Keychain | Lead Backend Engineer | Interview Experience | Rejected

The interview process involved an initial discussion with a Senior Engineering Manager, an HLD round, a take-home assignment, a review of the assignment, and a final round with the CTO.

Exploratory Call - Senior Engineering Manager

I began with an introductory call with a Senior Engineering Manager. We discussed my professional background, previous projects, current responsibilities, and reasons for exploring new opportunities.

I also learned more about the company, the team, and the expectations associated with the role.

Round 1: HLD

I was asked to design a job scheduling system that executes tasks at a specified future time, with a maximum delay of 10 seconds.

The discussion focused on the system's architecture and how scheduled jobs would be managed and executed within the required time window.

Take-Home Assignment (Distributed Priority Queue)

For the assignment, I had to implement a distributed priority queue.

I made the solution more complicated than it needed to be. Instead of starting with a simple in-memory queue running on a single node, I designed a gateway and distributed the queue across multiple nodes.

This introduced additional challenges, especially when handling concurrent operations. I ended up spending more effort managing the complexity of the architecture than necessary.

In hindsight, I should have started with a simpler implementation and added distribution only if the requirements called for it.

Round 2: Assignment Review

The discussion mainly revolved around:

• Thread safety and concurrent access

• Locking mechanisms

• Scaling the queue across multiple nodes

• Maximum achievable throughput

• Identifying performance bottlenecks

The interviewer expected me to understand the implementation in depth and justify the decisions I had made.

One thing I would do differently next time is benchmark the maximum throughput before the interview so I could support my performance claims with actual measurements.

Since I had introduced a distributed architecture, I also had to reason about concurrency issues that wouldn't have been as complicated in a single-node implementation.

Round 3: CTO Interview

The final round started with a discussion about my background and previous projects before moving into technical questions.

MongoDB and Database Architecture

I was asked why I had selected MongoDB over PostgreSQL for my use case and what factors influenced that decision.

Another question explored how MongoDB Atlas supports migration between AWS and GCP while maintaining zero downtime.

Data Aggregation Across Microservices

I was presented with a scenario involving two microservices: one returning user information and the other providing order details.

I was asked how I would combine the responses and expose the resulting data to consumers.

The discussion then became more challenging when I was asked how I would support filtering on the combined dataset.

This required me to consider where the aggregation and filtering logic should reside and how to handle data coming from independent services efficiently.

Final Result

I was ultimately rejected for the position, but I didn't receive any detailed feedback explaining the decision.

What I Learned

A few things stood out to me after going through this process:

• Keep the design aligned with the requirements. I could have avoided unnecessary complexity by starting with a single-node queue before considering a distributed solution.

• Prepare to defend every design choice. During assignment reviews, understanding concurrency, locking, and scaling in detail is just as important as completing the implementation.

• Validate performance claims. Benchmarking throughput beforehand would have helped me explain the system's actual capabilities more confidently.

• Think carefully about service boundaries. Combining and filtering data across microservices requires clear decisions about where aggregation should happen and what trade-offs that introduces.

Overall, the experience reminded me that senior engineering interviews are not just about building sophisticated systems. I also need to demonstrate sound judgment, understand the implications of my design choices, and explain the trade-offs clearly.