Requirements of WhatsApp’s Design

Requirements

Let’s scope the WhatsApp design by identifying the following requirements.

Functional requirements

• Conversation: Support one-on-one and group conversations.

• Acknowledgment: Distinguish durable server acceptance, receipt by a recipient device, and a client-reported read event. Define how these states aggregate across devices and group members.

• Sharing: Enable sharing of media files, including images, videos, and audio.

• Chat storage: Persist undelivered encrypted messages until the delivery policy is satisfied or the retention deadline expires.

• Push notifications: Use platform notifications to alert a reachable device when the app lacks an active connection. The client retrieves pending messages through the messaging service.

Here, “offline” means no active application session on a device. The device may still have network access and receive a platform notification. A device with no connectivity cannot receive live traffic until connectivity returns. A push notification is not a substitute for durable message storage or recipient acknowledgment.

Non-functional requirements