Low-Level Design: Polling System : Basic

Table of Contents

1. Problem Statement
2. Assumptions
3. Requirements
   • Create Polls
   • Manage Polls
   • Vote in Polls
   • View Poll Results
   • Poll Data
4. Implementation Details
   • Functions/Methods
     • Create Poll
     • Update Poll
     • Delete Poll
     • Vote In Poll
     • View Poll Results
5. Data Models
   • Poll
   • Vote
6. Implementation Details
7. CODE

Problem Statement

You are required to design and implement an online polling system. The system should allow users to create, manage, and participate in polls. Each poll consists of a question and multiple options for answers. Users can vote on polls and view the results.

Assumptions :

• We are usin a Class based implementation
• We are not using additional Database to store this information
• As not DB & server is involved, no REST API
• simple Low Level approch on Polling System

Requirements

Create Polls:

• Users should be able to create a new poll with a question and multiple answer options.
• Each poll must have a unique identifier, a question, a list of options, and a timestamp of creation.

Manage Polls:

• Users should be able to update the question or options of an existing poll.
• Users should be able to delete a poll.

Vote in Polls:

• Users should be able to cast a vote for one of the options in a poll.
• Each user can only vote once per poll.

View Poll Results:

• Users should be able to view the current results of a poll, including the number of votes for each option.

Poll Data:

• Store polls, options, and votes in a way that allows efficient retrieval and updates.
• Ensure data integrity and consistency, especially when multiple users are voting simultaneously.

Implementation Details

Functions/Methods:

Create Poll

createPoll :

• Input: question (string), options (array of strings)
• Output: pollId (string), message (string)
• Example: createPoll("What is your favorite color?", ["Red", "Blue", "Green", "Yellow"]) returns {"pollId": "123", "message": "Poll created successfully."}

Update Poll

updatePoll :

• Input: pollId (string), question (string), options (array of strings)
• Output: message (string)
• Example: updatePoll("123", "Updated question?", ["Option1", "Option2"]) returns {"message": "Poll updated successfully."}

Delete Poll

deletePoll :

• Input: pollId (string)
• Output: message (string)
• Example: deletePoll("123") returns {"message": "Poll deleted successfully."}

Vote In Poll

voteInPoll :

• Input: pollId (string), userId (string), option (string)
• Output: message (string)
• Example: voteInPoll("123", "user1", "Option1") returns {"message": "Vote cast successfully."}

View Poll Results

viewPollResults :

• Input: pollId (string)
• Output: pollId (string), question (string), results (object with option keys and vote count values)
• Example: viewPollResults("123") returns {"pollId": "123", "question": "What is your favorite color?", "results": {"Red": 10, "Blue": 5, "Green": 3, "Yellow": 2}}

Data Models

• Poll:

{
  "pollId": "123",
  "question": "What is your favorite color?",
  "options": ["Red", "Blue", "Green", "Yellow"],
  "createdAt": "2024-07-11T00:00:00Z"
}

• Vote:

{
  "pollId": "123",
  "userId": "user1",
  "option": "Red",
  "timestamp": "2024-07-11T01:00:00Z"
}

Implementation Details

This code defines a basic polling system using JavaScript classes. It allows the creation, management, and voting in polls. Let's break down each part of the code:

1. Class Definitions

Poll Class

• Purpose: Represents an individual poll.
• Constructor Parameters:
  • id: A unique identifier for the poll.
  • question: The question being asked in the poll.
  • options: An array of possible options that users can vote on.
• Properties:
  • pollId: Stores the unique ID of the poll.
  • question: Stores the poll question.
  • options: Stores the array of options.
  • createdAt: Stores the creation date and time of the poll.

Vote Class

• Purpose: Represents an individual vote cast by a user.
• Constructor Parameters:
  • pollId: The ID of the poll the vote is associated with.
  • userId: The ID of the user who cast the vote.
  • option: The option that the user voted for.
• Properties:
  • pollId: Stores the poll ID for which the vote was cast.
  • userId: Stores the user ID of the voter.
  • option: Stores the option that the user voted for.
  • timestamp: Stores the date and time when the vote was cast.

2. PollManager Class

Purpose: Manages the entire polling system, including creating polls, managing votes, and viewing results.

Constructor:

• Properties:
  • polls: A Map that stores all polls, where the key is the poll ID and the value is the Poll object.
  • pollResults: A Map that stores the results of each poll, where the key is the poll ID and the value is another Map that tracks votes for each option.
  • userVotes: A Map that stores the votes by each user, where the key is the poll ID and the value is another Map that tracks whether a user has voted in that poll.

Methods:

• createPoll(question, options)

  • Generates a new poll with a unique ID.
  • Initializes the poll results, setting the vote count for each option to 0.
  • Stores the poll and returns the generated poll ID.

• updatePoll(pollId, question, options)

  • Updates the question and options for an existing poll.
  • Resets the poll results to 0 for the updated options.
  • Returns a success message if the poll is found, otherwise returns "Poll not found."

• deletePoll(pollId)

  • Deletes a poll by its ID.
  • Also removes associated poll results and user votes.
  • Returns a success message if the poll is found, otherwise returns "Poll not found."

• voteInPoll(pollId, userId, option)

  • Allows a user to cast a vote in a specific poll.
  • Ensures a user can only vote once per poll.
  • Updates the vote count for the selected option if valid.
  • Returns appropriate messages depending on whether the vote was successful, the user had already voted, or the poll or option was invalid.

