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System Design questions require understanding of core concepts and practice. PracHub provides solutions with explanations to help you master system design interviews.

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This is a medium difficulty System Design question, commonly asked during Onsite rounds at StubHub.

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This question is commonly asked for Software Engineer candidates at StubHub during technical interviews.

Design Internal Chat and Re-engagement Emails | StubHub Interview Question

Design Internal Chat and Re-engagement Emails

Last updated: Apr 6, 2026

Quick Overview

This question evaluates a candidate's competency in designing scalable, real-time messaging systems and re-engagement workflows, covering distributed systems concepts such as WebSocket-based bidirectional communication, presence, cross-server message routing, message persistence, APIs, storage trade-offs, inactivity detection, and email-triggering workflows. Commonly asked in the System Design category, it tests high-level architectural reasoning and practical application of scalability, reliability, and failure-handling trade-offs when integrating persistent WebSocket connections, cross-server routing, and background email jobs in a distributed platform.

StubHub

Feb 16, 2026, 12:00 AM

Software Engineer

Onsite

System Design

Design an internal chat system for an event marketplace company.

Requirements:

Employees can send and receive one-to-one messages in real time.
Clients use persistent WebSocket connections.
A user may connect to any chat server, so sender and recipient may be connected to different servers.
Messages should be delivered immediately to online users and stored for later retrieval if a user is offline.
Support presence information and basic conversation history.
Explain how bidirectional WebSocket communication is routed across multiple servers.
In the same overall platform, add a workflow that sends an email to customers who have not logged in for 10 days.

Discuss the high-level architecture, APIs, storage choices, message routing, inactivity detection, scaling, reliability, and failure handling.

Solution

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