Fastly Software Engineer Interview Questions + Guide in 2025

Fastly is a leading edge cloud platform that empowers customers to create fast, secure, and reliable digital experiences by processing and serving applications as close to their end-users as possible.

As a Software Engineer at Fastly, you will play a vital role in developing and maintaining the technology that drives the company’s edge cloud platform. This involves writing high-quality code, collaborating with cross-functional teams, and utilizing your expertise in programming languages such as C++, Rust, and JavaScript. Your responsibilities may include enhancing language-specific SDKs, implementing the latest standards, and improving the performance and feature completeness of the Fastly platform. The ideal candidate will have extensive software engineering experience, a deep understanding of the JavaScript ecosystem, and an eagerness to engage with open-source communities.

Fastly values communication, collaboration, and a user-driven approach, making it essential for you to be a great teammate who can contribute positively to team dynamics. The role not only demands technical skills but also a proactive attitude toward mentoring others and driving project progress.

This guide will equip you with insights into the expectations and culture at Fastly, ensuring you are well-prepared to demonstrate your fit for the Software Engineer role during your interview.

The interview process for a Software Engineer position at Fastly is structured to assess both technical skills and cultural fit within the company. It typically consists of several key stages:

1. Initial Recruiter Call

The process begins with a brief phone call with a recruiter. This conversation usually lasts around 30 minutes and serves as an opportunity for the recruiter to gauge your interest in the role, discuss your background, and provide insights into Fastly's work culture and values. The recruiter may also clarify any logistical details regarding the position, such as location preferences and work arrangements.

2. Technical Screening

Following the initial call, candidates typically undergo a technical screening, which may be conducted via video call. This session is often led by a hiring manager or a senior engineer and focuses on assessing your coding skills and problem-solving abilities. You can expect to solve coding challenges in real-time, which may involve languages relevant to the role, such as C++, Rust, or JavaScript. Additionally, you may be asked to discuss your previous projects and the technical decisions you made.

3. Onsite Interviews

The onsite interview is a more comprehensive evaluation, usually consisting of multiple rounds. Candidates can expect to participate in several one-on-one interviews, each lasting approximately 45 minutes. These interviews typically cover a mix of technical and behavioral questions. You may be asked to demonstrate your knowledge of systems programming, distributed systems, and performance optimization. Behavioral interviews will assess your teamwork, communication skills, and how you align with Fastly's values.

4. Collaborative Problem-Solving

During the onsite process, there may also be a collaborative problem-solving session where you work with interviewers on a coding problem or a design challenge. This step is designed to evaluate how you approach problem-solving in a team environment and your ability to communicate your thought process effectively.

5. Final Interview

In some cases, a final interview may be conducted with senior leadership or cross-functional team members. This interview focuses on your long-term career goals, your fit within the company culture, and how you can contribute to Fastly's mission.

As you prepare for your interview, it's essential to be ready for a variety of questions that will test both your technical expertise and your ability to work collaboratively in a fast-paced environment.

Next, let's delve into the specific interview questions that candidates have encountered during the process.

Here are some tips to help you excel in your interview.

Understand the Interview Process

Fastly's interview process typically includes an initial chat with a recruiter, followed by a coding screen with the hiring manager, and then a full onsite interview. Familiarize yourself with this structure and prepare accordingly. Expect a mix of technical assessments, behavioral questions, and discussions about your coding knowledge, particularly in C++ and Rust. Knowing the flow of the interview can help you feel more at ease and allow you to focus on showcasing your skills.

Showcase Your Technical Skills

As a Software Engineer at Fastly, you will be expected to demonstrate a strong command of programming languages, particularly C++ and Rust. Brush up on your coding skills and be prepared to solve problems on the spot. Practice coding challenges that involve systems programming and language runtimes, as these are likely to be focal points during your technical assessments. Additionally, be ready to discuss your previous projects and how they relate to the role you are applying for.

Emphasize Collaboration and Communication

Fastly values teamwork and collaboration. During your interview, highlight your experiences working in cross-functional teams and how you effectively communicated with colleagues from different backgrounds. Be prepared to discuss how you handle feedback and contribute to a positive team culture. This will demonstrate that you are not only a skilled engineer but also a great teammate who can thrive in Fastly's collaborative environment.

Align with Company Values

Fastly is committed to building a trustworthy internet and values diversity and inclusion. Familiarize yourself with the company's mission and values, and think about how your personal values align with theirs. Be ready to discuss how you can contribute to their goals, whether through your technical expertise or your commitment to fostering an inclusive workplace. This alignment can set you apart from other candidates.

Prepare for Behavioral Questions

Expect behavioral questions that assess your problem-solving abilities, adaptability, and how you handle challenges. Use the STAR (Situation, Task, Action, Result) method to structure your responses. Reflect on past experiences where you faced obstacles and how you overcame them, particularly in a team setting. This will help you convey your thought process and demonstrate your resilience.

Be Ready to Discuss Open Source Contributions

Fastly encourages contributions to open source projects. If you have experience in this area, be prepared to discuss your involvement, the projects you've contributed to, and the impact of your work. If you haven't contributed to open source yet, consider discussing your interest in doing so and how you plan to engage with the community.

Ask Insightful Questions

At the end of your interview, you will likely have the opportunity to ask questions. Use this time to inquire about the team dynamics, ongoing projects, and how Fastly supports professional development. Asking thoughtful questions not only shows your interest in the role but also helps you assess if Fastly is the right fit for you.

