cswork / libraryOpen practice workspace

Interview guides · REsurety

REsurety Software Engineer Interview Guide

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

The 60-second brief

As a Software Engineer at REsurety, you are positioned at the intersection of high-scale software engineering and the critical transition to renewable energy. This role is not merely about writing code; it is about building the sophisticated, data-intensive web applications that provide market-leading insights into renewable energy generation and financial performance. You will be instrumental in translating complex, raw energy data into actionable intelligence for clients navigating the modern power grid. The work you do here has a direct impact on how the industry understands and values renewable assets. Whether you are optimizing data pipelines or crafting intuitive interfaces with modern frameworks, your contributions will directly influence the scalability and reliability of REsurety products.

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

Focus areas

  • Software Engineering
  • Coding Assignments
  • Scalable Web Applications

The REsurety interview process

REsurety candidates report 5 rounds over 4-6 weeks. The stages below are what candidates describe, not a published process.

Initial Screening

The process begins with an initial screening to assess your fit for the role.

What to demonstrate

  • The process begins with an initial screening to assess your fit for the role
  • Depth in Software Engineering

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 Skills Assessment

You will undergo a practical assessment to evaluate your technical skills.

What to demonstrate

  • You will undergo a practical assessment to evaluate your technical skills
  • Depth in Software Engineering

How to prepare

  • Answer aloud and timed: How would you optimize the performance of the data processing script you submitted?
  • Answer aloud and timed: Describe a situation where you had to debug a complex data-related issue in your code.

Deep-Dive Interviews

Participate in a series of in-depth interviews with various team members.

What to demonstrate

  • Participate in a series of in-depth interviews with various team members
  • Depth in Software Engineering

How to prepare

  • Answer aloud and timed: Walk me through your most significant technical project to date.
  • Answer aloud and timed: What were the most challenging technical trade-offs you made in your previous role?

Project Presentation

Present your project, showcasing your thought process and design decisions.

What to demonstrate

  • Present your project, showcasing your thought process and design decisions
  • Depth in Software Engineering

How to prepare

  • Answer aloud and timed: How do you handle project requirements that change mid-development?
  • Answer aloud and timed: Tell me about a time you had to learn a new technology stack quickly to deliver a feature.

Final Rounds

The focus shifts to collaborative problem-solving and your potential for long-term growth.

What to demonstrate

  • The focus shifts to collaborative problem-solving and your potential for long-term growth
  • Depth in Software Engineering

How to prepare

  • Answer aloud and timed: What do you know about the current challenges in the renewable energy market?
  • Answer aloud and timed: How do you approach collaboration with non-technical stakeholders, such as Data Scientists?

Where candidates lose points

Own your projects

When asked about past work, be specific about your contribution. Use the "I" instead of "we" to ensure the interviewer understands your direct impact.

Prepare for the "Why

The team values engineers who understand the business context. Always link your technical decisions back to the user or the business goal.

Be ready for the presentation

Practice presenting your most complex project. Aim for a balance between technical depth and high-level strategy.

Engage with the interviewer

Treat the interviews as a conversation. If you are asked a question about a technology you haven't used, explain how you would go about learning it.

Questions & practice

Plot a histogram representing power generation and revenue trends for a specific timeframe.

Medium · Coding and Technical Assignments

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?

Explain the logic behind your chosen approach for the take-home coding assignment.

Medium · Coding and Technical Assignments

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 optimize the performance of the data processing script you submitted?

Medium · Coding and Technical Assignments

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?

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?

Explain why the owner filter ignores the listing index

Medium · composite index · query plans · sort elimination

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

  1. 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.
  2. 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.
  3. 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.
  4. 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?

Walk me through your most significant technical project to date.

Medium · Project Experience and Resume Walkthroug

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 were the most challenging technical trade-offs you made in your previous role?

Medium · Project Experience and Resume Walkthroug

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 a situation where you had to debug a complex data-related issue in your code.

Medium · Coding and Technical Assignments

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 REsurety candidates report.

Day 1 — Map the REsurety loop

  • Write out the reported sequence: Initial Screening, Technical Skills Assessment, Deep-Dive Interviews, Project Presentation, Final Rounds.
  • 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 5 reported rounds, with the weakest marked.

Day 2 — Work Software Engineering

  • Spend the session on Software Engineering, which REsurety candidates report being tested on.
  • Write one worked example in Software Engineering and time yourself on it.

Deliverable: One timed worked example in Software Engineering.

Day 3 — Work Coding Assignments

  • Spend the session on Coding Assignments, which REsurety candidates report being tested on.
  • Write one worked example in Coding Assignments and time yourself on it.

Deliverable: One timed worked example in Coding Assignments.

Day 4 — Work Scalable Web Applications

  • Spend the session on Scalable Web Applications, which REsurety candidates report being tested on.
  • Write one worked example in Scalable Web Applications and time yourself on it.

Deliverable: One timed worked example in Scalable Web Applications.

Day 5 — Answer out loud: Coding and Technical Assignments

  • Answer aloud, timed: Plot a histogram representing power generation and revenue trends for a specific timeframe.
  • Answer aloud, timed: Explain the logic behind your chosen approach for the take-home coding assignment.

Deliverable: Spoken answers to 2 reported Coding and Technical Assignments question(s), under time.

Day 6 — Answer out loud: Project Experience and Resume Walkthrough

  • Answer aloud, timed: Walk me through your most significant technical project to date.
  • Answer aloud, timed: What were the most challenging technical trade-offs you made in your previous role?

Deliverable: Spoken answers to 2 reported Project Experience and Resume Walkthrough question(s), under time.

Day 7 — Answer out loud: Behavioral and Domain Awareness

  • Answer aloud, timed: What do you know about the current challenges in the renewable energy market?
  • Answer aloud, timed: How do you approach collaboration with non-technical stakeholders, such as Data Scientists?

Deliverable: Spoken answers to 2 reported Behavioral and Domain Awareness question(s), under time.

Behavioural preparation

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

How do you handle project requirements that change mid-development?

Medium · Project Experience and Resume Walkthroug

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?

Tell me about a time you had to learn a new technology stack quickly to deliver a feature.

Medium · Project Experience and Resume Walkthroug

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 do you know about the current challenges in the renewable energy market?

Medium · Behavioral and Domain Awareness

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 approach collaboration with non-technical stakeholders, such as Data Scientists?

Medium · Behavioral and Domain Awareness

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?

Describe a time you disagreed with a technical decision; how did you navigate that conflict?

Medium · Behavioral and Domain Awareness

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?

Why do you want to apply your engineering skills to the clean energy sector specifically?

Medium · Behavioral and Domain Awareness

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?