Interview guides · Parallon

Parallon Software Engineer Interview Guide

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

The 60-second brief

Parallon is a major provider of healthcare business and operational services, serving as a critical backbone for hospitals and healthcare systems nationwide. As a Software Engineer at Parallon, you will build, optimize, and maintain the highly secure, enterprise-scale software systems that power revenue cycle management, clinical billing, and operational workflows. Your work directly impacts the efficiency of healthcare delivery, ensuring that providers can focus on patient care while the underlying financial and administrative systems run seamlessly. The engineering team at Parallon operates in a high-transaction environment where data integrity, security, and system reliability are paramount. You will work on modernizing legacy platforms, building robust APIs, and developing scalable microservices that process millions of transactions daily.

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

Focus areas

Coding skills (general)
Spring Boot fundamentals
Problem solving

The Parallon interview process

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

Recruiter Screen

Initial discussion about your background, career goals, and role expectations.

What to demonstrate

Initial discussion about your background, career goals, and role expectations
Depth in Coding skills (general)

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

Includes an online assessment or a technical phone interview covering MVC concepts, SQL, and programming languages.

What to demonstrate

Includes an online assessment or a technical phone interview covering MVC concepts, SQL, and programming languages
Depth in Coding skills (general)

How to prepare

Answer aloud and timed: How do you manage transaction boundaries across multiple microservices in a Spring Boot application?
Answer aloud and timed: Describe your experience implementing dependency injection and how it improves code testability.

Technical Panel Interviews

Deeper technical interviews and discussions with hiring managers about system design and architectural patterns.

What to demonstrate

Deeper technical interviews and discussions with hiring managers about system design and architectural patterns
Depth in Coding skills (general)

How to prepare

Answer aloud and timed: How would you handle backpressure in a reactive stream application?
Answer aloud and timed: Explain the MVC pattern and how it is structured within a C#/.NET web application.

Where candidates lose points

Clarify the employment terms early

If you are interviewing for a contract-to-hire position, ensure you have explicit, written confirmation regarding your hourly rate, equivalent annual salary, benefits eligibility, and the typical timeline for conversion to a permanent employee.

Pay close attention to the job ID and compensation details listed on your offer letter

Double-check that all terms match what you discussed with your recruiter before signing to avoid administrative confusion later.

Prepare for rigorous background checks

Because Parallon operates in the heavily regulated healthcare sector, the onboarding process involves comprehensive background screening, drug testing, and education verification.

Questions & practice

How would you approach refactoring a legacy method that has a high cyclomatic complexity?

Medium · Problem Solving & Algorithmic Logic

Approach

Restate the input: its shape, its size, and what is guaranteed about it.
Name the brute-force solution and its complexity before improving on it.
Choose the data structure from the access pattern, not from familiarity.
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?

Walk through how you would optimize an algorithm with an O(N^2) time complexity to run in linear or logarithmi

Medium · Problem Solving & Algorithmic Logic

Walk through how you would optimize an algorithm with an O(N^2) time complexity to run in linear or logarithmic time.

Approach

Restate the input: its shape, its size, and what is guaranteed about it.
Name the brute-force solution and its complexity before improving on it.
Choose the data structure from the access pattern, not from familiarity.
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?

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?

Keep soft-deleted accounts from blocking re-registration

Easy · soft delete · unique constraints · engine differences

app_user holds user_id, tenant_id, email CITEXT, password_hash (NULL for SSO principals), email_verified_at, auth_version, status ('invited','active','suspended','deactivated'), created_at, updated_at, deleted_at. Two live accounts for one address inside a tenant must be impossible, but an address freed by a soft delete must be reusable, and the same tenant may delete and re-register it repeatedly. Write the uniqueness DDL for PostgreSQL 16, then the equivalent for MySQL 8 where partial indexes do not exist, and say what each permits once three deleted rows already hold that address.

Approach

Start from what is actually unique: not (tenant_id, email), but (tenant_id, email) among live rows. PostgreSQL says that directly — CREATE UNIQUE INDEX app_user_live_email ON app_user (tenant_id, email) WHERE deleted_at IS NULL. A full constraint over the same two columns burns the address permanently the first time someone deletes an account.
Keep case-insensitivity in the type or the index, never in the application: CITEXT as given, or UNIQUE (tenant_id, lower(email)) as an expression index where the extension is unavailable. A case-sensitive unique column is exactly how two accounts for one human appear.
For MySQL 8 the predicate has to move inside the key: add a discriminator column that is a constant 0 while the row is live and is set to user_id on delete, with UNIQUE (tenant_id, email, deleted_marker). Live rows share the constant and still collide; deleted rows differ from each other and stop colliding.
State the NULL variant and its dependency: leaving the marker NULL for deleted rows also works, because a unique index treats NULLs as distinct — true in MySQL, and true in PostgreSQL only under the default NULLS DISTINCT, which PostgreSQL 15 lets you reverse. Check the polarity against the three existing deleted rows: constant-on-live is what preserves the collision you want, and reversing it silently admits duplicate live accounts.

