As a Software Engineer at Felix, you will play a crucial role in designing, developing, and maintaining software systems that power our innovative solutions. This position is integral to our mission of delivering high-quality products that enhance user experiences and drive business success. You will engage with various teams across the organization to tackle complex technical challenges, ensuring that our products are not only functional but also scalable and efficient. The work of a Software Engineer at Felix is both impactful and rewarding. You will contribute to significant projects that influence the overall functionality of our products, such as enhancing our data processing capabilities or refining user interfaces. This role allows you to engage with cutting-edge technologies and methodologies, making it a unique opportunity for growth and professional development. You can expect to work in a dynamic environment that values collaboration, creativity, and continuous improvement. At Felix, your contributions will directly affect our users and the broader business landscape, making your role both critical and fulfilling.
Coding Challenge
reportedComplete a coding challenge on HackerRank to assess technical aptitude.
What to demonstrate
- Complete a coding challenge on HackerRank to assess technical aptitude
- Depth in Problem Solving
How to prepare
- Answer aloud and timed: What are the key differences between an abstract class and an interface?
- Answer aloud and timed: Explain the concept of dependency injection and its benefits.
Technical Assessment
reportedParticipate in a technical interview focusing on core software engineering principles.
What to demonstrate
- Participate in a technical interview focusing on core software engineering principles
- Depth in Problem Solving
How to prepare
- Answer aloud and timed: How do you manage memory in a programming environment?
- Answer aloud and timed: What is the significance of version control in software development?
Behavioral Interview
reportedEngage in a behavioral interview to evaluate interpersonal skills and cultural fit.
What to demonstrate
- Engage in a behavioral interview to evaluate interpersonal skills and cultural fit
- Depth in Problem Solving
How to prepare
- Prepare three examples from your own work, each with a decision you made and an outcome you can quantify.
- Re-read the description of the behavioral interview above and write down what you would ask to confirm before it.
PracHub editorial advice for the preparation topics above.
Practice Coding
Regularly solve coding challenges on platforms like HackerRank or LeetCode to sharpen your skills.
Understand the Company Culture
Research Felix's mission and values to articulate why you are a good fit during interviews.
Prepare for Behavioral Questions
Reflect on your past experiences and how they align with the values of Felix; practice articulating these stories.
Engage with Interviewers
Show curiosity and engagement during interviews by asking thoughtful questions about the team and projects.
Choose a category, try a prompt, then open its approach, worked solution or follow-up when you need it.
How do you manage memory in a programming environment?
How do you manage memory in a programming environment?
Approach
- Say what the runtime actually does before reasoning about the code.
- Name what is shared across threads and what owns each piece of state.
- Identify the window where an invariant is briefly untrue.
- Distinguish a value from a reference to it, and say which one you handed out.
Follow-up
- What happens if two callers reach this at the same time?
- Where could this allocate more than you expect?
How would you approach optimizing an algorithm that is running slowly?
How would you approach optimizing an algorithm that is running slowly?
Approach
- Restate the input: its shape, its size, and what is guaranteed about it.
- Name the brute-force solution and its complexity before improving on it.
- Choose the data structure from the access pattern, not from familiarity.
- State the target complexity and say which constraint rules the naive version out.
Follow-up
- How does this change if the input no longer fits in memory?
- What is the worst case, and how likely is it on real data?
Write a function to reverse a linked list.
Write a function to reverse a linked list.
Approach
- Restate the input: its shape, its size, and what is guaranteed about it.
- Name the brute-force solution and its complexity before improving on it.
- Choose the data structure from the access pattern, not from familiarity.
- State the target complexity and say which constraint rules the naive version out.
Follow-up
- How does this change if the input no longer fits in memory?
- What is the worst case, and how likely is it on real data?
Given an array of integers, find two numbers such that they add up to a specific target.
Given an array of integers, find two numbers such that they add up to a specific target.
Approach
- Restate the input: its shape, its size, and what is guaranteed about it.
- Name the brute-force solution and its complexity before improving on it.
- Choose the data structure from the access pattern, not from familiarity.
- State the target complexity and say which constraint rules the naive version out.
Follow-up
- How does this change if the input no longer fits in memory?
- What is the worst case, and how likely is it on real data?
Explain the time complexity of your solution for a sorting algorithm you implemented.
Explain the time complexity of your solution for a sorting algorithm you implemented.
