The Children'S Hospital Of Philadelphia Software Engineer Interview Guide

The 60-second brief

A Software Engineer at The Children'S Hospital Of Philadelphia (CHOP) plays a vital role in bridging the gap between cutting-edge technology and world-class pediatric healthcare. In this role, you are not just writing code; you are building and maintaining the digital infrastructure that supports clinical applications, medical research, patient portals, and data pipelines. The software you develop directly impacts the efficiency of medical staff, the accuracy of patient data, and ultimately, the lives of thousands of children. Unlike traditional tech companies, engineering at The Children'S Hospital Of Philadelphia requires a deep appreciation for system reliability, data security, and regulatory compliance.

This guide is scoped to a Software Engineer candidate at The Children'S Hospital Of Philadelphia.

Focus areas

• Data Structures
• Algorithms
• DSA (Interview-style problem solving)

The The Children'S Hospital Of Philadelphia interview process

The Children'S Hospital Of Philadelphia candidates report 3 rounds over 3-5 weeks. The stages below are what candidates describe, not a published process.

Recruiter Screening

Initial discussion about your background, interest in the role, and salary expectations.

What to demonstrate

• Initial discussion about your background, interest in the role, and salary expectations
• Depth in Data Structures

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 Evaluation

Includes coding assessments and system design discussions.

What to demonstrate

• Includes coding assessments and system design discussions
• Depth in Data Structures

How to prepare

• Answer aloud and timed: Write a function to validate if a given string representing a clinical code format is structured correctly.
• Answer aloud and timed: Reverse a linked list, or find the shortest path in a network of connected clinical databases.

Panel Interviews

Presenting to or answering questions from a group of three to four team members.

What to demonstrate

• Presenting to or answering questions from a group of three to four team members
• Depth in Data Structures

How to prepare

• Answer aloud and timed: Write an algorithm to merge two sorted lists of patient IDs while eliminating duplicates.
• Answer aloud and timed: Design a highly available patient portal that can handle sudden spikes in traffic during seasonal vaccination drives.

Where candidates lose points

Prepare for Silent Note-Taking

During panel interviews, do not be discouraged if the interviewers write silently while you speak. They are following a structured evaluation process and taking detailed notes to ensure a fair assessment.

Emphasize Data Accuracy and Testing

In a hospital setting, software quality is paramount. Whenever you discuss your coding or design decisions, emphasize how you test your code and ensure data integrity.

Align with the Mission

Take some time to research CHOP's recent achievements, research initiatives, and patient-care values. Weaving this knowledge into your behavioral answers demonstrates genuine interest.

Questions & practice

Given an array of patient check-in times, find the maximum number of overlapping appointments.

Medium · Coding & Algorithmic Problem Solving

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?

Implement a basic caching mechanism to store frequently accessed medical records.

Medium · Coding & Algorithmic Problem Solving

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?

Write a function to validate if a given string representing a clinical code format is structured correctly.

Medium · Coding & Algorithmic Problem Solving

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?

Reverse a linked list, or find the shortest path in a network of connected clinical databases.

Medium · Coding & Algorithmic Problem Solving

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?

Write an algorithm to merge two sorted lists of patient IDs while eliminating duplicates.

Medium · Coding & Algorithmic Problem Solving

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 ensure data privacy and compliance with regulations like HIPAA when designing database schemas?

Medium · Domain & Job-Specific 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?

Design a highly available patient portal that can handle sudden spikes in traffic during seasonal vaccination

Medium · System Design & Architecture

Design a highly available patient portal that can handle sudden spikes in traffic during seasonal vaccination drives.

Approach

1. Fix the scope first: who calls this, how often, and what they do when it fails.
2. Name the read and write paths separately; they rarely have the same bottleneck.
3. Choose a partition key and say what query it makes expensive.
4. State the consistency you need, and where you are willing to be stale.

Follow-up

• What breaks first when traffic grows ten times?
• How does this behave when that dependency is down for an hour?

How would you design a secure data pipeline to ingest and process real-time telemetry data from patient monito

Medium · System Design & Architecture

How would you design a secure data pipeline to ingest and process real-time telemetry data from patient monitors?

