Interview guides · United Therapeutics

United Therapeutics Software Engineer Interview Guide

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

The 60-second brief

At United Therapeutics, a Software Engineer does not just write code; they build the digital backbone that supports life-saving medical innovations. As the first publicly traded biotech company structured as a public benefit corporation, United Therapeutics is dedicated to addressing unmet medical needs, from developing novel therapies for pulmonary arterial hypertension (PAH) to pioneering the manufacturing of transplantable organs. Software engineers here design, develop, and scale the critical applications, AI-enabled tools, and data pipelines that accelerate scientific discovery and streamline clinical and manufacturing operations. This role sits at the intersection of cutting-edge technology and biotechnology. You will work on software that directly impacts researchers, clinical scientists, and manufacturing engineers.

This guide is scoped to a Software Engineer candidate at United Therapeutics.

Focus areas

AI-enabled Web Applications
GPT / LLMs (Large Language Models)
.NET

The United Therapeutics interview process

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

HR Screening Call

Initial call focusing on your background, daily tasks, and interest in the company.

What to demonstrate

Initial call focusing on your background, daily tasks, and interest in the company
Depth in AI-enabled Web Applications

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 Screening

Call with the hiring manager, potentially involving a structured resume walkthrough.

What to demonstrate

Call with the hiring manager, potentially involving a structured resume walkthrough
Depth in AI-enabled Web Applications

How to prepare

Answer aloud and timed: How do you approach documenting your software architecture? Walk me through how you use UML diagrams to communicate design patterns.
Answer aloud and timed: Present a past research project or major software deliverable. Explain the problem, your technical approach, and the ultimate business or scientific outcome.

Presentation of Work

Deliver a 30-to-60-minute presentation of past work or research to a panel, followed by Q&A.

What to demonstrate

Deliver a 30-to-60-minute presentation of past work or research to a panel, followed by Q&A
Depth in AI-enabled Web Applications

How to prepare

Answer aloud and timed: How would you design a scalable web application using.NET and Azure to handle high-throughput data processing?
Answer aloud and timed: Explain your experience with RESTful APIs and how you ensure secure, reliable communication across multiple distributed services.

Panel Interview

Comprehensive interview with a mix of software engineers, department heads, and scientific team leaders.

What to demonstrate

Comprehensive interview with a mix of software engineers, department heads, and scientific team leaders
Depth in AI-enabled Web Applications

How to prepare

Answer aloud and timed: How do you optimize database performance and query efficiency when working with large relational databases or star schemas?
Answer aloud and timed: Describe how you would implement a CI/CD pipeline to automate testing and deployment across development, staging, and production environments.

Where candidates lose points

Understand the Mission

Spend time researching United Therapeutics’ pipeline, including their work in pulmonary arterial hypertension (PAH) and their organ manufacturing initiatives. Showing that you understand and care about their public benefit mission will set you apart from other candidates.

Prepare for Non-Technical Panelists

Your panel interviewers will likely include leaders from process engineering, chemical R&D, or production. Frame your technical achievements in terms of how they improve operational efficiency, data reliability, or scientific progress, rather than just code quality.

Document Your Work

Be ready to show how you approach the software development lifecycle. If you have examples of UML diagrams, technical specifications, or architectural documentation you have written, be prepared to discuss them.

If you are interviewing in person, dress professionally and wear comfortable shoes

Onsite interviews often include a comprehensive tour of their advanced manufacturing or research facilities.

Questions & practice

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?

Identify the heaviest tenants in a five-minute window under memory pressure

Medium · top-k · heavy hitters · streaming

The edge service handles about 3,000 requests per second across roughly 50,000 tenants, peaking near 9,000. Expose the 50 heaviest tenants by request count over the trailing five minutes so limits can be tightened before one tenant's backfill starves the fleet. You may not retain five minutes of raw records. Give the exact solution and its memory, then the bounded-memory approximation with its error stated as a formula, and say which you would ship and at what tenant cardinality that choice changes.

Approach

Do the exact version first, because it is affordable at this cardinality: a ring of 300 one-second counters per tenant, advanced lazily, is 1,200 bytes of counters per tenant and roughly 60 to 90 MB for 50,000 tenants with overhead. Carry a running total and subtract the bucket you overwrite so a window read is O(1) rather than 300 adds.
Extract the top 50 with a size-k min-heap over the tenant sums: O(d log k) for d tenants, against O(d log d) to sort them all. Maintaining the heap continuously instead of on query requires a tenant-to-heap-index map, because incrementing a count already inside the heap means sifting from a known position, and without that map you rebuild the heap on every request.
State the approximation precisely rather than gesturing at sketches. Misra-Gries with m counters retains every item whose true count exceeds N/(m+1), and each retained count underestimates the truth by at most N/(m+1). With m = 1,000 and N = 900,000 requests in the window the error is roughly 900 requests, which is fine for spotting a tenant sending 50,000 and useless for ranking two tenants 200 apart.
Say what breaks when the window slides: Misra-Gries and Space-Saving are insert-only and cannot be decremented as records age out. The workable construction is one summary per sub-window, say ten seconds, with 30 summaries merged at query time, and the merged error is the sum of the per-summary errors, so the bound degrades linearly in the number of sub-windows.

