HRL Laboratories Software Engineer Interview Guide

The 60-second brief

A Software Engineer at HRL Laboratories operates at the fascinating intersection of cutting-edge scientific research and robust systems engineering. Originally founded as Hughes Research Laboratories and now a joint venture between Boeing and General Motors, HRL Laboratories is a physical sciences and engineering research powerhouse. In this role, you will not simply be building standard web applications or enterprise SaaS products; instead, you will design, develop, and deploy software that powers quantum computers, neuromorphic processors, autonomous vehicles, advanced sensors, and next-generation microelectronics. The software you write directly impacts the physical world. Your code might control a high-voltage semiconductor testing apparatus, simulate complex electromagnetic fields, or process real-time sensor data on an embedded aerospace platform.

This guide is scoped to a Software Engineer candidate at HRL Laboratories.

Focus areas

• High Voltage Engineering
• Take-home Project Development
• Power Electronics

The HRL Laboratories interview process

HRL Laboratories candidates report 4 rounds over 3-5 weeks. The stages below are what candidates describe, not a published process.

Recruiter Call

Initial call to establish baseline qualifications, availability, and alignment.

What to demonstrate

• Initial call to establish baseline qualifications, availability, and alignment
• Depth in High Voltage 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 Phone Screens

One or more technical phone screens focusing on background, coursework, and programming concepts.

What to demonstrate

• One or more technical phone screens focusing on background, coursework, and programming concepts
• Depth in High Voltage 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.

Take-Home Project

Some teams may assign a targeted, take-home technical project relevant to the role.

What to demonstrate

• Some teams may assign a targeted, take-home technical project relevant to the role
• Depth in High Voltage Engineering

How to prepare

• Answer aloud and timed: Tell me about a time you had to learn a highly specialized scientific or mathematical concept to write a piece of software.
• Answer aloud and timed: What are the primary differences between C and C++ when it comes to memory management and performance?

Full-Day Interview

Comprehensive interview day with formal presentation, multiple interviews, team lunch, and facility tour.

What to demonstrate

• Comprehensive interview day with formal presentation, multiple interviews, team lunch, and facility tour
• Depth in High Voltage Engineering

How to prepare

• Answer aloud and timed: How do you handle concurrency and multi-threading in Python versus C++?
• Answer aloud and timed: Describe a scenario where you had to optimize code that was running too slowly on a hardware target. What profiling tools did you use?

Where candidates lose points

Pace your energy

The onsite interview is a marathon of 30-to-40-minute sessions. You must make a great first impression a dozen times over. Eat a good breakfast, stay hydrated, and remember that each interviewer is a fresh start—even if you feel you struggled in a previous session.

Treat lunch as an interview

You will have lunch with several members of the team, often overlooking the Pacific Ocean in Malibu. While the setting is beautiful and the conversation will feel casual, remember that your potential teammates are still evaluating your communication skills, cultural fit, and professionalism.

Show genuine curiosity

HRL Laboratories is a place of invention. Ask your interviewers about their current research, the challenges they are facing, and the technologies they are excited about. Showing a passion for learning is highly valued.

Highlight hardware familiarity

Even if you are a pure software engineer, any experience you have working with physical hardware, sensors, or lab equipment is highly attractive to HRL hiring managers. Make sure to emphasize these experiences.

Questions & practice

What are the primary differences between C and C++ when it comes to memory management and performance?

Medium · Core Software Engineering & Programming

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?

How do you handle concurrency and multi-threading in Python versus C++?

Medium · Core Software Engineering & Programming

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?

Describe a scenario where you had to optimize code that was running too slowly on a hardware target. What prof

Medium · Core Software Engineering & Programming

Describe a scenario where you had to optimize code that was running too slowly on a hardware target. What profiling tools did you use?

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?

How do you ensure your code is maintainable and well-documented when working on a highly experimental research

Medium · Core Software Engineering & Programming

How do you ensure your code is maintainable and well-documented when working on a highly experimental research project?

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?

Describe a situation where a project's requirements were highly ambiguous or constantly shifting. How did you

Medium · Behavioral & Team Collaboration

Describe a situation where a project's requirements were highly ambiguous or constantly shifting. How did you maintain progress?

