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Lantern Software Engineer Interview Guide

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

The 60-second brief

At Lantern, the Software Engineer role is central to building the infrastructure and financial systems that define our platform. You are not just writing code; you are architecting robust, scalable solutions that handle complex financial data, cloud platform stability, or critical GRC (Governance, Risk, and Compliance) requirements. This position requires a blend of deep technical rigor and an ability to navigate the high-stakes environment of financial technology. You will contribute to high-impact projects that require both precision and strategic thinking. Whether you are optimizing cloud architecture, ensuring the integrity of our financial systems, or consulting on platform-wide developer experiences, your work directly influences the reliability and growth of the Lantern ecosystem.

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

Focus areas

  • Cloud Platforms (Implied by Senior Cloud Platform Engineer)
  • GRC / Governance, Risk & Compliance (Implied by Senior GRC E
  • Financial Systems Engineering (Implied by Senior Financial S

The Lantern interview process

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

Application Review

Initial review of candidate applications to assess qualifications and fit.

What to demonstrate

  • Initial review of candidate applications to assess qualifications and fit
  • Depth in Cloud Platforms (Implied by Senior Cloud Platform Engineer)

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.

Phone Screen

Initial screening call to discuss the candidate's background and role expectations.

What to demonstrate

  • Initial screening call to discuss the candidate's background and role expectations
  • Depth in Cloud Platforms (Implied by Senior Cloud Platform Engineer)

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 Interview

Deep-dive technical sessions focusing on coding skills and problem-solving abilities.

What to demonstrate

  • Deep-dive technical sessions focusing on coding skills and problem-solving abilities
  • Depth in Cloud Platforms (Implied by Senior Cloud Platform Engineer)

How to prepare

  • Answer aloud and timed: Given a specific input set, write a function that handles edge cases efficiently.
  • Answer aloud and timed: Design a service that handles high-frequency financial transactions.

Design Interview

Assessment of design capability through system design discussions.

What to demonstrate

  • Assessment of design capability through system design discussions
  • Depth in Cloud Platforms (Implied by Senior Cloud Platform Engineer)

How to prepare

  • Answer aloud and timed: How would you ensure data consistency across multiple distributed services?
  • Answer aloud and timed: Describe the architecture of a cloud platform you have previously built or maintained.

Behavioral Interview

Evaluation of cultural alignment and soft skills through behavioral questions.

What to demonstrate

  • Evaluation of cultural alignment and soft skills through behavioral questions
  • Depth in Cloud Platforms (Implied by Senior Cloud Platform Engineer)

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.

Final Round

Interviews with leadership to assess overall fit and alignment with company values.

What to demonstrate

  • Interviews with leadership to assess overall fit and alignment with company values
  • Depth in Cloud Platforms (Implied by Senior Cloud Platform Engineer)

How to prepare

  • Answer aloud and timed: Describe a time you disagreed with a technical decision made by a peer or manager.
  • Answer aloud and timed: How do you handle a situation where requirements are vague or constantly changing?

Where candidates lose points

Think out loud

This is the most critical advice. Even if you are unsure of the answer, narrating your thought process allows the interviewer to see how you analyze problems.

Own your gaps

If you don't know an answer, be honest. Explain how you would go about finding the answer or what your logical process would be to solve it.

Prepare your own questions

Use the final rounds to ask about our engineering culture, our biggest technical challenges, and how we measure success.

Focus on the 'Why'

When discussing past projects, don't just list what you did. Explain why you chose a specific technology or architectural pattern over the alternatives.

Questions & practice

Implement a basic data structure or algorithm without relying on external libraries.

Medium · Technical 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?

Walk through the time and space complexity of your proposed solution.

Medium · Technical 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 optimize this function for a high-concurrency environment?

Medium · Technical Coding & Algorithms

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?

Given a specific input set, write a function that handles edge cases efficiently.

Medium · Technical 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?

What are the trade-offs between choosing SQL versus NoSQL for our specific use case?

Medium · System Design & Architecture

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?

Stop tag and share joins from fanning out a page

Medium · join fan-out · semi-joins · many-to-many

resource_tag is (resource_id, tag_id) with PK (resource_id, tag_id); resource_share is (resource_id, shared_with_user_id, permission). The tagged-and-shared listing inner-joins resource to both, filters tenant_id, tag_id = ANY($2) and shared_with_user_id = $3, orders by updated_at DESC and takes 50. Pages come back with fewer than 50 distinct resources and the total in the header is far too high. Explain the row multiplication, rewrite both the page query and the count query so each is correct, and name the index each one needs. PostgreSQL 16.