Follow-up

The heaviest tenant is heavy because of one export job rather than user traffic. Should the limiter treat those as the same tenant?
Two tenants sit tied at the boundary of the top 50. Does your answer flap, and does the flapping matter?

Track a rolling failure rate per destination for circuit decisions

Easy · sliding window · ring buffer · circuit breaker

The egress service delivers about 1,500 webhooks per second across roughly 40,000 destinations, each call bounded by a 10 second timeout. Maintain, per destination, the failure rate over the trailing 60 seconds so a caller can ask before dispatch whether the circuit should open. Attempts arrive as (destination_id, finished_at_ms, outcome). Requirement: amortised O(1) per attempt, with total memory bounded by the destination count rather than by traffic. Give the structure, its exact memory, and the rule that stops a destination with three attempts from opening a circuit.

Approach

Name the exact-deque version and then reject it as the default. Holding timestamps and advancing a tail pointer past anything older than now minus 60 seconds is a correct two-pointer window at amortised O(1) per attempt, but its memory tracks in-window traffic, so one destination in a retry storm holds hundreds of thousands of entries while thousands of quiet destinations hold none.
Use a ring of 60 one-second buckets per destination, each bucket a pair of counters for attempts and failures. On an attempt, advance the ring by the elapsed whole seconds, zeroing at most min(elapsed, 60) buckets, then increment the head. That is amortised O(1) with a fixed footprint per destination.
State the footprint: 60 buckets times two 4-byte counters is 480 bytes of payload per destination, so 40,000 destinations is roughly 20 to 25 MB with per-entry overhead, bounded by the catalogue rather than by the rate. The cost is granularity, since the oldest bucket ages out in whole seconds, which is far tighter than the decision needs.
Require a minimum sample before the circuit may open. A destination with three attempts and three failures reads as 100 percent and is not evidence; a floor of roughly 20 attempts in the window makes the ratio meaningful, and below that floor use a run of consecutive failures as the trigger instead.

Follow-up

The fleet is 30 instances and each sees roughly a thirtieth of a destination's traffic. Where does the rate actually live, and what does a per-instance answer get wrong?
A destination answers in 9.5 seconds and succeeds. It is not failing but it is consuming your per-destination concurrency. What signal should open the circuit here?

How do you optimize database performance and query efficiency when working with large relational databases or

Medium · Technical & System Architecture

How do you optimize database performance and query efficiency when working with large relational databases or star schemas?

Approach

Name the grain you start from and join outward from it.
Check whether any join is one-to-many before aggregating, or the sums inflate.
Say which index the query would use, and what makes it unusable.
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?

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

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.
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.
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.
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 a complex software project you designed and implemented from scratch. What were the architectu

Medium · Project Walkthroughs & Technical Present

Walk me through a complex software project you designed and implemented from scratch. What were the architectural challenges?

Approach

Fix the scope first: who calls this, how often, and what they do when it fails.
Name the read and write paths separately; they rarely have the same bottleneck.
Choose a partition key and say what query it makes expensive.
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 documenting your software architecture? Walk me through how you use UML diagrams to commun

Medium · Project Walkthroughs & Technical Present

How do you approach documenting your software architecture? Walk me through how you use UML diagrams to communicate design patterns.

Approach

Fix the scope first: who calls this, how often, and what they do when it fails.
Name the read and write paths separately; they rarely have the same bottleneck.
Choose a partition key and say what query it makes expensive.
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?

Present a past research project or major software deliverable. Explain the problem, your technical approach, a

Medium · Project Walkthroughs & Technical Present

Present a past research project or major software deliverable. Explain the problem, your technical approach, and the ultimate business or scientific outcome.

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 would you design a scalable web application using.NET and Azure to handle high-throughput data processing?

Medium · Technical & System Architecture

Approach

Fix the scope first: who calls this, how often, and what they do when it fails.
Name the read and write paths separately; they rarely have the same bottleneck.
Choose a partition key and say what query it makes expensive.
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 how you would implement a CI/CD pipeline to automate testing and deployment across development, stagi

Medium · Technical & System Architecture

Describe how you would implement a CI/CD pipeline to automate testing and deployment across development, staging, and production environments.

Approach

Fix the scope first: who calls this, how often, and what they do when it fails.
Name the read and write paths separately; they rarely have the same bottleneck.
Choose a partition key and say what query it makes expensive.
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 strategies do you use to monitor, analyze, and optimize the costs and performance of custom AI applicatio

Medium · Technical & System Architecture

What strategies do you use to monitor, analyze, and optimize the costs and performance of custom AI applications utilizing large language models (LLMs)?

Approach

Fix the scope first: who calls this, how often, and what they do when it fails.
Name the read and write paths separately; they rarely have the same bottleneck.
Choose a partition key and say what query it makes expensive.
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?

