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Fanduel Software Engineer Interview Guide

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

The 60-second brief

A Software Engineer at Fanduel is at the heart of one of the world's most dynamic sports-tech and gaming ecosystems. You will be responsible for building, scaling, and maintaining the robust platforms that millions of users rely on for real-time betting, gaming, and sports engagement. This role is not just about writing code; it is about managing massive concurrency, ensuring sub-second latency, and delivering high-availability services that operate under intense pressure during major sporting events. The work you do here has a direct, visible impact on the business and the end-user experience. Whether you are working on backend microservices, real-time data pipelines, or interactive frontend components, your contributions drive the reliability of the Fanduel product suite.

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

Focus areas

  • Python
  • Coding Assessments
  • System Design

The Fanduel interview process

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

Initial Alignment

Begin the interview process with an alignment on expectations and role fit.

What to demonstrate

  • Begin the interview process with an alignment on expectations and role fit
  • Depth in Python

How to prepare

  • Answer aloud and timed: How would you design a data structure to handle real-time sports odds or player statistics?
  • Answer aloud and timed: Explain the difference between pass-by-value and pass-by-reference in your preferred language.

Technical Assessment

Engage in a deep-dive technical assessment focusing on coding challenges and architectural intuition.

What to demonstrate

  • Engage in a deep-dive technical assessment focusing on coding challenges and architectural intuition
  • Depth in Python

How to prepare

  • Answer aloud and timed: Can you optimize a function that processes large arrays of objects?
  • Answer aloud and timed: Walk me through how you would handle concurrency in a high-traffic API.

Cross-Functional Team Assessment

Evaluate your ability to function within a cross-functional team through discussions and problem-solving.

What to demonstrate

  • Evaluate your ability to function within a cross-functional team through discussions and problem-solving
  • Depth in Python

How to prepare

  • Answer aloud and timed: Describe your approach to refactoring legacy code while adding new features.
  • Answer aloud and timed: Design a platform similar to a content-sharing site (e.g., Medium) or a streaming service.

Final Assessment

Conclude with a final assessment that may include discussions on team-specific challenges.

What to demonstrate

  • Conclude with a final assessment that may include discussions on team-specific challenges
  • Depth in Python

How to prepare

  • Answer aloud and timed: How would you design a database schema for a high-frequency transactional system?
  • Answer aloud and timed: Explain how you would ensure system availability during a major sporting event with massive traffic spikes.

Where candidates lose points

Communicate your thought process

The most important part of the interview is how you reach your answer. If you are silent, we cannot evaluate your reasoning.

Ask questions

Use your time to interview us as well. Ask about the team’s current technical challenges, how they handle on-call responsibilities, or how they balance feature work with technical debt.

Know your resume

Be ready to speak in detail about any project or technology you have listed. If you mention it, be prepared to answer deep-dive questions about it.

Be honest about your experience

If you do not have experience with a specific technology, state that, but follow up with how you would go about learning it or how your existing knowledge applies to the problem.

Questions & practice

Explain the difference between pass-by-value and pass-by-reference in your preferred language.

Medium · Technical & Coding Proficiency

Approach

  1. Say what the runtime actually does before reasoning about the code.
  2. Name what is shared across threads and what owns each piece of state.
  3. Identify the window where an invariant is briefly untrue.
  4. Distinguish a value from a reference to it, and say which one you handed out.

Follow-up

  • What happens if two callers reach this at the same time?
  • Where could this allocate more than you expect?

Can you optimize a function that processes large arrays of objects?

Medium · Technical & Coding Proficiency

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?

Describe your approach to refactoring legacy code while adding new features.

Medium · Technical & Coding Proficiency

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?

What trade-offs do you consider when choosing between SQL and NoSQL databases for a new service?

Medium · System Design & Architecture

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?

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

  1. 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.
  2. 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.
  3. 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.
  4. 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?

How would you design a data structure to handle real-time sports odds or player statistics?

Medium · Technical & Coding Proficiency

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?

Walk me through how you would handle concurrency in a high-traffic API.

Medium · Technical & Coding Proficiency

Approach

  1. Say who the caller is and what they do when the call fails halfway.
  2. Define the identity of a request so a retry cannot double-apply it.
  3. Separate accepted, pending, failed and confirmed; they are different facts.
  4. Design the error taxonomy before the success shape; callers branch on it.

Follow-up

  • What happens if the caller retries after a timeout?
  • How does a client discover it is on an old version of this contract?

Design a platform similar to a content-sharing site (e.g., Medium) or a streaming service.

Medium · System Design & Architecture

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 database schema for a high-frequency transactional system?

Medium · System Design & Architecture

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 ensure system availability during a major sporting event with massive traffic spikes.

Medium · System Design & Architecture

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 do you handle failure in a distributed microservices environment?

Medium · System Design & Architecture

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?

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 Fanduel candidates report.

Day 1 — Map the Fanduel loop

  • Write out the reported sequence: Initial Alignment, Technical Assessment, Cross-Functional Team Assessment, Final Assessment.
  • 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 Python

  • Spend the session on Python, which Fanduel candidates report being tested on.
  • Write one worked example in Python and time yourself on it.

Deliverable: One timed worked example in Python.

Day 3 — Work Coding Assessments

  • Spend the session on Coding Assessments, which Fanduel candidates report being tested on.
  • Write one worked example in Coding Assessments and time yourself on it.

Deliverable: One timed worked example in Coding Assessments.

Day 4 — Work System Design

  • Spend the session on System Design, which Fanduel candidates report being tested on.
  • Write one worked example in System Design and time yourself on it.

Deliverable: One timed worked example in System Design.

Day 5 — Answer out loud: Technical & Coding Proficiency

  • Answer aloud, timed: How would you design a data structure to handle real-time sports odds or player statistics?
  • Answer aloud, timed: Explain the difference between pass-by-value and pass-by-reference in your preferred language.

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

Day 6 — Answer out loud: System Design & Architecture

  • Answer aloud, timed: Design a platform similar to a content-sharing site (e.g., Medium) or a streaming service.
  • Answer aloud, timed: How would you design a database schema for a high-frequency transactional system?

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

Day 7 — Answer out loud: Behavioral & Leadership

  • Answer aloud, timed: Tell me about a time you disagreed with a colleague on a technical approach; how did you resolve it?
  • Answer aloud, timed: How do you handle a project that is falling behind its deadline?

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

Behavioural preparation

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

Tell me about a time you disagreed with a colleague on a technical approach; how did you resolve it?

Medium · Behavioral & Leadership

Approach

  1. Pick a story where you made the decision, not one where you watched it.
  2. State the situation in two sentences and spend the rest on the reasoning.
  3. Give the blast radius: what could have broken, and what you measured.
  4. Name the disagreement and how you resolved it with evidence.

Follow-up

  • What would you do differently if you ran that again?
  • How did you know your change caused the improvement?

How do you handle a project that is falling behind its deadline?

Medium · Behavioral & Leadership

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 advocate for the user’s needs over a quick technical fix.

Medium · Behavioral & Leadership

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 process for conducting code reviews, and how do you give constructive feedback?

Medium · Behavioral & Leadership

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?

Why do you want to work for Fanduel specifically?

Medium · Behavioral & Leadership

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?