Interview guides · Lawrence Berkeley Lab

Lawrence Berkeley Lab Software Engineer Interview Guide

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

The 60-second brief

As a Software Engineer at Lawrence Berkeley Lab, you will build and maintain the digital infrastructure, control systems, and data-analysis platforms that drive world-class scientific research. This role directly supports critical domains like genomics, beamline operations, and advanced energy research, requiring you to bridge complex scientific computing needs with robust software engineering practices. Your work enables researchers to process massive datasets, automate physical instruments, and accelerate discoveries that impact national science initiatives. The position sits at the intersection of high-performance computing, systems engineering, and collaborative scientific inquiry. You might find yourself working on infrastructure engineering for distributed systems, developing software for the Joint BioEnergy Institute, or optimizing codebases for large-scale analytical pipelines.

This guide is scoped to a Software Engineer candidate at Lawrence Berkeley Lab.

Focus areas

Behavioral Interviewing (STAR method)
Coding Exercises (write code during interview)
Communication Skills (technical + presentation)

The Lawrence Berkeley Lab interview process

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

Phone Screen

Initial screening call to evaluate candidate's background and fit for the role.

What to demonstrate

Initial screening call to evaluate candidate's background and fit for the role
Depth in Behavioral Interviewing (STAR method)

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

One or more rounds of technical evaluations to assess relevant skills.

What to demonstrate

One or more rounds of technical evaluations to assess relevant skills
Depth in Behavioral Interviewing (STAR method)

How to prepare

Answer aloud and timed: What is your experience reviewing filesystems, writing Linux commands, and managing server infrastructure?
Answer aloud and timed: How do you approach software development when building applications for research or scientific projects?

Behavioral Evaluations

Interviews focusing on behavioral questions, cultural fit, and teamwork.

What to demonstrate

Interviews focusing on behavioral questions, cultural fit, and teamwork
Depth in Behavioral Interviewing (STAR method)

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 evaluations above and write down what you would ask to confirm before it.

Final Interviews

Concluding interviews that may include additional assessments and discussions.

What to demonstrate

Concluding interviews that may include additional assessments and discussions
Depth in Behavioral Interviewing (STAR method)

How to prepare

Answer aloud and timed: How did you contribute to your previous research projects, and what was the ultimate impact of your software on those goals?
Answer aloud and timed: Tell us about a complex personal project you built and how you managed its technical scope.

Where candidates lose points

Embrace the scientific mission

Research the lab's recent publications or high-profile projects in your target domain before your interview, and be ready to explain why you want to apply your software skills to scientific research rather than commercial enterprise.

Master your project narrative

Expect every interviewer to read your resume closely and ask granular questions about past work; prepare 2 or 3 comprehensive project stories that highlight your technical decisions, trade-offs, and measurable outcomes.

Practice whiteboard and verbal system design

Even in virtual formats, interviewers will ask you to talk through how you would architect a system, control an instrument, or debug a memory leak on the fly.

Prepare for behavioral structure

Do not wing your answers to situational questions about safety, difficult coworkers, or procurement; use the STAR method to keep your responses focused, professional, and impactful.

Ask insightful questions about infrastructure

Show your engineering maturity by asking the panel about their technical debt, CI/CD pipelines, hardware constraints, and how they handle cross-functional collaboration with research scientists.

Questions & practice

Can you explain how garbage collection works in Java, and what methods you use to find memory leaks?

Medium · Technical and Domain Knowledge

Approach

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

Diff a projection against the primary without per-row point reads

Hard · reconciliation · range hashing · throttling

The listing projection has drifted and some rows show a stale version. The primary holds 40,000,000 resource rows across 12,000 tenants while serving 1,200 writes and 14,000 reads per second. The obvious repair, reading each resource row and comparing its version against the projection, is correct and would eventually finish. Explain precisely why it is unacceptable here, then give a diff that finds the differing rows, state its complexity, and make it safe to run against a live primary. Replication lag is usually under 100 ms and is not bounded.

Approach

Quantify the naive cost rather than calling it slow: 40,000,000 point reads at even 0.5 ms each is over five hours serialised, and the only lever is concurrency, which is exactly what you cannot spend. The primary's pool is sized for the write path, and 40,000,000 random reads evict the buffer cache that sustains the 85 percent cache hit rate, so the audit degrades the system it is auditing.
Replace random access with one ordered pass per side. Both sides can be read in (tenant_id, resource_id) order, which is a sequential scan on each and a merge join in O(n) time and O(1) memory. For a dense diff that is the whole answer, and it reads the primary once instead of 40,000,000 times.
For the expected sparse case, compare range hashes instead of rows: partition the key space, compute per range an order-independent aggregate over hash(resource_id, version), compare aggregates, and descend only into ranges that differ. With d differing rows and branching factor B, at most d ranges mismatch per level, so the drill-down examines O(d log_B(n/d)) ranges and reads full rows only in mismatching leaves.
Aggregate with a sum modulo 2^64 or a multiset hash, never XOR. XOR is order-independent but self-cancelling, so two rows wrong in the same way, or a row duplicated on one side, leave the range aggregate matching and the range is declared clean.