Exports duplicate a row range about once a week

Hard · leases · clock skew · idempotency

Roughly once a week an export writes a file containing a duplicated range of rows. The affected job_run rows show attempt = 1, status = succeeded, one started_at, and a lease_owner naming a different host from the one whose logs show the job starting. Leases last 30 seconds and are heartbeated every 10 from inside the handler; lease_expires_at is computed on the worker and compared against the database's now(). Find the mechanism, and give a fix that holds even if you cannot fix the clocks.

Approach

Start from the fact that eliminates the obvious answer. attempt = 1 means no retry was recorded, so this is not a re-run after failure; two workers ran the same row concurrently and the takeover path never touched the counter. lease_owner naming a host other than the one that started the job is the same statement from the other side.
Enumerate the mechanisms that cause a premature takeover, then find the signal that separates them. Either the lease genuinely expired because the heartbeat did not fire, which is what happens when the heartbeat runs on the handler's own thread and the handler makes a long blocking call, or it only appeared expired because two clocks disagree, since lease_expires_at is written from the worker's clock and evaluated against the database's. The discriminator is the distribution: incidents clustered on the longest exports indict the heartbeat, incidents clustered on one host indict skew. Measure both, and measure each host's offset against the database directly.
Read the reclaim query precisely. In PostgreSQL now() is transaction start time, not statement time, so a reclaimer holding a long transaction compares against an older timestamp than expected; clock_timestamp() is the statement-time function. This is worth ruling in or out before you redesign anything, because it changes which rows look expired.
Remove the second clock rather than trying to synchronise it. Issue and extend the lease in the database, with lease_expires_at = now() + interval '30 seconds' in both the claim and the heartbeat, so exactly one clock is ever compared and worker skew stops mattering to this predicate.

Follow-up

The displaced worker has already streamed half the file to object storage. What makes that side effect safe to repeat?
You now count takeovers. What alert fires on that counter, and at what threshold?

Suggested practice checklist

Built from the rounds and topics United Therapeutics candidates report.

Day 1 — Map the United Therapeutics loop

Write out the reported sequence: HR Screening Call, Technical Screening, Presentation of Work, Panel 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 AI-enabled Web Applications

Spend the session on AI-enabled Web Applications, which United Therapeutics candidates report being tested on.
Write one worked example in AI-enabled Web Applications and time yourself on it.

Deliverable: One timed worked example in AI-enabled Web Applications.

Day 3 — Work GPT / LLMs (Large Language Models)

Spend the session on GPT / LLMs (Large Language Models), which United Therapeutics candidates report being tested on.
Write one worked example in GPT / LLMs (Large Language Models) and time yourself on it.

Deliverable: One timed worked example in GPT / LLMs (Large Language Models).

Day 4 — Work .NET

Spend the session on .NET, which United Therapeutics candidates report being tested on.
Write one worked example in .NET and time yourself on it.

Deliverable: One timed worked example in .NET.

Day 5 — Answer out loud: Project Walkthroughs & Technical Presentation

Answer aloud, timed: Walk me through a complex software project you designed and implemented from scratch. What were the architectural challenges?
Answer aloud, timed: Describe a time when you had to integrate a new technology (such as an AI model or a cloud service) into an existing legacy system.

Deliverable: Spoken answers to 2 reported Project Walkthroughs & Technical Presentation question(s), under time.

Day 6 — Answer out loud: Technical & System Architecture

Answer aloud, timed: How would you design a scalable web application using.NET and Azure to handle high-throughput data processing?
Answer aloud, timed: Explain your experience with RESTful APIs and how you ensure secure, reliable communication across multiple distributed services.

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

Day 7 — Answer out loud: Behavioral & Mission Alignment

Answer aloud, timed: Why do you want to work at a public benefit biotechnology corporation rather than a traditional tech company?
Answer aloud, timed: Describe a situation where you had to gather requirements from non-technical stakeholders, such as scientists or manufacturing operators. How did you translate their needs into technical specifications?

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

Behavioural preparation

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

Describe a time when you had to integrate a new technology (such as an AI model or a cloud service) into an ex

Medium · Project Walkthroughs & Technical Present

Describe a time when you had to integrate a new technology (such as an AI model or a cloud service) into an existing legacy system.

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?

Explain your experience with RESTful APIs and how you ensure secure, reliable communication across multiple di

Medium · Technical & System Architecture

Explain your experience with RESTful APIs and how you ensure secure, reliable communication across multiple distributed services.

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?

Why do you want to work at a public benefit biotechnology corporation rather than a traditional tech company?

Medium · Behavioral & Mission Alignment

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?

Describe a situation where you had to gather requirements from non-technical stakeholders, such as scientists

Medium · Behavioral & Mission Alignment

Describe a situation where you had to gather requirements from non-technical stakeholders, such as scientists or manufacturing operators. How did you translate their needs into technical specifications?

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?

Tell me about a time you disagreed with a team member on an architectural decision. How did you resolve the co

Medium · Behavioral & Mission Alignment

Tell me about a time you disagreed with a team member on an architectural decision. How did you resolve the conflict and move the project forward?

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 shifting priorities or ambiguous requirements in an agile development environment?

Medium · Behavioral & Mission Alignment

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?