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Salesforce·Software Engineer·Onsite - System Design / Architecture·Senior

Senior
May 2026

Summary

System design round at Salesforce for a software engineer role, single question about building a 911 emergency call routing platform. Pretty intense scope for one question and I definitely underestimated how many moving parts there were.

Questions Asked (1)

Q1

Design a 911 emergency calling service that routes calls from mobile, landline, and VoIP devices to the correct emergency dispatch center.

System DesignTechnical Trade-offsAPI & Integrations
Author's notes

I started with the happy path, caller dials 911, signal hits a tower, gets routed to a PSAP.

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AI HintsAI Generated

Suggested Approach

Start by clarifying requirements and scale, then design a high-level architecture that handles location determination, routing, and failover for mobile, landline, and VoIP calls. Focus on the critical path: call ingestion, location lookup, dispatch center selection, and call delivery, while addressing reliability, latency, and regulatory constraints.

Pro tip: Emphasize that 911 systems require extreme reliability and low latency, so discuss redundancy, failover, and how you would test the system under failure scenarios. Also, mention that you would collaborate with local authorities and adhere to standards like NENA i3.

1. Clarify Requirements and Constraints

Ask about scale (calls per second), latency requirements, regulatory compliance, and supported device types. Define functional and non-functional requirements.

2. High-Level Architecture

Outline the main components: call ingestion (from carriers/VoIP providers), location service (using GPS, cell tower, or registered address), routing engine, dispatch center database, and call delivery. Use a diagram if possible.

3. Location Determination and Routing Logic

Explain how to obtain location for each device type: mobile (GPS/AGPS, cell ID), landline (ALI database), VoIP (registered address or dynamic location). Describe how to map location to the correct dispatch center (PSAP) using geospatial data and routing rules.

4. Reliability, Failover, and Scalability

Discuss redundancy at every layer, active-active data centers, automatic failover, and how to handle network partitions. Address scalability with horizontal scaling and load balancing.

5. Integration and Standards

Mention integration with carrier networks, VoIP providers, and PSAP systems. Reference standards like NENA i3, SIP, and location protocols (e.g., HELD). Discuss API design for third-party integration.

Key Points to Mention

  • Location accuracy and determination methods for different device types (GPS, cell tower triangulation, ALI, registered address).
  • Routing to the correct PSAP using geospatial databases and emergency service numbers (ESN).
  • High availability and fault tolerance: redundant data centers, automatic failover, and disaster recovery.
  • Low latency and real-time processing: use of in-memory databases, efficient geospatial indexing, and optimized network paths.
  • Regulatory compliance: FCC requirements, NENA i3 standards, and local regulations.
  • Security and privacy: encryption of call data, authentication of carriers, and protection against denial-of-service attacks.

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.