Follow-up

The diff reports 900 stale rows. How do you decide between patching those rows and rebuilding the projection from resource_revision?
Same job, but the projection lives in a search index that cannot be scanned in key order. What changes?

Archive a resource graph without breaking live references or recursing

Medium · graph traversal · topological order · tenant isolation

Resources reference other resources within a tenant; for the largest tenant the reference table holds up to 2,000,000 nodes and 8,000,000 edges. Archiving a resource must archive everything reachable from it that nothing outside the set still references, refuse when a live external referrer exists, and terminate when references form cycles, which they legitimately do. Produce the archive order and the refusal list, targeting O(V+E). Say what stops the traversal crossing a tenant boundary, and why recursion is the wrong control structure at this size.

Approach

Load the subgraph with the tenant predicate on both endpoints of the edge, not only on the side you started from. Scoping the left table alone is the classic cross-tenant leak: one mis-entered edge then pulls another tenant's resources into the traversal and, worse, into the archive.
Traverse iteratively with an explicit stack. A 2,000,000-node graph can hold a chain deep enough to exhaust a native stack in the low tens of thousands of frames, and that failure is a process crash rather than an error you can return.
Treat cycles as data rather than corruption: compute strongly connected components with Tarjan in O(V+E) using its own explicit stack, then condense. The condensation is a DAG, so a topological order over it gives the archive order, and every member of a component archives in one transaction because no order within a cycle is valid.
Decide refusals with reverse edges. A candidate is archivable only if every in-edge originates inside the candidate set, so build the transpose or count in-degrees restricted to the visited set, and emit each blocked resource with the id of the external referrer, which is the only part of the answer an operator can act on.

Follow-up

The graph is read in one query and the archive writes a minute later. What can change in between, and how do you make the write safe?
The candidate set is 400,000 resources. Is that one transaction, and if not, what does a half-finished archive look like to a reader?

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

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.
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.
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.
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?

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

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.
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.
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.
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 would you handle overridden versus overloaded methods in your code design?

Medium · Technical and Domain Knowledge

Approach

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

Follow-up

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

How do you approach software development when building applications for research or scientific projects?

Medium · Technical and Domain Knowledge

Approach

Clarify what is being asked and what a complete answer contains.
State your assumptions explicitly before working the problem.
Say what you would check first and why it is the highest-information step.
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 basic programming design principles do you apply when starting a new codebase from scratch?

Medium · Technical and Domain Knowledge

Approach

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

Follow-up

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

Can you walk us through your resume and describe how you have applied software engineering to projects outside

Medium · Background and Project Deep Dive

Can you walk us through your resume and describe how you have applied software engineering to projects outside of academia?

Approach

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

Follow-up

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

How did you contribute to your previous research projects, and what was the ultimate impact of your software o

Medium · Background and Project Deep Dive

How did you contribute to your previous research projects, and what was the ultimate impact of your software on those goals?

Approach

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

Follow-up

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

Tell us about a complex personal project you built and how you managed its technical scope.

Medium · Background and Project Deep Dive

Approach

Clarify what is being asked and what a complete answer contains.
State your assumptions explicitly before working the problem.
Say what you would check first and why it is the highest-information step.
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, languages, and frameworks are you most comfortable using in a production environment?

Medium · Background and Project Deep Dive

Approach

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

Follow-up

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

How do you explain technical software concepts to researchers or stakeholders who lack a computer science back

Medium · Background and Project Deep Dive

How do you explain technical software concepts to researchers or stakeholders who lack a computer science background?

Approach

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

Follow-up

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

Read latency spikes on a sixty-second sawtooth

Medium · cache stampede · single-flight · ttl

The cached listing read path serves about 14k reads/second at an 85% hit rate. p99 sits at 35 ms for 57 seconds, jumps to 900 ms for 3, and repeats. During each spike the primary shows several hundred identical listing queries starting within the same millisecond, all carrying one large tenant's id. Cache entries use a 60-second TTL. Give the mechanism, the ordered checks, the fix, and the correctness hazard your fix must not introduce.