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?

Design an access control system that manages permissions for doctors, nurses, and administrative staff across

Medium · System Design & Architecture

Design an access control system that manages permissions for doctors, nurses, and administrative staff across different hospital departments.

Approach

1. Fix the scope first: who calls this, how often, and what they do when it fails.
2. Name the read and write paths separately; they rarely have the same bottleneck.
3. Choose a partition key and say what query it makes expensive.
4. State the consistency you need, and where you are willing to be stale.

Follow-up

• What breaks first when traffic grows ten times?
• How does this behave when that dependency is down for an hour?

How would you architect a system to integrate external lab results into our internal database while ensuring z

Medium · System Design & Architecture

How would you architect a system to integrate external lab results into our internal database while ensuring zero data loss?

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?

Explain how you would approach testing a critical clinical application before deploying it to production.

Medium · Domain & Job-Specific 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?

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 The Children'S Hospital Of Philadelphia candidates report.

Day 1 — Map the The Children'S Hospital Of Philadelphia loop

• Write out the reported sequence: Recruiter Screening, Technical Evaluation, Panel 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 3 reported rounds, with the weakest marked.

Day 2 — Work Data Structures

• Spend the session on Data Structures, which The Children'S Hospital Of Philadelphia candidates report being tested on.
• Write one worked example in Data Structures and time yourself on it.

Deliverable: One timed worked example in Data Structures.

Day 3 — Work Algorithms

• Spend the session on Algorithms, which The Children'S Hospital Of Philadelphia candidates report being tested on.
• Write one worked example in Algorithms and time yourself on it.

Deliverable: One timed worked example in Algorithms.

Day 4 — Work DSA (Interview-style problem solving)

• Spend the session on DSA (Interview-style problem solving), which The Children'S Hospital Of Philadelphia candidates report being tested on.
• Write one worked example in DSA (Interview-style problem solving) and time yourself on it.

Deliverable: One timed worked example in DSA (Interview-style problem solving).

Day 5 — Answer out loud: Coding & Algorithmic Problem Solving

• Answer aloud, timed: Given an array of patient check-in times, find the maximum number of overlapping appointments.
• Answer aloud, timed: Implement a basic caching mechanism to store frequently accessed medical records.

Deliverable: Spoken answers to 2 reported Coding & Algorithmic Problem Solving question(s), under time.

Day 6 — Answer out loud: System Design & Architecture

• Answer aloud, timed: Design a highly available patient portal that can handle sudden spikes in traffic during seasonal vaccination drives.
• Answer aloud, timed: How would you design a secure data pipeline to ingest and process real-time telemetry data from patient monitors?

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

Day 7 — Answer out loud: Behavioral & Scenario-Based

• Answer aloud, timed: Describe a time when you had a significant technical disagreement with a teammate. How did you resolve it?
• Answer aloud, timed: Tell me about a time when you had to work with a stakeholder who did not have a technical background. How did you communicate complex concepts to them?

Deliverable: Spoken answers to 2 reported Behavioral & Scenario-Based question(s), under time.

Behavioural preparation

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

Describe a time when you had a significant technical disagreement with a teammate. How did you resolve it?

Medium · Behavioral & Scenario-Based

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 when you had to work with a stakeholder who did not have a technical background. How did

Medium · Behavioral & Scenario-Based

Tell me about a time when you had to work with a stakeholder who did not have a technical background. How did you communicate complex concepts to them?

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?

Describe a situation where you had to quickly adapt to a sudden change in project requirements or priorities.

Medium · Behavioral & Scenario-Based

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?

Give an example of a time you made a mistake on a project. What did you do to correct it, and what did you lea

Medium · Behavioral & Scenario-Based

Give an example of a time you made a mistake on a project. What did you do to correct it, and what did you learn?

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 healthcare data standards such as HL7 or FHIR?

Medium · Domain & Job-Specific 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?

Describe your experience with managing and configuring clinical software or equipment integration platforms.

Medium · Domain & Job-Specific 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?