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?

Denormalise tenant onto revisions and backfill it live

Hard · denormalisation · online migration · backfill

resource_revision (revision_id, resource_id, version, actor_user_id, change_kind, patch, request_id, created_at) has 400M rows and no tenant column; tenant_id lives only on resource. Two reads need it: a tenant-scoped audit feed ordered by created_at DESC, and an offboarding purge. Both join back to resource today. Justify adding tenant_id to resource_revision against those two reads, name the anomaly the copy introduces and the constraint that prevents it, then give the ordered migration for a live table taking 1.2k writes/second — the lock each step takes, how the backfill is batched, and where each step stops being reversible. PostgreSQL 16.

Approach

1. Justify from the access path rather than from taste. Without the column, the audit feed either scans resource_revision by created_at and discards other tenants' rows, or resolves the tenant's resource_ids first and probes with them — both proportional to the tenant's whole history rather than to one page. With (tenant_id, created_at DESC, revision_id DESC) it is a seek that stops at 50 rows, and the purge becomes a ranged delete instead of a join.
2. Name the cost exactly: a second copy of a fact can disagree with the first. Make the disagreement unwritable rather than documented — add UNIQUE (resource_id, tenant_id) on resource so it can serve as a foreign-key target, then FOREIGN KEY (resource_id, tenant_id) REFERENCES resource (resource_id, tenant_id) on the revision table. A revision can then only ever carry its parent's tenant.
3. Step one, expand: ALTER TABLE resource_revision ADD COLUMN tenant_id BIGINT NULL, with no default, so it is a catalogue change and no rewrite. It still needs ACCESS EXCLUSIVE for an instant, and that instant queues behind the longest open transaction on the table while every later query queues behind it — set lock_timeout to 2s and retry rather than wait.
4. Step two, dual-write: deploy the writer that populates tenant_id on every new revision while reads still use the join. Reversible by redeploying the previous build, because nothing reads the column yet.

Follow-up

• The backfill is half finished and a rollback is required. What state is the table in, and what does the previous build do with a half-populated column?
• How do you verify the backfill actually finished, given rows are still being inserted while it runs?

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?

Can you walk me through the architecture of the most complex software project on your resume?

Medium · Past Research & Technical Background

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?

What was your specific contribution to your past research group or school project, and how did you handle data

Medium · Past Research & Technical Background

What was your specific contribution to your past research group or school project, and how did you handle data collection and analysis?

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?

Explain the physical or hardware constraints you had to write software for in your previous role.

Medium · Past Research & Technical Background

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?

Why did you choose to stop your studies at a Master's degree (or BS/PhD), and how does your academic progressi

Medium · Past Research & Technical Background

Why did you choose to stop your studies at a Master's degree (or BS/PhD), and how does your academic progression align with your career goals?

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?

Every query on one table stalls for forty seconds mid-deploy

Medium · lock contention · ddl · migrations

During a release on PostgreSQL, every query touching resource times out for about 40 seconds and then recovers with no intervention. The release ran one migration, ALTER TABLE resource ADD COLUMN archived_reason TEXT, and the migration log shows it completing in 6 ms. Unrelated tables showed no change in error rate. Explain how a 6 ms statement caused a 40-second stall, give the ordered checks you would run on a live system to confirm it, and give the migration procedure that prevents a repeat.

Approach

1. Separate the statement's duration from the lock's duration. ADD COLUMN with no default is a catalogue-only change and genuinely runs in milliseconds, but it requires ACCESS EXCLUSIVE, and it cannot acquire that until every transaction already touching the table has finished.
2. Account for the queueing, which is the part that surprises people. A lock request that is waiting blocks later requests for conflicting modes behind it rather than letting them overtake, so one long-open transaction holds the DDL and the DDL holds all the traffic. The stall length is set by the longest open transaction, not by the size of the change.
3. Confirm on a live system in this order: pg_stat_activity for that table ordered by xact_start, looking for the oldest transaction and specifically for state = idle in transaction; then pg_locks where granted = false to find the waiter; then join them on pid to name blocker and blocked. pg_blocking_pids() does that join for you and is the fastest single call.
4. Prevent rather than merely time it better. Set lock_timeout to a second or two on the migration session so the DDL abandons the queue after a bounded wait and is retried, instead of holding it for as long as the oldest transaction lives. Be exact about what that buys: queries arriving during the wait still queue behind the pending ACCESS EXCLUSIVE request, so each attempt costs them up to one lock_timeout of added latency. The outage goes from 40 seconds to about one second per attempt, not to zero. Also run migrations away from deploy-time peaks, and put a statement timeout and an idle-in-transaction timeout on the analytics role that opens the long transactions.