Approach
- Restate the input: its shape, its size, and what is guaranteed about it.
- Name the brute-force solution and its complexity before improving on it.
- Choose the data structure from the access pattern, not from familiarity.
- State the target complexity and say which constraint rules the naive version out.
Follow-up
- How does this change if the input no longer fits in memory?
- What is the worst case, and how likely is it on real data?
What factors would you consider when designing a database schema?
What factors would you consider when designing a database schema?
Approach
- Name the grain you start from and join outward from it.
- Check whether any join is one-to-many before aggregating, or the sums inflate.
- Say which index the query would use, and what makes it unusable.
- Handle the rows that do not match: that is usually the actual question.
Follow-up
- How does the query change if that join becomes one-to-many?
- What happens to this when the table is ten times larger?
Explain why the owner filter ignores the listing index
The only index on resource is (tenant_id, status, updated_at DESC, resource_id DESC). A new endpoint returns one user's resources across all statuses, newest created first: WHERE tenant_id = $1 AND owner_user_id = $2 ORDER BY created_at DESC LIMIT 20. On a tenant with 2M rows it takes 900 ms and EXPLAIN shows a sort above a large scan. Explain precisely why the existing index cannot serve it, give the index that can, and state which of these the new index still will not help: owner_user_id alone across tenants; the same query ordered by updated_at. PostgreSQL 16.
Approach
- Separate the two jobs an index does. For filtering, a composite btree is seekable only on a left prefix, so with no predicate on status the scan can at best range over tenant_id and test owner_user_id per row; PostgreSQL 16 has no btree skip scan to jump the unconstrained column.
- For ordering, the index is sorted by (status, updated_at) within a tenant and not by created_at, so the LIMIT cannot stop early: every matching row is read and then sorted. That is the 'Sort Method: top-N heapsort' line, and it is why the plan reads 2M rows to answer with 20.
- Derive the replacement from the access path — equality, equality, then the ordering column: CREATE INDEX CONCURRENTLY ON resource (tenant_id, owner_user_id, created_at DESC). The scan seeks to the (tenant, owner) range and walks 20 entries in order, so the Sort node disappears along with the row-read.
- Treat INCLUDE (title, status) as conditional, not free. An index-only scan still visits the heap for any row whose page is not marked all-visible, so on a table taking 1.2k writes/second the win depends on autovacuum keeping the visibility map current, and the wider index costs more on every insert.
Follow-up
- 90% of rows are status='active'. Would a partial index WHERE status = 'active' change your answer, and for which of the three queries?
- A dashboard runs this for 40 owners in one page load. What changes about the design?
What are the key differences between an abstract class and an interface?
What are the key differences between an abstract class and an interface?
Approach
- Clarify what is being asked and what a complete answer contains.
- State your assumptions explicitly before working the problem.
- Say what you would check first and why it is the highest-information step.
- Work from the requirement backwards to the design.
Follow-up
- What assumption would you test first?
- How would you know your answer was wrong?
Explain the concept of dependency injection and its benefits.
Explain the concept of dependency injection and its benefits.
Approach
- Clarify what is being asked and what a complete answer contains.
- State your assumptions explicitly before working the problem.
- Say what you would check first and why it is the highest-information step.
- Work from the requirement backwards to the design.
Follow-up
- What assumption would you test first?
- How would you know your answer was wrong?
What is the significance of version control in software development?
What is the significance of version control in software development?
Approach
- Clarify what is being asked and what a complete answer contains.
- State your assumptions explicitly before working the problem.
- Say what you would check first and why it is the highest-information step.
- Work from the requirement backwards to the design.
Follow-up
- What assumption would you test first?
- How would you know your answer was wrong?
Given a set of data, how would you determine the most efficient way to process it?
Given a set of data, how would you determine the most efficient way to process it?
Approach
- Clarify what is being asked and what a complete answer contains.
- State your assumptions explicitly before working the problem.
- Say what you would check first and why it is the highest-information step.
- Work from the requirement backwards to the design.
Follow-up
- What assumption would you test first?
- How would you know your answer was wrong?
Describe a complex problem you solved in a previous project and the steps you took to resolve it.
Describe a complex problem you solved in a previous project and the steps you took to resolve it.
Approach
- Clarify what is being asked and what a complete answer contains.
- State your assumptions explicitly before working the problem.
- Say what you would check first and why it is the highest-information step.
- Work from the requirement backwards to the design.
