System Design Interview: Friends in Proximity

Friends in Proximity is a mobile app that allows users to view friends who are currently nearby. Let’s discuss how a scalable backend system for this app is designed.

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Requirements and Design Scope

Before initiating the design process, let’s outline the scope of the app.

• “Proximity” is defined as a configurable parameter, with a default value set to 5 miles.
• The app targets an audience of 1 billion users, with 10% expected to actively use the feature.
• The app stores location history.
• If a friend remains inactive for 10 minutes, their entry will be removed from the Friends in Proximity list.
• For simplicity, privacy considerations such as GDPR and CCPA are excluded from the scope of this design.

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Functional Requirements