Kairos Backend Engineer interview questions & guide 2026

The following questions reflect patterns observed in real Kairos interview processes. While specific inquiries will vary based on your interviewer and the specific team, these categories represent the core competencies we evaluate.

Java Expertise & Core Technical Knowledge

These questions test your mastery of the language, memory management, and best practices essential for high-quality development at Kairos.

• Explain the difference between an interface and an abstract class in Java.
• How do you manage memory leaks in a Java application?

Success at Kairos requires a balance of deep technical proficiency and a proactive, collaborative mindset. Your preparation should go beyond syntax; focus on how your technical skills solve real business problems.

Role-related knowledge – We expect a high level of comfort with Java and related frameworks. You should be prepared to discuss how you write testable, maintainable, and efficient code in a production environment.

Problem-solving ability – When faced with a complex system challenge, show us your methodology. We want to see how you break down a problem, identify potential bottlenecks, and evaluate trade-offs between different technical approaches.

Collaboration & Communication – Engineering is a team sport at Kairos. You should be able to articulate your thought process clearly, listen to feedback, and demonstrate how you contribute to a positive team culture.

The Kairos interview process is designed to be rigorous yet transparent. It typically begins with a technical screening to assess your foundational knowledge, followed by a series of deep-dive sessions covering system design, coding, and behavioral alignment. We prioritize candidates who demonstrate not only technical excellence but also a genuine interest in our product mission.

The pace is deliberate, ensuring that both you and our team have sufficient time to assess the potential fit. We value candidates who ask insightful questions about our tech stack, our development lifecycle, and our engineering culture.

This visual timeline illustrates the typical sequence of stages you will navigate. Use this to structure your study plan, ensuring you are prepared for the technical depth of the mid-stage interviews while keeping your behavioral examples polished for the final rounds.

Technical Proficiency

We evaluate your ability to write clean, efficient Java code. You should be prepared to demonstrate expertise in data structures, algorithms, and concurrency.

Be ready to go over:

• Concurrency & Multi-threading – Crucial for handling high-load backend tasks.
• Dependency Injection – Understanding frameworks like Spring is highly beneficial.
• RESTful API design – Demonstrating how you build secure and scalable endpoints.

Example scenarios:

• "How do you handle race conditions in a multi-threaded environment?"
• "Walk me through the process of optimizing a service that is hitting API rate limits."

System Design

This area tests your ability to think at scale. We want to see how you structure services that are resilient and easy to maintain.

Be ready to go over:

• Database design – Choosing between SQL and NoSQL based on specific use cases.
• Message queues – When and why to implement asynchronous communication.
• Microservices vs. Monoliths – Understanding the trade-offs of different architectural styles.

Example scenarios:

• "Design a notification system that handles millions of events per day."
• "How would you handle a sudden spike in traffic for our primary login service?"

As a Backend Engineer, you will spend your time building and maintaining the services that keep Kairos running. Your daily work includes writing production-level Java code, participating in code reviews to ensure quality, and collaborating with frontend and product teams to define API contracts.

You will often be involved in the full development lifecycle, from initial design and prototyping to deployment and monitoring. You will also participate in on-call rotations or incident management, which helps you gain a deep understanding of our production systems and how to improve their reliability over time.

We are looking for individuals who bring both technical rigor and a mindset for continuous improvement.

• Must-have skills: Deep experience with Java, strong understanding of relational databases, and experience working with RESTful APIs.
• Nice-to-have skills: Familiarity with cloud platforms, containerization tools like Docker or Kubernetes, and experience with CI/CD pipelines.

Candidates should ideally have prior experience working in agile environments where cross-team collaboration is the norm. The ability to communicate effectively in English is essential, as you will be working within an international team.

Q: How difficult is the technical interview? The interviews are challenging but fair. We focus on practical application rather than "trick" questions, so if you have strong Java fundamentals and system design experience, you will be well-positioned.

Q: Is there a coding test? Yes, you can expect a coding assessment or a live coding session where you will be asked to solve a problem related to our day-to-day work. Focus on writing readable code and explaining your logic as you go.

Q: What is the culture like at Kairos? We value transparency, technical excellence, and a collaborative spirit. We encourage engineers to take ownership of their work and share knowledge across teams.

• Think out loud: During technical sessions, explain your thought process. It helps the interviewer understand your problem-solving approach, even if you don't reach the perfect solution immediately.
• Be honest about trade-offs: There is rarely one "right" answer in architecture. Acknowledge the pros and cons of your chosen solution to demonstrate senior-level thinking.
• Prepare your questions: Use your interview time to learn about the team's current challenges. It shows that you are already thinking like a member of the team.

The Backend Engineer position at Kairos offers a unique opportunity to work on complex, high-impact systems within a collaborative environment. By focusing on your Java proficiency, system design fundamentals, and clear communication, you will be well-prepared to demonstrate your value to our team.

We encourage you to review your past projects, reflect on the technical decisions you have made, and practice articulating your process. Preparation is the key to turning your experience into a successful interview performance. We look forward to seeing the unique perspective you can bring to Kairos.

The salary data provided represents current market expectations for this role in Spain. Use this to understand the compensation landscape, but remember that total packages often include benefits and growth opportunities that are specific to your level and experience.