembership;
  private String name;

  private Profile profile;
  private HashSet<Integer> memberFollows;
  private HashSet<Integer> memberConnections;
  private HashSet<Integer> pageFollows;
  private HashSet<Integer> memberSuggestions;
  private HashSet<ConnectionInvitation> connectionInvitations;
  private HashSet<Integer> groupFollows;

  public boolean sendMessage(Message message);
  public boolean createPost(Post post);
  public boolean sendConnectionInvitation(ConnectionInvitation invitation);

  private Map<Integer, Integer> searchMemberSuggestions() {
    Map<Integer, Integer> suggestions = new HashMap<>();
    for(Integer memberId : this.memberConnections) {
      HashSet<Integer> firstLevelConnections = new Member(memberId).getMemberConnections());
      for(Integer firstLevelConnectionId : firstLevelConnections) {
        this.findMemberSuggestion(suggestions, firstLevelConnectionId);
        HashSet<Integer> secondLevelConnections = new Member(firstLevelConnectionId).getMemberConnections());
        for(Integer secondLevelConnectionId : secondLevelConnections) {
          this.findMemberSuggestion(suggestions, secondLevelConnectionId);
        }
      }
	  }

    // sort by value (increasing count), i.e., by highest number of mutual connection count
    Map<Integer, Integer> result = new LinkedHashMap<>();
    suggestions.entrySet().stream()
        .sorted(reverseOrder(Map.Entry.comparingByValue()))
        .forEachOrdered(x -> result.put(x.getKey(), x.getValue()));

    return result;
  }

  private void findMemberSuggestion(Map<Integer, Integer> suggestions, Integer connectionId) {
    // return if the proposed suggestion is already a connection or if there is a
    // pending connection invitation
    if(this.memberConnections.contains(connectionId) ||
        this.connectionInvitations.contains(connectionId)) {
      return;
    }

    int count = suggestions.containsKey(connectionId) ? suggestions.get(connectionId) : 0;
    suggestions.put(connectionId, count + 1);
  }
}

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