Follow Up

After your interview, send a thank-you email to express your appreciation for the opportunity to interview. Reiterate your enthusiasm for the role and briefly mention a key point from your conversation that resonated with you. This small gesture can leave a positive impression and keep you top of mind as they make their decision.

By following these tips and preparing thoroughly, you can approach your interview with confidence and increase your chances of success at Fastly. Good luck!

In this section, we’ll review the various interview questions that might be asked during a Software Engineer interview at Fastly. The interview process will likely assess your technical skills, problem-solving abilities, and your fit within the collaborative culture of the company. Be prepared to discuss your experience with programming languages, systems design, and your approach to teamwork and communication.

Technical Skills

1. Can you explain the differences between C++ and Rust, and when you would choose one over the other?

Understanding the nuances between these languages is crucial for a role that involves both.

How to Answer

Discuss the strengths and weaknesses of each language, focusing on memory safety, performance, and concurrency. Highlight scenarios where one might be more advantageous than the other.

Example

"C++ offers more control over system resources, which is beneficial for performance-critical applications. However, Rust provides memory safety guarantees without a garbage collector, making it ideal for concurrent programming. I would choose Rust for projects where safety and concurrency are paramount, while C++ might be preferred for legacy systems or performance-intensive applications."

2. Describe a challenging bug you encountered in a large-scale system. How did you diagnose and resolve it?

This question assesses your problem-solving skills and experience with complex systems.

How to Answer

Outline the steps you took to identify the issue, the tools you used, and how you implemented a solution. Emphasize your analytical skills and persistence.

Example

"I once faced a memory leak in a distributed application that caused performance degradation. I used profiling tools to identify the leak's source, which was a forgotten reference in a caching mechanism. After isolating the issue, I refactored the code to ensure proper memory management, which resolved the performance issues."

3. How do you ensure code quality and maintainability in your projects?

Fastly values high-quality code, so demonstrating your commitment to best practices is essential.

How to Answer

Discuss your approach to code reviews, testing, and documentation. Mention any specific methodologies you follow, such as Agile or Test-Driven Development (TDD).

Example

"I prioritize code quality by adhering to coding standards and conducting thorough code reviews with my team. I also implement unit tests and integration tests to catch issues early. Additionally, I maintain clear documentation to ensure that future developers can easily understand and build upon my work."

4. What strategies do you use for optimizing performance in your applications?

Performance is critical in Fastly's edge cloud platform, so be prepared to discuss optimization techniques.

How to Answer

Talk about profiling, benchmarking, and specific techniques you’ve used to improve performance, such as caching strategies or algorithm optimization.

Example

"I regularly use profiling tools to identify bottlenecks in my applications. For instance, in a recent project, I implemented caching for frequently accessed data, which reduced load times by 40%. I also analyze algorithm complexity to ensure that my solutions scale effectively with increased data."

5. Can you explain how you would design a system to handle high availability and fault tolerance?

This question tests your understanding of system design principles.

How to Answer

Discuss concepts like redundancy, load balancing, and failover strategies. Provide a high-level overview of how you would structure the system.

Example

"I would design a system with multiple redundant servers across different geographic locations to ensure high availability. Implementing load balancers would distribute traffic evenly, while health checks would monitor server status. In case of a failure, automatic failover mechanisms would redirect traffic to healthy servers, minimizing downtime."

Collaboration and Communication

1. Describe a time when you had to work closely with a team to achieve a goal. What was your role?

Collaboration is key at Fastly, so they want to see how you work with others.

How to Answer

Share a specific example that highlights your teamwork skills, your contributions, and the outcome of the project.

Example

"In my last project, I collaborated with a cross-functional team to develop a new feature for our application. I took the lead on the backend development, ensuring that our API met the frontend requirements. Regular stand-ups and open communication helped us stay aligned, and we successfully launched the feature ahead of schedule."

2. How do you handle conflicts or disagreements within a team?

Fastly values a collaborative environment, so demonstrating your conflict resolution skills is important.

How to Answer

Discuss your approach to addressing conflicts, emphasizing communication and understanding different perspectives.

Example

"When conflicts arise, I believe in addressing them directly and respectfully. I encourage open dialogue to understand each person's viewpoint. For instance, during a project, two team members disagreed on the implementation approach. I facilitated a meeting where we discussed the pros and cons of each approach, leading us to a consensus that combined the best elements of both ideas."

3. How do you prioritize tasks when working on multiple projects?

This question assesses your time management and organizational skills.

How to Answer

Explain your method for prioritizing tasks, such as using project management tools or frameworks like Agile.

Example

"I prioritize tasks based on urgency and impact. I use tools like Jira to track progress and deadlines. For example, when managing multiple projects, I assess which tasks align with our team's goals and deadlines, allowing me to focus on high-impact work first while ensuring that all projects progress smoothly."

4. Can you give an example of how you mentored a junior engineer?

Fastly values mentorship and knowledge sharing, so be prepared to discuss your experience in this area.

How to Answer

Share a specific instance where you provided guidance, highlighting the impact of your mentorship.

Example

"I mentored a junior engineer who was struggling with understanding our codebase. I organized regular one-on-one sessions to walk them through the architecture and best practices. Over time, they became more confident and even contributed to a major feature, which was rewarding for both of us."

5. What do you think is the most important aspect of team communication?

Effective communication is vital in a collaborative environment like Fastly.

How to Answer

Discuss the importance of clarity, transparency, and active listening in team communication.

Example

"I believe clarity is the most important aspect of team communication. Ensuring that everyone understands their roles and responsibilities helps prevent misunderstandings. I also value transparency, as it fosters trust within the team. Regular check-ins and feedback loops are essential for maintaining open lines of communication."