Follow-up
- What assumption would you test first?
- How would you know your answer was wrong?
Describe how you would design a scalable API for a mobile application.
Describe how you would design a scalable API for a mobile application.
Approach
- Fix the scope first: who calls this, how often, and what they do when it fails.
- Name the read and write paths separately; they rarely have the same bottleneck.
- Choose a partition key and say what query it makes expensive.
- State the consistency you need, and where you are willing to be stale.
Follow-up
- What breaks first when traffic grows ten times?
- How does this behave when that dependency is down for an hour?
How would you ensure data consistency in a distributed system?
How would you ensure data consistency in a distributed system?
Approach
- Fix the scope first: who calls this, how often, and what they do when it fails.
- Name the read and write paths separately; they rarely have the same bottleneck.
- Choose a partition key and say what query it makes expensive.
- State the consistency you need, and where you are willing to be stale.
Follow-up
- What breaks first when traffic grows ten times?
- How does this behave when that dependency is down for an hour?
Read latency spikes on a sixty-second sawtooth
The cached listing read path serves about 14k reads/second at an 85% hit rate. p99 sits at 35 ms for 57 seconds, jumps to 900 ms for 3, and repeats. During each spike the primary shows several hundred identical listing queries starting within the same millisecond, all carrying one large tenant's id. Cache entries use a 60-second TTL. Give the mechanism, the ordered checks, the fix, and the correctness hazard your fix must not introduce.
Approach
- Match the period to a configured number before theorising about load. A spike every 60 seconds against a 60-second TTL is an entry expiring, and you confirm it by correlating spike timestamps with the entry's write time rather than with the traffic curve. If the period had matched a cron or a GC interval instead, this is a different investigation.
- Establish the concurrency of the miss. Several hundred identical queries in one millisecond means the miss path has no coalescing: every request that arrives between expiry and repopulation recomputes. The herd size is that key's arrival rate times its recompute time, so at 1.2k reads/second for the hot key and a 250 ms recompute you expect about 300 concurrent misses, which matches what is observed.
- Add single-flight on the miss path so one caller per key recomputes under a short-lived lock while the rest wait for its result. Prefer stale-while-revalidate where the read tolerates it: return the expired value immediately and refresh asynchronously, which removes the latency spike rather than serialising it into a queue of waiters.
- De-synchronise the keys. Write TTLs with jitter, for example 60 seconds plus or minus 10%, so a deploy or a mass invalidation does not align every key on the same second and turn a per-key herd into a fleet-wide one.
Follow-up
- The same sawtooth appears on a key that is invalidated on write rather than expired. Is that the same bug?
- How does your answer change if the recompute takes 4 seconds instead of 250 ms?
Built from the rounds and topics Felix candidates report.
Prepare, practise & reflect
One practical outcome each day. Spend longer where you need it.
0 / 7 done01Map the Felix loop
- Write out the reported sequence: Coding Challenge, Technical Assessment, Behavioral Interview.
- For each round, write one sentence on what it is judging, from the description above, and mark the one you are least ready for.
Deliverable: A one-page map of the 3 reported rounds, with the weakest marked.
02Work Problem Solving
- Spend the session on Problem Solving, which Felix candidates report being tested on.
- Write one worked example in Problem Solving and time yourself on it.
Deliverable: One timed worked example in Problem Solving.
03Work Data Structures
- Spend the session on Data Structures, which Felix candidates report being tested on.
- Write one worked example in Data Structures and time yourself on it.
Deliverable: One timed worked example in Data Structures.
04Work Algorithms
- Spend the session on Algorithms, which Felix candidates report being tested on.
- Write one worked example in Algorithms and time yourself on it.
Deliverable: One timed worked example in Algorithms.
05Answer out loud: Technical / Domain Questions
- Answer aloud, timed: What are the key differences between an abstract class and an interface?
- Answer aloud, timed: Explain the concept of dependency injection and its benefits.
Deliverable: Spoken answers to 2 reported Technical / Domain Questions question(s), under time.
06Answer out loud: Problem-Solving / Case Studies
- Answer aloud, timed: How would you approach optimizing an algorithm that is running slowly?
- Answer aloud, timed: Given a set of data, how would you determine the most efficient way to process it?
Deliverable: Spoken answers to 2 reported Problem-Solving / Case Studies question(s), under time.