Approach

Match the period to a configured number before theorising about load. A spike every 60 seconds against a 60-second TTL is an entry expiring, and you confirm it by correlating spike timestamps with the entry's write time rather than with the traffic curve. If the period had matched a cron or a GC interval instead, this is a different investigation.
Establish the concurrency of the miss. Several hundred identical queries in one millisecond means the miss path has no coalescing: every request that arrives between expiry and repopulation recomputes. The herd size is that key's arrival rate times its recompute time, so at 1.2k reads/second for the hot key and a 250 ms recompute you expect about 300 concurrent misses, which matches what is observed.
Add single-flight on the miss path so one caller per key recomputes under a short-lived lock while the rest wait for its result. Prefer stale-while-revalidate where the read tolerates it: return the expired value immediately and refresh asynchronously, which removes the latency spike rather than serialising it into a queue of waiters.
De-synchronise the keys. Write TTLs with jitter, for example 60 seconds plus or minus 10%, so a deploy or a mass invalidation does not align every key on the same second and turn a per-key herd into a fleet-wide one.

Follow-up

The same sawtooth appears on a key that is invalidated on write rather than expired. Is that the same bug?
How does your answer change if the recompute takes 4 seconds instead of 250 ms?

Suggested practice checklist

Built from the rounds and topics Lawrence Berkeley Lab candidates report.

Day 1 — Map the Lawrence Berkeley Lab loop

Write out the reported sequence: Phone Screen, Technical Assessments, Behavioral Evaluations, Final Interviews.
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 Behavioral Interviewing (STAR method)

Spend the session on Behavioral Interviewing (STAR method), which Lawrence Berkeley Lab candidates report being tested on.
Write one worked example in Behavioral Interviewing (STAR method) and time yourself on it.

Deliverable: One timed worked example in Behavioral Interviewing (STAR method).

Day 3 — Work Coding Exercises (write code during interview)

Spend the session on Coding Exercises (write code during interview), which Lawrence Berkeley Lab candidates report being tested on.
Write one worked example in Coding Exercises (write code during interview) and time yourself on it.

Deliverable: One timed worked example in Coding Exercises (write code during interview).

Day 4 — Work Communication Skills (technical + presentation)

Spend the session on Communication Skills (technical + presentation), which Lawrence Berkeley Lab candidates report being tested on.
Write one worked example in Communication Skills (technical + presentation) and time yourself on it.

Deliverable: One timed worked example in Communication Skills (technical + presentation).

Day 5 — Answer out loud: Technical and Domain Knowledge

Answer aloud, timed: Can you explain how garbage collection works in Java, and what methods you use to find memory leaks?
Answer aloud, timed: How would you handle overridden versus overloaded methods in your code design?

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

Day 6 — Answer out loud: Background and Project Deep Dive

Answer aloud, timed: Can you walk us through your resume and describe how you have applied software engineering to projects outside of academia?
Answer aloud, timed: How did you contribute to your previous research projects, and what was the ultimate impact of your software on those goals?

Deliverable: Spoken answers to 2 reported Background and Project Deep Dive question(s), under time.

Day 7 — Answer out loud: Behavioral and Situational Leadership

Answer aloud, timed: What would you do if you saw a co-worker violate lab safety protocols?
Answer aloud, timed: How do you handle a situation where a co-worker becomes an obstacle rather than a resource on a project team?

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

Behavioural preparation

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

What is your experience reviewing filesystems, writing Linux commands, and managing server infrastructure?

Medium · Technical and Domain Knowledge

Approach

Pick a story where you made the decision, not one where you watched it.
State the situation in two sentences and spend the rest on the reasoning.
Give the blast radius: what could have broken, and what you measured.
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 would you do if you saw a co-worker violate lab safety protocols?

Medium · Behavioral and Situational Leadership

Approach

Pick a story where you made the decision, not one where you watched it.
State the situation in two sentences and spend the rest on the reasoning.
Give the blast radius: what could have broken, and what you measured.
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 a co-worker becomes an obstacle rather than a resource on a project team?

Medium · Behavioral and Situational Leadership

Approach

Pick a story where you made the decision, not one where you watched it.
State the situation in two sentences and spend the rest on the reasoning.
Give the blast radius: what could have broken, and what you measured.
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 managed vendor relationships or procurement processes for a technical initiat

Medium · Behavioral and Situational Leadership

Can you describe a time when you managed vendor relationships or procurement processes for a technical initiative?

Approach

Pick a story where you made the decision, not one where you watched it.
State the situation in two sentences and spend the rest on the reasoning.
Give the blast radius: what could have broken, and what you measured.
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 navigate disagreements with team members over architectural direction or code standards?

Medium · Behavioral and Situational Leadership

Approach

Pick a story where you made the decision, not one where you watched it.
State the situation in two sentences and spend the rest on the reasoning.
Give the blast radius: what could have broken, and what you measured.
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 are your weakest professional qualities, and how do you actively work to improve them?

Medium · Behavioral and Situational Leadership

Approach

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