cswork / libraryOpen practice workspace

Interview guides · Censys

Censys Software Engineer Interview Guide

Censys Software Engineer interview guide: reported rounds, questions and how candidates describe the process.

The 60-second brief

As a Software Engineer at Censys, you will play a pivotal role in developing and maintaining critical software systems that underpin the company's mission of enhancing internet security and visibility for its users. This position is integral to the design and implementation of scalable backend services that process and analyze vast amounts of data, ensuring that Censys can deliver accurate and timely information to its customers. You will work on innovative projects that leverage cutting-edge technologies, collaborating with cross-functional teams to deliver solutions that enhance the performance and reliability of Censys products. Your contributions will directly impact users by providing them with enhanced insights into cybersecurity threats and vulnerabilities.

This guide is scoped to a Software Engineer candidate at Censys.

Focus areas

  • Kubernetes (K8s)
  • API Development
  • Coding Assessments (Take-home/Timed)

The Censys interview process

Censys candidates report 3 rounds over 3-5 weeks. The stages below are what candidates describe, not a published process.

Initial Screening

The first step involves a review of your application and a preliminary discussion about your background.

What to demonstrate

  • The first step involves a review of your application and a preliminary discussion about your background
  • Depth in Kubernetes (K8s)

How to prepare

  • Be able to walk your CV end to end in two minutes, and say why this company specifically.
  • Have your salary expectations, notice period and location constraints ready, and ask for the rest of the loop in writing.

Technical Assessments

Candidates will undergo technical evaluations to assess their relevant skills and knowledge.

What to demonstrate

  • Candidates will undergo technical evaluations to assess their relevant skills and knowledge
  • Depth in Kubernetes (K8s)

How to prepare

  • Answer aloud and timed: Can you explain the differences between SQL and NoSQL databases?
  • Answer aloud and timed: Describe a challenging technical problem you faced and how you resolved it.

Interviews with Team Members

Engagement with various team members to evaluate both technical skills and cultural fit.

What to demonstrate

  • Engagement with various team members to evaluate both technical skills and cultural fit
  • Depth in Kubernetes (K8s)

How to prepare

  • Answer aloud and timed: What tools and methodologies do you use for debugging and testing your code?
  • Answer aloud and timed: How would you approach designing a system to handle millions of concurrent users?

Where candidates lose points

Clarify Expectations

Always seek to understand the role and expectations clearly during your interviews. Asking insightful questions can demonstrate your interest and engagement.

Showcase Collaboration

Highlight experiences where you worked effectively with others, as teamwork is highly valued at Censys.

Be Prepared for Technical Depth

Expect to dive deep into technical topics, so ensure you can discuss your past work and projects in detail.

Express a Growth Mindset

Demonstrate your willingness to learn and adapt, as Censys values continuous improvement.

Questions & practice

How would you approach designing a system to handle millions of concurrent users?

Medium · Problem-Solving / Case Studies

Approach

  1. Say what the runtime actually does before reasoning about the code.
  2. Name what is shared across threads and what owns each piece of state.
  3. Identify the window where an invariant is briefly untrue.
  4. 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?

Write a function to reverse a linked list.

Medium · Coding / Algorithms

Approach

  1. Restate the input: its shape, its size, and what is guaranteed about it.
  2. Name the brute-force solution and its complexity before improving on it.
  3. Choose the data structure from the access pattern, not from familiarity.
  4. 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?

How would you implement an algorithm to find the shortest path in a graph?

Medium · Coding / Algorithms

Approach

  1. Restate the input: its shape, its size, and what is guaranteed about it.
  2. Name the brute-force solution and its complexity before improving on it.
  3. Choose the data structure from the access pattern, not from familiarity.
  4. 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?

Can you explain Big O notation and provide examples of its use?

Medium · Coding / Algorithms

Approach

  1. Restate the input: its shape, its size, and what is guaranteed about it.
  2. Name the brute-force solution and its complexity before improving on it.
  3. Choose the data structure from the access pattern, not from familiarity.
  4. 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?

Can you explain the differences between SQL and NoSQL databases?

Medium · Technical / Domain Questions

Approach

  1. Name the grain you start from and join outward from it.
  2. Check whether any join is one-to-many before aggregating, or the sums inflate.
  3. Say which index the query would use, and what makes it unusable.
  4. 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?

Find version gaps and relay lag with window functions

Medium · window functions · event tables · frame semantics

outbox_event holds event_id, aggregate_type, aggregate_id, aggregate_version, event_type, payload, status ('pending','published','dead'), attempts, created_at, published_at. A projection is missing rows and you must decide whether the relay skipped events or the consumer dropped them. Write three queries over the last seven days: one listing every aggregate_id whose published aggregate_version sequence has a hole, one giving per-day counts with a running total, and one returning the newest published event per aggregate. For each, say where the window function is evaluated relative to WHERE and LIMIT. PostgreSQL 16.

Approach

  1. Gaps: compute lead(aggregate_version) OVER (PARTITION BY aggregate_id ORDER BY aggregate_version) in a subquery, then filter next_version <> aggregate_version + 1 in the outer query. Window functions are evaluated after WHERE, GROUP BY and HAVING and before the outer ORDER BY and LIMIT, so the predicate cannot sit in the same WHERE clause and PostgreSQL 16 has no QUALIFY.
  2. Say what the seven-day filter does to the answer: it truncates every partition, so the first row per aggregate has no predecessor inside the window and a hole spanning the boundary is invisible. Widen the window, or join to resource.version as the authority for the true maximum.
  3. Running total: SELECT date_trunc('day', created_at) AS d, count() AS n, sum(count()) OVER (ORDER BY date_trunc('day', created_at) ROWS UNBOUNDED PRECEDING). An aggregate inside a window call is legal because grouping runs before windowing. The grouping key is unique per row here so ROWS and RANGE agree, but write the frame anyway — over ungrouped rows with tied timestamps the default RANGE frame pulls in every peer row and the total jumps.
  4. Newest per aggregate: DISTINCT ON (aggregate_id) ... ORDER BY aggregate_id, aggregate_version DESC is the cheap PostgreSQL-only form when an index matches that order; row_number() OVER (PARTITION BY aggregate_id ORDER BY aggregate_version DESC) = 1 is the portable form and needs a subquery for the same evaluation-order reason as the gap query.