• viewPollResults(pollId)

  • Returns the results of a specific poll in an array format, where each entry is a tuple of the option and its vote count.
  • Returns "Poll not found" if the poll doesn't exist.

3. Example Usage

• Poll Creation: A poll is created asking, "What is your favorite color?" with options ["Red", "Blue", "Green", "Yellow"]. The poll ID is generated as "1".
• Voting: Multiple users vote in the poll, and the system ensures users can only vote once.
• Viewing Results: The poll results are displayed, showing the vote counts for each option.
• Updating the Poll: The poll's question and options are updated, resetting the results.
• Voting in Updated Poll: A user votes in the updated poll, and the results are displayed.
• Poll Deletion: The poll is deleted, and an attempt to view the results afterward confirms the poll no longer exists.

This implementation provides a basic but functional polling system, handling common scenarios such as poll creation, updating, voting, and deletion.

CODE

class Poll {
    constructor(id, question, options) {
        this.pollId = id;
        this.question = question;
        this.options = options;
        this.createdAt = new Date();
    }
}

class Vote {
    constructor(pollId, userId, option) {
        this.pollId = pollId;
        this.userId = userId;
        this.option = option;
        this.timestamp = new Date();
    }
}

class PollManager {
    constructor() {
        this.polls = new Map();
        this.pollResults = new Map();
        this.userVotes = new Map();
    }

    createPoll(question, options) {
        const pollId = (this.polls.size + 1).toString();
        const poll = new Poll(pollId, question, options);
        this.polls.set(pollId, poll);

        const result = new Map();
        options.forEach(option => result.set(option, 0));
        this.pollResults.set(pollId, result);

        return pollId;
    }

    updatePoll(pollId, question, options) {
        const poll = this.polls.get(pollId);
        if (poll) {
            poll.question = question;
            poll.options = options;

            // Update results for the new options
            const result = new Map();
            options.forEach(option => result.set(option, 0));
            this.pollResults.set(pollId, result);

            return "Poll updated successfully.";
        }
        return "Poll not found.";
    }

    deletePoll(pollId) {
        if (this.polls.delete(pollId)) {
            this.pollResults.delete(pollId);
            this.userVotes.delete(pollId);
            return "Poll deleted successfully.";
        }
        return "Poll not found.";
    }

    voteInPoll(pollId, userId, option) {
        const poll = this.polls.get(pollId);
        if (poll) {
            if (!this.userVotes.has(pollId)) {
                this.userVotes.set(pollId, new Map());
            }

            const userVote = this.userVotes.get(pollId);
            if (userVote.get(userId)) {
                return "User has already voted.";
            }

            const result = this.pollResults.get(pollId);
            if (result.has(option)) {
                result.set(option, result.get(option) + 1);
                userVote.set(userId, true);
                return "Vote cast successfully.";
            } else {
                return "Invalid option.";
            }
        }
        return "Poll not found.";
    }

    viewPollResults(pollId) {
        const results = this.pollResults.get(pollId);
        if (results) {
            return Array.from(results.entries());
        }
        return "Poll not found.";
    }
}

// Example usage
const pollManager = new PollManager();

// Creating a poll
const pollId = pollManager.createPoll("What is your favorite color?", ["Red", "Blue", "Green", "Yellow"]);
console.log("Poll created with ID:", pollId);

// Voting in the poll
let voteMessage = pollManager.voteInPoll(pollId, "user1", "Red");
console.log(voteMessage);

voteMessage = pollManager.voteInPoll(pollId, "user2", "Blue");
console.log(voteMessage);

voteMessage = pollManager.voteInPoll(pollId, "user1", "Green");
console.log(voteMessage); // Should inform the user has already voted

// Viewing poll results
let results = pollManager.viewPollResults(pollId);
console.log("Poll results for poll ID", pollId, ":", results);

// Updating the poll
let updateMessage = pollManager.updatePoll(pollId, "What is your favorite primary color?", ["Red", "Blue", "Yellow"]);
console.log(updateMessage);

// Voting in the updated poll
voteMessage = pollManager.voteInPoll(pollId, "user3", "Yellow");
console.log(voteMessage);

// Viewing updated poll results
results = pollManager.viewPollResults(pollId);
console.log("Updated poll results for poll ID", pollId, ":", results);

// Deleting the poll
let deleteMessage = pollManager.deletePoll(pollId);
console.log(deleteMessage);

// Attempting to view results of a deleted poll
results = pollManager.viewPollResults(pollId);
if (typeof results === "string") {
    console.log(results);
}


// Response
// Poll created with ID: 1
// Vote cast successfully.
// Vote cast successfully.
// User has already voted.
// Poll results for poll ID 1 : [['Red', 1], ['Blue', 1], ['Green', 0], ['Yellow', 0]]
// Poll updated successfully.
// Vote cast successfully.
// Updated poll results for poll ID 1 : [['Red', 0], ['Blue', 0], ['Yellow', 1]]
// Poll deleted successfully.
// Poll not found.

Note: Please follow for more detail & Enchanced version of this article with DB, API & other system Design Concept.
Low-Level Design: Polling System - API's Using Nodejs & SQL

Low-Level Design: Polling System - Edge Cases

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