Approach

  1. Do the arithmetic against the predicates that are actually there. An inner join emits one row per matching child row, and both joins are filtered: tag_id = ANY($2) admits only the requested tags, shared_with_user_id = $3 admits one user's share rows. So a resource holding three of the requested tags and shared with $3 once yields three rows, not one — the multiplier is its count of matching tags times its share rows for that single user, and that second factor is 1 unless the table admits duplicate (resource_id, shared_with_user_id) pairs. LIMIT 50 then limits rows rather than resources, and COUNT(*) counts pairs — the header is the product, not the population.
  2. Reject DISTINCT as the fix. It deduplicates after the product has been built, so the planner must materialise and sort the fanned-out set before the LIMIT can apply, and it leaves any SUM or AVG in the same select list wrong.
  3. Rewrite both filters as semi-joins, keeping resource as the only row source: AND EXISTS (SELECT 1 FROM resource_tag rt WHERE rt.resource_id = r.resource_id AND rt.tag_id = ANY($2)) and the same shape against resource_share. A semi-join stops at the first match per resource and preserves the driving index order, so ORDER BY updated_at DESC, resource_id DESC LIMIT 50 still stops after 50 rows.
  4. Count with the same predicates and no join at all: SELECT count(*) FROM resource r WHERE r.tenant_id = $1 AND r.status = 'active' AND EXISTS (...) AND EXISTS (...). Nothing multiplies a resource, so the number is the population.

Follow-up

  • The filter changes from 'any of these tags' to 'all of these tags'. Rewrite it and state what it costs relative to the ANY form.
  • A resource can be shared with the same user twice under different permissions. Does your count change, and should it?

Design a service that handles high-frequency financial transactions.

Medium · System Design & Architecture

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?

How would you ensure data consistency across multiple distributed services?

Medium · System Design & Architecture

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 the architecture of a cloud platform you have previously built or maintained.

Medium · System Design & Architecture

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?

How do you approach monitoring and alerting for a critical production service?

Medium · System Design & Architecture

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 situation where you had to debug a complex performance bottleneck in production.

Medium · Technical Coding & Algorithms

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 Lantern candidates report.

Day 1 — Map the Lantern loop

  • Write out the reported sequence: Application Review, Phone Screen, Technical Interview, Design Interview, Behavioral Interview, Final Round.
  • 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 6 reported rounds, with the weakest marked.

Day 2 — Work Cloud Platforms (Implied by Senior Cloud Platform Engineer)

  • Spend the session on Cloud Platforms (Implied by Senior Cloud Platform Engineer), which Lantern candidates report being tested on.
  • Write one worked example in Cloud Platforms (Implied by Senior Cloud Platform Engineer) and time yourself on it.

Deliverable: One timed worked example in Cloud Platforms (Implied by Senior Cloud Platform Engineer).

Day 3 — Work GRC / Governance, Risk & Compliance (Implied by Senior GRC Engineer)

  • Spend the session on GRC / Governance, Risk & Compliance (Implied by Senior GRC Engineer), which Lantern candidates report being tested on.
  • Write one worked example in GRC / Governance, Risk & Compliance (Implied by Senior GRC Engineer) and time yourself on it.

Deliverable: One timed worked example in GRC / Governance, Risk & Compliance (Implied by Senior GRC Engineer).

Day 4 — Work Financial Systems Engineering (Implied by Senior Financial Systems Engineer)

  • Spend the session on Financial Systems Engineering (Implied by Senior Financial Systems Engineer), which Lantern candidates report being tested on.
  • Write one worked example in Financial Systems Engineering (Implied by Senior Financial Systems Engineer) and time yourself on it.

Deliverable: One timed worked example in Financial Systems Engineering (Implied by Senior Financial Systems Engineer).

Day 5 — Answer out loud: Technical Coding & Algorithms

  • Answer aloud, timed: Implement a basic data structure or algorithm without relying on external libraries.
  • Answer aloud, timed: Walk through the time and space complexity of your proposed solution.

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

Day 6 — Answer out loud: System Design & Architecture

  • Answer aloud, timed: Design a service that handles high-frequency financial transactions.
  • Answer aloud, timed: How would you ensure data consistency across multiple distributed services?

Deliverable: Spoken answers to 2 reported System Design & Architecture question(s), under time.

Day 7 — Answer out loud: Behavioral & Leadership

  • Answer aloud, timed: Describe a time you disagreed with a technical decision made by a peer or manager.
  • Answer aloud, timed: How do you handle a situation where requirements are vague or constantly changing?

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.

Describe a time you disagreed with a technical decision made by a peer or manager.

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 a situation where requirements are vague or constantly changing?

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?

Tell me about a project where you had to mentor a junior engineer or take the lead on a technical initiative.

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 communicate technical debt to non-technical stakeholders?

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?

Tell me about a time you failed to meet a deadline. What did you learn?

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?