Databricks·Software Engineer·Technical Phone Screen·Senior

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Jun 2026

Summary

Databricks software engineering interview that went deep on fault tolerance patterns. The main problem was a double-layer circuit breaker and it was more involved than I expected, especially around getting the state transitions right under mixed failure sequences.

Questions Asked (1)

Design and implement a double-layer circuit breaker that wraps calls to multiple backend servers, including the full state machine (Closed, Open, Half-Open), a `call(server_lambda)` wrapper API, and a demonstration of behavior across mixed success and failure sequences.

System DesignTechnical Trade-offsAPI & Integrations

Author's notes

The state machine part felt manageable at first.

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AI HintsAI Generated

Suggested Approach

Start by clarifying requirements and defining the circuit breaker state machine and its transitions. Then design the API and data structures, and walk through a concrete example with mixed success/failure sequences to demonstrate correctness. Finally, discuss trade-offs and extensions like per-server breakers and concurrency handling.

Pro tip: Emphasize that the circuit breaker should be per-server to avoid cascading failures, and discuss how to handle concurrency (e.g., locking or atomic operations) to ensure thread safety in a multi-threaded environment.

1. Clarify Requirements and Scope

Ask clarifying questions about expected behavior, such as failure thresholds, timeout durations, and whether the breaker is per-server or global. Confirm the API signature and demonstration expectations.

2. Design the State Machine and Data Structures

Define the three states (Closed, Open, Half-Open) and transitions: Closed to Open on failure threshold, Open to Half-Open after timeout, Half-Open to Closed on success or Open on failure. Outline data structures like failure count, last failure time, and state.

3. Implement the call() Wrapper and State Transitions

Write pseudocode or actual code for the call(server_lambda) method, handling state checks, invoking the lambda, updating counters, and transitioning states. Ensure thread safety with locks or atomic operations.

4. Demonstrate with Mixed Success/Failure Sequences

Walk through a concrete example: start Closed, cause failures to trip Open, wait for timeout to Half-Open, then show success closing it or failure reopening it. Use a timeline or step-by-step simulation.

5. Discuss Trade-offs and Extensions

Talk about trade-offs like per-server vs global breakers, timeout tuning, and fallback strategies. Mention extensions like metrics, logging, and integration with retries or bulkheads.

Key Points to Mention

State transitions and thresholds (e.g., failure count, timeout duration)
Thread safety and concurrency control (locks, atomics)
Per-server circuit breakers to isolate failures
Fallback behavior when circuit is open (e.g., throw exception, return default)
Monitoring and metrics for observability
Integration with retry logic and backoff strategies

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