Follow-up

  • Relay failover redelivers events. Does a duplicate break the gap query, and how would you detect one from this table alone?
  • Turn the gap check into a continuous monitor rather than a query someone runs after an incident. What does it watch?

How do you ensure the scalability of a backend system?

Medium · Technical / Domain Questions

Approach

  1. Fix the scope first: who calls this, how often, and what they do when it fails.
  2. Name the read and write paths separately; they rarely have the same bottleneck.
  3. Choose a partition key and say what query it makes expensive.
  4. 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?

Describe a challenging technical problem you faced and how you resolved it.

Medium · Technical / Domain Questions

Approach

  1. Clarify what is being asked and what a complete answer contains.
  2. State your assumptions explicitly before working the problem.
  3. Say what you would check first and why it is the highest-information step.
  4. Work from the requirement backwards to the design.

Follow-up

  • What assumption would you test first?
  • How would you know your answer was wrong?

If given a performance issue in a service, what steps would you take to diagnose and fix it?

Medium · Problem-Solving / Case Studies

Approach

  1. Clarify what is being asked and what a complete answer contains.
  2. State your assumptions explicitly before working the problem.
  3. Say what you would check first and why it is the highest-information step.
  4. 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 prioritize features for a product in a tight deadline scenario.

Medium · Problem-Solving / Case Studies

Approach

  1. Clarify what is being asked and what a complete answer contains.
  2. State your assumptions explicitly before working the problem.
  3. Say what you would check first and why it is the highest-information step.
  4. 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 tools and methodologies do you use for debugging and testing your code?

Medium · Technical / Domain Questions

Approach

  1. Establish what changed and when, before forming any theory.
  2. Pick a bisection that eliminates candidates whichever way it turns out.
  3. Check the instrumentation before believing the symptom.
  4. Separate the trigger from the cause; the deploy is rarely the bug.

Follow-up

  • What would you look at first, and what would it rule out?
  • How would you tell a cause from a coincidence here?

Suggested practice checklist

Built from the rounds and topics Censys candidates report.

Day 1 — Map the Censys loop

  • Write out the reported sequence: Initial Screening, Technical Assessments, Interviews with Team Members.
  • 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.

Day 2 — Work Kubernetes (K8s)

  • Spend the session on Kubernetes (K8s), which Censys candidates report being tested on.
  • Write one worked example in Kubernetes (K8s) and time yourself on it.

Deliverable: One timed worked example in Kubernetes (K8s).

Day 3 — Work API Development

  • Spend the session on API Development, which Censys candidates report being tested on.
  • Write one worked example in API Development and time yourself on it.

Deliverable: One timed worked example in API Development.

Day 4 — Work Coding Assessments (Take-home/Timed)

  • Spend the session on Coding Assessments (Take-home/Timed), which Censys candidates report being tested on.
  • Write one worked example in Coding Assessments (Take-home/Timed) and time yourself on it.

Deliverable: One timed worked example in Coding Assessments (Take-home/Timed).

Day 5 — Answer out loud: Technical / Domain Questions

  • Answer aloud, timed: What is your experience with cloud services and their implementation?
  • Answer aloud, timed: How do you ensure the scalability of a backend system?

Deliverable: Spoken answers to 2 reported Technical / Domain Questions question(s), under time.

Day 6 — Answer out loud: Problem-Solving / Case Studies

  • Answer aloud, timed: How would you approach designing a system to handle millions of concurrent users?
  • Answer aloud, timed: If given a performance issue in a service, what steps would you take to diagnose and fix it?

Deliverable: Spoken answers to 2 reported Problem-Solving / Case Studies question(s), under time.

Day 7 — Answer out loud: Behavioral / Leadership

  • Answer aloud, timed: Can you describe a time when you had to work collaboratively with a team to achieve a goal?
  • Answer aloud, timed: How do you handle conflicts within a team?

Deliverable: Spoken answers to 2 reported Behavioral / Leadership question(s), under time.

Behavioural preparation

Behavioural rounds judge the decision you made and what it cost.

What is your experience with cloud services and their implementation?

Medium · Technical / Domain Questions

Approach

  1. Pick a story where you made the decision, not one where you watched it.
  2. State the situation in two sentences and spend the rest on the reasoning.
  3. Give the blast radius: what could have broken, and what you measured.
  4. 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?

Can you describe a time when you had to work collaboratively with a team to achieve a goal?

Medium · Behavioral / Leadership

Approach

  1. Pick a story where you made the decision, not one where you watched it.
  2. State the situation in two sentences and spend the rest on the reasoning.
  3. Give the blast radius: what could have broken, and what you measured.
  4. 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 handle conflicts within a team?

Medium · Behavioral / Leadership

Approach

  1. Pick a story where you made the decision, not one where you watched it.
  2. State the situation in two sentences and spend the rest on the reasoning.
  3. Give the blast radius: what could have broken, and what you measured.
  4. 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?

What motivates you to deliver high-quality work?

Medium · Behavioral / Leadership

Approach

  1. Pick a story where you made the decision, not one where you watched it.
  2. State the situation in two sentences and spend the rest on the reasoning.
  3. Give the blast radius: what could have broken, and what you measured.
  4. 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?