Design Internal Chat and Re-engagement Emails

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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.

Design Internal Chat and Re-engagement Emails

Company: StubHub

Role: Software Engineer

Category: System Design

Difficulty: medium

Interview Round: Onsite

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.

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.

Read the full StubHub Software Engineer interview experience this question came from

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StubHub
Feb 16, 2026
mediumSoftware EngineerOnsiteSystem Design
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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.

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