Follow-up

• The same release also wants NOT NULL on that column. What is the sequence that gets there without a long lock?
• Your lock_timeout retry fails ten times in a row because the analytics transaction is always open. What do you change?

Suggested practice checklist

Built from the rounds and topics HRL Laboratories candidates report.

Day 1 — Map the HRL Laboratories loop

• Write out the reported sequence: Recruiter Call, Technical Phone Screens, Take-Home Project, Full-Day 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 4 reported rounds, with the weakest marked.

Day 2 — Work High Voltage Engineering

• Spend the session on High Voltage Engineering, which HRL Laboratories candidates report being tested on.
• Write one worked example in High Voltage Engineering and time yourself on it.

Deliverable: One timed worked example in High Voltage Engineering.

Day 3 — Work Take-home Project Development

• Spend the session on Take-home Project Development, which HRL Laboratories candidates report being tested on.
• Write one worked example in Take-home Project Development and time yourself on it.

Deliverable: One timed worked example in Take-home Project Development.

Day 4 — Work Power Electronics

• Spend the session on Power Electronics, which HRL Laboratories candidates report being tested on.
• Write one worked example in Power Electronics and time yourself on it.

Deliverable: One timed worked example in Power Electronics.

Day 5 — Answer out loud: Past Research & Technical Background

• Answer aloud, timed: Can you walk me through the architecture of the most complex software project on your resume?
• Answer aloud, timed: What was your specific contribution to your past research group or school project, and how did you handle data collection and analysis?

Deliverable: Spoken answers to 2 reported Past Research & Technical Background question(s), under time.

Day 6 — Answer out loud: Core Software Engineering & Programming Languages

• Answer aloud, timed: What are the primary differences between C and C++ when it comes to memory management and performance?
• Answer aloud, timed: How do you handle concurrency and multi-threading in Python versus C++?

Deliverable: Spoken answers to 2 reported Core Software Engineering & Programming Languages question(s), under time.

Day 7 — Answer out loud: Behavioral & Team Collaboration

• Answer aloud, timed: How do you handle conflicts or differing technical opinions within a multidisciplinary team (e.g., when working with a physicist or hardware designer)?
• Answer aloud, timed: Tell me about a time you faced a significant technical roadblock. How did you pivot, and who did you reach out to for help?

Deliverable: Spoken answers to 2 reported Behavioral & Team Collaboration question(s), under time.

Behavioural preparation

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

Tell me about a time you had to learn a highly specialized scientific or mathematical concept to write a piece

Medium · Past Research & Technical Background

Tell me about a time you had to learn a highly specialized scientific or mathematical concept to write a piece of software.

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 is your experience with software-hardware integration, such as reading data from sensors or controlling e

Medium · Core Software Engineering & Programming

What is your experience with software-hardware integration, such as reading data from sensors or controlling external instruments?

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 or differing technical opinions within a multidisciplinary team (e.g., when workin

Medium · Behavioral & Team Collaboration

How do you handle conflicts or differing technical opinions within a multidisciplinary team (e.g., when working with a physicist or hardware designer)?

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 faced a significant technical roadblock. How did you pivot, and who did you reach out

Medium · Behavioral & Team Collaboration

Tell me about a time you faced a significant technical roadblock. How did you pivot, and who did you reach out to for help?

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 explain highly complex software concepts to non-software stakeholders, such as managers or research

Medium · Behavioral & Team Collaboration

How do you explain highly complex software concepts to non-software stakeholders, such as managers or research scientists?

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 type of work environment allows you to do your best work, and how do you handle constructive criticism on

Medium · Behavioral & Team Collaboration

What type of work environment allows you to do your best work, and how do you handle constructive criticism on your code or research?

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?