Follow-up

A deleted account re-registers with the same address the next day. Do the old resource rows follow the new user_id, and how does the API keep the two principals apart?
How do you honour an erasure request while resource_revision.actor_user_id still references this table?

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?

Explain the core Spring Boot annotations you use daily and how they manage the application context.

Medium · Java & Spring Boot Ecosystem

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 is the difference between reactive programming and traditional functional programming in a microservices

Medium · Java & Spring Boot Ecosystem

What is the difference between reactive programming and traditional functional programming in a microservices environment?

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 manage transaction boundaries across multiple microservices in a Spring Boot application?

Medium · Java & Spring Boot Ecosystem

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 would you handle backpressure in a reactive stream application?

Medium · Java & Spring Boot Ecosystem

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?

Explain the MVC pattern and how it is structured within a C#/.NET web application.

Medium · .NET, C#, & Web 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?

How do you optimize database queries and manage active connections in a high-throughput SQL environment?

Medium · .NET, C#, & Web 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?

What is the difference between abstract classes and interfaces, and when would you use each in a system design

Medium · .NET, C#, & Web Architecture

What is the difference between abstract classes and interfaces, and when would you use each in a system design?

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 ensure secure data transmission when building RESTful APIs?

Medium · .NET, C#, & Web 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?

Write a program or design an algorithm to solve a logic-based board game.

Medium · Problem Solving & Algorithmic Logic

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?

Explain how you would design an efficient caching mechanism for frequently accessed lookup data.

Medium · Problem Solving & Algorithmic Logic

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 a scenario where you had to debug a memory leak or performance bottleneck in a C# application.

Medium · .NET, C#, & Web Architecture

Approach

Establish what changed and when, before forming any theory.
Pick a bisection that eliminates candidates whichever way it turns out.
Check the instrumentation before believing the symptom.
Separate the trigger from the cause; the deploy is rarely the bug.

Follow-up

What would you look at first, and what would it rule out?
How would you tell a cause from a coincidence here?

Suggested practice checklist

Built from the rounds and topics Parallon candidates report.

Day 1 — Map the Parallon loop

Write out the reported sequence: Recruiter Screen, Technical Screening, Technical 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 Coding skills (general)

Spend the session on Coding skills (general), which Parallon candidates report being tested on.
Write one worked example in Coding skills (general) and time yourself on it.

Deliverable: One timed worked example in Coding skills (general).

Day 3 — Work Spring Boot fundamentals

Spend the session on Spring Boot fundamentals, which Parallon candidates report being tested on.
Write one worked example in Spring Boot fundamentals and time yourself on it.

Deliverable: One timed worked example in Spring Boot fundamentals.

Day 4 — Work Problem solving

Spend the session on Problem solving, which Parallon candidates report being tested on.
Write one worked example in Problem solving and time yourself on it.

Deliverable: One timed worked example in Problem solving.

Day 5 — Answer out loud: Java & Spring Boot Ecosystem

Answer aloud, timed: Explain the core Spring Boot annotations you use daily and how they manage the application context.
Answer aloud, timed: What is the difference between reactive programming and traditional functional programming in a microservices environment?

Deliverable: Spoken answers to 2 reported Java & Spring Boot Ecosystem question(s), under time.

Day 6 — Answer out loud: .NET, C#, & Web Architecture

Answer aloud, timed: Explain the MVC pattern and how it is structured within a C#/.NET web application.
Answer aloud, timed: How do you optimize database queries and manage active connections in a high-throughput SQL environment?

Deliverable: Spoken answers to 2 reported .NET, C#, & Web Architecture question(s), under time.

Day 7 — Answer out loud: Problem Solving & Algorithmic Logic

Answer aloud, timed: Write a program or design an algorithm to solve a logic-based board game.
Answer aloud, timed: How would you approach refactoring a legacy method that has a high cyclomatic complexity?

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

Behavioural preparation

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

Describe your experience implementing dependency injection and how it improves code testability.

Medium · Java & Spring Boot Ecosystem

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 manage your schedule, prioritize tasks, and remain productive in a remote or hybrid work environmen

Medium · Behavioral & Team Collaboration

How do you manage your schedule, prioritize tasks, and remain productive in a remote or hybrid work environment?

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 time you had to solve a complex production issue under a tight deadline with limited documentation.

Medium · Behavioral & Team Collaboration

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 constructive feedback from a peer or a manager during a code review?

Medium · Behavioral & Team Collaboration

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 had to collaborate with a non-technical stakeholder to define system requirements.

Medium · Behavioral & Team Collaboration

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?