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Gallagher Software Engineer Interview Guide

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

The 60-second brief

As a Software Engineer at Gallagher, you play a vital role in building and maintaining the technological infrastructure that supports a global leader in insurance, risk management, and consulting. Your work directly impacts how internal and external stakeholders interact with complex data, ensuring that the firm remains agile and efficient in a highly regulated, fast-paced industry. You will likely work within cross-functional teams, contributing to the development of robust software solutions that solve real-world problems. Whether you are focusing on backend systems, internal tools, or client-facing applications, your contribution is critical to the firm’s operational excellence. You can expect a professional, collaborative environment where your technical proficiency is balanced with a need for clear communication and an understanding of the business domain.

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

Focus areas

  • SQL
  • OOP concepts
  • Aptitude testing

The Gallagher interview process

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

Initial Screening

An initial screening to gauge interest and baseline qualifications.

What to demonstrate

  • An initial screening to gauge interest and baseline qualifications
  • Depth in SQL

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 Deep-Dive

In-depth technical discussions with hiring managers or architecture teams.

What to demonstrate

  • In-depth technical discussions with hiring managers or architecture teams
  • Depth in SQL

How to prepare

  • Answer aloud and timed: How do you ensure your code is scalable and maintainable?
  • Answer aloud and timed: Can you explain the core principles of Object-Oriented Programming (OOP)?

Aptitude Test

Possibility of an early aptitude test covering reasoning, SQL, and programming fundamentals.

What to demonstrate

  • Possibility of an early aptitude test covering reasoning, SQL, and programming fundamentals
  • Depth in SQL

How to prepare

  • Answer aloud and timed: How do you approach database design and SQL query optimization?
  • Answer aloud and timed: Can you walk us through a challenging project you worked on and the specific role you played?

Where candidates lose points

Be transparent about your expectations

If you are in the middle of other processes, be professional about your timeline, but prioritize clarity regarding your requirements and expectations from the start.

Prepare for the aptitude test

If your process includes an aptitude round, brush up on basic logic, SQL queries, and OOP concepts. These are often used as a baseline filter.

Research the firm

Understand the core business of Gallagher. Showing that you understand how your code supports their clients adds significant value to your candidacy.

Clarify the budget

If you have specific salary requirements, ensure these are addressed early in the screening process to avoid misalignment later.

Questions & practice

Walk me through your thought process when faced with an ambiguous technical requirement.

Medium · Problem Solving and Aptitude

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 do you approach solving a performance bottleneck in an application?

Medium · Problem Solving and Aptitude

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?

Describe a situation where you had to simplify a complex technical concept for a non-technical audience.

Medium · Problem Solving and Aptitude

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 is your strategy for testing and quality assurance during the development lifecycle?

Medium · Problem Solving and Aptitude

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 do you approach database design and SQL query optimization?

Medium · Technical and Domain Knowledge

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?

Write the update path that detects a concurrent edit

Medium · optimistic concurrency · transactional outbox · audit history

resource carries version INT NOT NULL DEFAULT 1. resource_revision holds revision_id, resource_id, version, actor_user_id, change_kind, patch JSONB, request_id, created_at with UNIQUE (resource_id, version). outbox_event holds aggregate_type, aggregate_id, aggregate_version, event_type, payload, status. A PUT carries the version the client read. Write the exact statements for the single transaction that applies the edit, records the revision and enqueues 'resource.updated', and give the handler's branch on zero affected rows. Then say what PostgreSQL 16 does under READ COMMITTED when two of these updates hit one row at once.

Approach

  1. One transaction, three writes, no network call inside it: UPDATE resource SET title = $3, version = version + 1, updated_at = now() WHERE resource_id = $1 AND tenant_id = $4 AND version = $2; then INSERT the resource_revision row at version $2 + 1; then INSERT the outbox_event row at the same aggregate_version. The event goes to a table rather than a broker because no transaction spans both.
  2. Branch on the affected-row count before doing anything else. Zero has three causes — stale version, wrong tenant, row gone — so re-read once and map to 409 carrying the current version, or 404 for an id outside the caller's tenant, which also stops the endpoint confirming that another tenant's id exists.
  3. State the engine behaviour instead of assuming it. Under READ COMMITTED the second UPDATE blocks on the row lock, and when the first commits PostgreSQL re-evaluates the WHERE clause against the newly committed row, so the version predicate now fails and the statement reports zero rows. Under REPEATABLE READ the identical collision raises SQLSTATE 40001 instead, so the handler must fold both shapes into one conflict response.
  4. Keep UNIQUE (resource_id, version) even though the predicate already serialises writers. It is what makes a lost update unwritable if any other path ever reaches the revision table, and it converts a logic bug into 23505 rather than into a silently missing history row.

Follow-up

  • A client sends the version it read ten minutes ago and the resource has moved three versions. What is in your 409 so it can resolve the conflict without a full re-fetch?
  • Two editors, two disjoint fields, no overlap. Does your answer still refuse the second write, and should it?

How do you ensure your code is scalable and maintainable?

Medium · Technical and Domain Knowledge

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?

Can you explain the core principles of Object-Oriented Programming (OOP)?

Medium · Technical and Domain Knowledge

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 your process for debugging complex issues within a production environment.

Medium · Technical and Domain Knowledge

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 Gallagher candidates report.

Day 1 — Map the Gallagher loop

  • Write out the reported sequence: Initial Screening, Technical Deep-Dive, Aptitude Test.
  • 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 SQL

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

Deliverable: One timed worked example in SQL.

Day 3 — Work OOP concepts

  • Spend the session on OOP concepts, which Gallagher candidates report being tested on.
  • Write one worked example in OOP concepts and time yourself on it.

Deliverable: One timed worked example in OOP concepts.

Day 4 — Work Aptitude testing

  • Spend the session on Aptitude testing, which Gallagher candidates report being tested on.
  • Write one worked example in Aptitude testing and time yourself on it.

Deliverable: One timed worked example in Aptitude testing.

Day 5 — Answer out loud: Technical and Domain Knowledge

  • Answer aloud, timed: Explain your experience with.NET frameworks and how you have utilized them in previous projects.
  • Answer aloud, timed: Describe your process for debugging complex issues within a production environment.

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

Day 6 — Answer out loud: Behavioral and Culture Fit

  • Answer aloud, timed: Can you walk us through a challenging project you worked on and the specific role you played?
  • Answer aloud, timed: How do you handle situations where you have a disagreement with a team member or stakeholder?

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

Day 7 — Answer out loud: Problem Solving and Aptitude

  • Answer aloud, timed: Walk me through your thought process when faced with an ambiguous technical requirement.
  • Answer aloud, timed: How do you approach solving a performance bottleneck in an application?

Deliverable: Spoken answers to 2 reported Problem Solving and Aptitude question(s), under time.

Behavioural preparation

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

Explain your experience with.NET frameworks and how you have utilized them in previous projects.

Medium · Technical and Domain Knowledge

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 walk us through a challenging project you worked on and the specific role you played?

Medium · Behavioral and Culture Fit

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 situations where you have a disagreement with a team member or stakeholder?

Medium · Behavioral and Culture Fit

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 are you interested in working for a global firm like Gallagher?

Medium · Behavioral and Culture Fit

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 us about a time you had to learn a new technology or tool under a tight deadline.

Medium · Behavioral and Culture Fit

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 prioritize your tasks when managing multiple competing deadlines?

Medium · Behavioral and Culture Fit

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?