Design a HR Interview Scheduling & Chat

Problem statement

Design an HR interview scheduling and real-time chat platform that coordinates recruiters, hiring managers, interview panel members, and candidates across timezones, calendars, and communication channels. The system must manage interviewer availability, propose and confirm time slots, sync with external calendars (Google Calendar, Microsoft 365, iCal), deliver real-time messages between recruiters and candidates, trigger automated reminders and rescheduling flows, integrate video-call links, and protect candidate PII under GDPR and equivalent regulations.

This is not a simple appointment-booking widget. A single onsite interview loop may involve five interviewers across three timezones, each with existing calendar commitments, preferred working hours, and buffer requirements. The candidate expects a self-service scheduling page, instant confirmation, and a chat channel for logistics questions. The recruiter expects conflict-free scheduling, automatic reminders, and a communication audit trail. The hiring manager expects visibility into pipeline velocity.

Why the problem is distinctive

A generic chat system handles message delivery. A generic calendar handles slot booking. This system must combine both while solving three hard coordination problems simultaneously:

1. Timezone correctness. Every timestamp must be stored in UTC, displayed in the viewer's local timezone, and validated against each participant's working-hours policy. A single DST transition or offset misconfiguration can shift an interview by an hour and waste six people's time.

1. Calendar bidirectional sync. The platform reads interviewer availability from external calendars and writes confirmed events back. OAuth token refresh, webhook-based change notification, conflict detection, and graceful degradation when a provider is down are all in scope.

1. Real-time chat with delivery guarantees. Recruiter-candidate messages must be delivered reliably, ordered correctly, and retained for compliance. Unlike social chat, these messages form a legal audit trail for hiring decisions.

The four architectural planes

1. Scheduling plane: availability aggregation, conflict detection, slot proposal, confirmation, rescheduling, and calendar write-back.
2. Communication plane: real-time chat, presence, typing indicators, message delivery, read receipts, and media sharing.
3. Notification plane: email, push, SMS, and in-app reminders triggered by scheduling events and chat messages.
4. Governance plane: PII protection, audit logging, tenant isolation, retention policies, and compliance reporting.

A strong interview answer keeps these planes separate. The scheduling plane can degrade (show stale availability) without breaking the communication plane (chat still works). The notification plane can lag without affecting scheduling correctness.

Public operating baseline

Calendly reports over 20 million users and millions of meetings scheduled monthly across 190+ countries. Greenhouse serves 6,500+ customers including HubSpot, Coinbase, and Booking.com. Lever (now Employ Inc.) powers hiring at Netflix, Shopify, and Deloitte. LinkedIn Recruiter reaches over 60 million hiring professionals. These are public figures establishing that the category operates at significant scale.

For capacity planning, this answer explicitly assumes a mid-market SaaS deployment: 5,000 tenant organizations, 200,000 active recruiter and hiring-manager users, 2 million candidates in active pipelines, 500,000 scheduling operations per day, 3 million chat messages per day, and a 4x event peak during hiring seasons. Unless tied to a citation, every number is a stated design assumption.