Functional Requirements:

User Registration and Authentication:

• The application should allow users to register using their phone numbers.
• A one-time password (OTP) will be sent to the user’s phone number for verification.
• Users should be able to log in using their phone number and OTP.
• The system must support multiple authentication methods such as phone number verification or integrating social media accounts for authentication.

Contacts and Groups:

• Users should be able to add and manage contacts, automatically synchronizing with the phone’s contact list.
• The app should allow users to create groups, add contacts to the groups, and manage group settings such as group name, group icon, and permissions.
• Group members should be able to send and receive messages in real-time.

Messaging:

• Real-time text messaging functionality is required.
• Users should be able to send text messages to individuals or groups.
• Support for multimedia messaging such as photos, videos, documents, voice notes, and location sharing.
• Delivery statuses (sent, delivered, read) should be visible for each message.
• The app must support sending and receiving messages while offline, with automatic syncing when the device comes online.

Voice and Video Calls:

• Users should be able to initiate one-on-one voice and video calls.
• Group video calls with up to 8 participants must be supported.
• The app should support high-quality audio and video with minimal delay.
• Users must be notified about incoming calls with the ability to accept or reject calls.

Notifications:

• The app should push notifications for new messages, incoming calls, and group notifications.
• Users must be able to configure notification settings (mute/unmute chats, change notification tones, etc.).
• Notification should work both in the foreground and in the background.

Status Updates:

• Users should be able to post a status update (text, image, or video) that disappears after 24 hours.
• Users should be able to view others’ status updates.
• The status update should allow users to see who viewed their status.

End-to-End Encryption:

• Messages, calls, and shared media should be encrypted end-to-end, ensuring that only the sender and recipient can access the data.
• The application should use encryption standards such as AES (Advanced Encryption Standard) and RSA (Rivest–Shamir–Adleman).

Media Sharing and Storage:

• Users should be able to send images, audio files, videos, and documents.
• The application should compress media files to reduce size for easier sharing.
• The app should have an internal media gallery for users to view, delete, or forward received files.
• The app should allow users to save received media files to their devices.

User Profiles:

• Users should have the ability to set a profile picture, display name, and status message.
• The app should allow users to update their profile information at any time.
• Users should be able to set privacy settings for who can view their profile picture, last seen, and status.

Search Functionality:

• The app should provide a search functionality that allows users to search for individual chats, group chats, contacts, and media files.
• Search results should be displayed instantly as the user types, ensuring a seamless user experience.

Non-Functional Requirements:

Performance:

• The app should support real-time message delivery with minimal latency.
• The app should be optimized for both high-speed and low-speed internet connections, ensuring good performance on 2G, 3G, 4G, and Wi-Fi networks.
• Voice and video calls should have minimal delay, with automatic bandwidth adjustments to maintain quality.

Scalability:

• The application should be scalable to support millions of active users worldwide.
• It should handle high volumes of concurrent users sending messages, making calls, and sharing media without degradation in performance.

Usability:

• The application should have an intuitive, user-friendly interface that is easy to navigate.
• The app should support multiple languages to cater to users worldwide.
• It should be compatible with both Android and iOS devices, with the user interface (UI) optimized for different screen sizes and orientations.

Security:

• The app must use HTTPS for all communication over the internet.
• All media shared on the platform should be encrypted.
• User data should be securely stored and comply with privacy regulations like GDPR.
• The application must have secure recovery methods in case of account hijacking, such as two-factor authentication (2FA).

Compatibility:

• The app should be compatible with both Android and iOS operating systems.
• It should support different screen resolutions, including high-definition displays.
• The app should provide an optimized experience on a variety of mobile devices and tablet sizes.

Reliability:

• The app should be available with minimal downtime and provide fault tolerance in the event of server failure.
• The app must include mechanisms to back up and restore messages and media, ensuring users do not lose their data.

Backup and Syncing:

• The app should allow users to back up their chat history, contacts, and media files to the cloud (Google Drive, iCloud).
• Users should have the option to restore their data when reinstalling the app or switching devices.

Battery and Data Usage:

• The app should be optimized for battery usage to prevent excessive consumption while running in the background.
• It should have the ability to adjust data usage settings, allowing users to limit data consumption (e.g., turning off auto-download of media over cellular networks).

Compliance with Regulatory Standards:

• The app must comply with applicable laws and regulations in each country, including data protection and privacy regulations.
• It should allow users to report inappropriate content and block abusive contacts or groups.

Additional Features (Future Enhancements):

Chatbots and Automation:

• Integration of chatbots for automating replies in business accounts.
• Ability for businesses to provide customer service through automated messaging.

Dark Mode:

• Providing a dark mode option for users who prefer a darker user interface, especially in low-light environments.

Augmented Reality (AR) Filters:

• Integration of AR filters for video calling to enhance user experience with fun effects.

Multi-Device Support:

• Allow users to access their WhatsApp account simultaneously across multiple devices, including desktop web apps or standalone desktop apps.

Conclusion:

WhatsApp is a feature-rich messaging platform that allows for seamless communication, media sharing, and real-time conversations. Its core functionalities revolve around easy user registration, instant messaging, media sharing, voice and video calling, and strong encryption to ensure privacy. By adhering to both functional and non-functional requirements, WhatsApp can maintain scalability, reliability, and security, which are essential for delivering a smooth user experience to millions of users globally.