07Answer out loud: Behavioral / Leadership
- Answer aloud, timed: Tell me about a time you faced a conflict within a team. How did you handle it?
- Answer aloud, timed: Describe a situation where you had to make a tough decision. What was the outcome?
Deliverable: Spoken answers to 2 reported Behavioral / Leadership question(s), under time.
Expand any day for tasks and deliverables. Your progress is saved on this device.
Behavioural rounds judge the decision you made and what it cost.
Describe your experience with cloud computing services.
Describe your experience with cloud computing services.
Approach
- Pick a story where you made the decision, not one where you watched it.
- State the situation in two sentences and spend the rest on the reasoning.
- Give the blast radius: what could have broken, and what you measured.
- Name the disagreement and how you resolved it with evidence.
Follow-up
- What would you do differently if you ran that again?
- How did you know your change caused the improvement?
Tell me about a time you faced a conflict within a team. How did you handle it?
Tell me about a time you faced a conflict within a team. How did you handle it?
Approach
- Pick a story where you made the decision, not one where you watched it.
- State the situation in two sentences and spend the rest on the reasoning.
- Give the blast radius: what could have broken, and what you measured.
- Name the disagreement and how you resolved it with evidence.
Follow-up
- What would you do differently if you ran that again?
- How did you know your change caused the improvement?
Describe a situation where you had to make a tough decision. What was the outcome?
Describe a situation where you had to make a tough decision. What was the outcome?
Approach
- Pick a story where you made the decision, not one where you watched it.
- State the situation in two sentences and spend the rest on the reasoning.
- Give the blast radius: what could have broken, and what you measured.
- Name the disagreement and how you resolved it with evidence.
Follow-up
- What would you do differently if you ran that again?
- How did you know your change caused the improvement?
How do you prioritize tasks when managing multiple projects?
How do you prioritize tasks when managing multiple projects?
Approach
- Pick a story where you made the decision, not one where you watched it.
- State the situation in two sentences and spend the rest on the reasoning.
- Give the blast radius: what could have broken, and what you measured.
- Name the disagreement and how you resolved it with evidence.
Follow-up
- What would you do differently if you ran that again?
- How did you know your change caused the improvement?
- 01
Describe your experience with cloud computing services.
- 02
Tell me about a time you faced a conflict within a team. How did you handle it?
- 03
Describe a situation where you had to make a tough decision. What was the outcome?
- 04
How do you prioritize tasks when managing multiple projects?
What is the typical difficulty level of the interviews at Felix?
The interviews are generally considered average in difficulty, focusing on both technical and behavioral aspects. Candidates should prepare thoroughly to demonstrate their skills and fit for the role.
Felix Software Engineer candidate reports ↗How much preparation time is typical for candidates?
Candidates often spend several weeks preparing, especially focusing on coding challenges and system design principles. Practice is key to feeling confident.
Felix Software Engineer candidate reports ↗What differentiates successful candidates at Felix?
Successful candidates often demonstrate a strong technical foundation, excellent problem-solving skills, and a clear alignment with the company’s values.
Felix Software Engineer candidate reports ↗What is the culture and working style at Felix?
The culture at Felix emphasizes collaboration, innovation, and continuous learning. You'll find a supportive environment that encourages professional growth.
Felix Software Engineer candidate reports ↗What is the typical timeline from the initial screen to an offer?
Candidates can expect the entire process to take a few weeks, but timelines may vary based on the specific team and role.
Felix Software Engineer candidate reports ↗Are remote work options available for this role?
Felix offers flexible working arrangements, including remote work opportunities, depending on team needs and individual preferences.
Felix Software Engineer candidate reports ↗What topics does Felix test in interviews?
Felix interviews most often cover Video interview (one-way/recorded), Problem Solving, Phone screening interview, Marketing Analytics, and Data Structures. The exact emphasis depends on the specific role you apply for.
Felix Software Engineer candidate reports ↗Sources & methodology 3 sources ↗
No official company page is cited. Rounds and questions come from candidate reports and PracHub editorial material; each source shows the date it was read.
- 01Felix Software Engineer candidate reports ↗
Company-reported rounds, questions and FAQ.
Candidate reports · Accessed 2026-09-22 - 02PracHub Software Engineer practice ↗
PracHub practice material, not company-reported.
PracHub page · Accessed 2026-09-22 - 03PracHub preparation framework ↗
PracHub preparation guidance.
PracHub page · Accessed 2026-09-22
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