1. What is the primary purpose of WhatsApp as a software system?

Answer:

WhatsApp is a real-time messaging application designed to allow users to exchange text, voice, images, videos, documents, and make voice/video calls over the internet. Its goal is to provide an instant, secure, and user-friendly communication platform using mobile numbers as user identifiers, ensuring seamless connectivity globally.

2. How does WhatsApp ensure secure communication between users?

Answer:

WhatsApp implements end-to-end encryption by default using the Signal Protocol. This means messages, calls, and shared media are encrypted on the sender’s device and decrypted only on the receiver’s device. Neither WhatsApp nor any third party can read the contents, which guarantees a high level of privacy and data security for users.

3. What happens when a user sends a message on WhatsApp?

Answer:

When a user sends a message:

• The app first encrypts the message using the recipient’s public encryption key.
• The message is then sent over the internet to WhatsApp’s servers.
• The server temporarily holds the message until the recipient is online.
• Once delivered, the message is decrypted on the receiver’s device and removed from the server.
• Message delivery status is tracked using ticks (✓ for sent, ✓✓ for delivered, ✓✓ blue for seen).

4. Can WhatsApp be used without a SIM card or phone number?

Answer:

No, WhatsApp uses the mobile number as the unique identifier for users. During registration, a one-time password (OTP) is sent to verify the number. However, users can use WhatsApp Web or desktop versions after linking it with the mobile app via a QR code, though the phone still acts as the primary device.

5. What are the main components of WhatsApp’s backend system?

Answer:

WhatsApp’s backend comprises:

• Authentication Service: Verifies phone numbers and manages sessions.
• Message Queues: For temporarily storing and routing messages.
• Media Server: Handles upload and retrieval of multimedia files.
• Database Servers: Store user profiles, chat metadata (not content), and group information.
• Notification Service: Sends push notifications for new messages and calls.

6. How does WhatsApp handle group chats differently from one-to-one chats?

Answer:

In group chats:

• Each message is individually encrypted and sent to all participants.
• The admin has special privileges like adding/removing members or assigning roles.
• Metadata such as group name, icon, and members list is stored in a structured format.
• Message delivery involves checking the status for each group member and ensuring sync across all devices.

7. What design principles help WhatsApp manage billions of messages daily?

Answer:

WhatsApp uses principles like:

• Asynchronous communication to improve responsiveness.
• Scalable architecture with load balancers, distributed servers, and data replication.
• Lightweight message protocols to reduce bandwidth usage.
• Message compression and minimal UI for efficient mobile performance.

8. What happens if a user’s phone is offline when a message is sent?

Answer:

If the user is offline:

• The message remains encrypted and temporarily stored on WhatsApp’s server.
• Once the user comes online, the message is delivered and deleted from the server.
• If the user doesn’t come online within a certain timeframe (usually 30 days), the message is discarded to maintain privacy.

9. What technologies and programming languages power WhatsApp?

Answer:

WhatsApp was originally developed using Erlang, a language known for handling distributed and fault-tolerant systems. Other technologies include:

• FreeBSD for server OS,
• MySQL or Cassandra for databases,
• XMPP protocol (in early versions) for real-time messaging.

10. How does WhatsApp support multimedia sharing and optimize performance?

Answer:

WhatsApp uses:

• Media compression to reduce file sizes before upload.
• Content Delivery Networks (CDNs) for fast access to images/videos.
• Preview generation and thumbnail caching for media files.
• Efficient file system management to store media with unique file IDs and timestamps.