Delta Air Lines Software Engineer Interview Guide

The 60-second brief

As a Software Engineer at Delta Air Lines, you play a vital role in powering one of the world's premier commercial airlines. Technology at Delta Air Lines is not merely a back-office utility; it is the core engine driving flight operations, passenger experience, baggage tracking, pricing models, and crew scheduling. Engineers at Delta Air Lines build and maintain high-scale enterprise systems that handle millions of transactions daily, ensuring that thousands of flights depart and arrive safely and on schedule. In this role, you will contribute to critical enterprise platforms modernizing aviation technology. Whether you are developing cloud-native microservices, optimizing low-latency flight data pipelines, or refining customer-facing digital platforms across web and mobile, your code directly affects millions of travelers worldwide.

This guide is scoped to a Software Engineer candidate at Delta Air Lines.

Focus areas

• Java
• Spring Boot
• REST APIs

The Delta Air Lines interview process

Delta Air Lines candidates report 5 rounds over 4-6 weeks. The stages below are what candidates describe, not a published process.

Application Review

Candidates submit their applications, which are reviewed by the hiring team.

What to demonstrate

• Candidates submit their applications, which are reviewed by the hiring team
• Depth in Java

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.

Initial Screening

Candidates undergo an initial screening to assess their qualifications and fit.

What to demonstrate

• Candidates undergo an initial screening to assess their qualifications and fit
• Depth in Java

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.

Deeper Interview Sessions

One or two in-depth interview sessions focusing on technical skills and behavioral responses.

What to demonstrate

• One or two in-depth interview sessions focusing on technical skills and behavioral responses
• Depth in Java

How to prepare

• Answer aloud and timed: How do you optimize database performance using indexing and structured SQL queries?
• Answer aloud and timed: Implement code to reverse or manipulate strings and arrays efficiently.

Final Interview

A combined session of technical discussion and behavioral questions with the hiring manager or panel.

What to demonstrate

• A combined session of technical discussion and behavioral questions with the hiring manager or panel
• Depth in Java

How to prepare

• Answer aloud and timed: How would you construct and traverse a custom Linked List structure?
• Answer aloud and timed: Given a set of operational constraints, write an algorithm to find the optimal path or route between two points.

Offer Discussion

Candidates may receive an offer shortly after the final interview.

What to demonstrate

• Candidates may receive an offer shortly after the final interview
• Depth in Java

How to prepare

• Answer aloud and timed: Explain basic Unix/Linux commands used for process management, file inspection, and server log monitoring.
• Answer aloud and timed: How do you optimize an algorithm's time complexity from $O(N^2)$ to $O(N \log N)$ or $O(N)$?

Where candidates lose points

Going into the loop without having done this.

Master the STAR Method: Evaluators at Delta Air Lines explicitly use structured behavioral rubrics. Ensure your answers explicitly cover the Situation, Task, Action, and measurable Result.

Going into the loop without having done this.

Prepare specific examples that highlight operational efficiency. Demonstrating how your code improved throughput, cut costs, or streamlined a workflow aligns directly with Delta's business goals.

Going into the loop without having done this.

Brush Up on Command Line Basics: Do not overlook fundamental Unix/Linux knowledge. Be comfortable describing basic terminal commands for file management, log filtering (grep), and process monitoring.

Questions & practice

How do thread pools work in Java, and how do you prevent resource exhaustion in concurrent execution?

Medium · Core Java,.NET & Full-Stack Fundamentals

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?

Implement code to reverse or manipulate strings and arrays efficiently.

Medium · Data Structures, Algorithms & Live Codin

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 would you construct and traverse a custom Linked List structure?

Medium · Data Structures, Algorithms & Live Codin

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?

Given a set of operational constraints, write an algorithm to find the optimal path or route between two point

Medium · Data Structures, Algorithms & Live Codin

Given a set of operational constraints, write an algorithm to find the optimal path or route between two points.

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?

Explain basic Unix/Linux commands used for process management, file inspection, and server log monitoring.

Medium · Data Structures, Algorithms & Live Codin

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 optimize an algorithm's time complexity from $O(N^2)$ to $O(N \log N)$ or $O(N)$?

Medium · Data Structures, Algorithms & Live Codin

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 optimize database performance using indexing and structured SQL queries?

Medium · Core Java,.NET & Full-Stack Fundamentals

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?

Replace offset paging on the resource feed with keyset

Easy · keyset pagination · composite index · sort order

resource holds resource_id, tenant_id, owner_user_id, title, body_ref, version, status ('draft','active','archived','deleted'), created_at, updated_at, deleted_at, with an index on (tenant_id, status, updated_at DESC, resource_id DESC). The listing endpoint returns active resources for one tenant, newest update first, 50 per page, today with LIMIT 50 OFFSET n. Tenants reach page 400 and rows are created while they read. Write the keyset query, define what the cursor carries and how it is encoded, and say which part of the index each predicate uses. Assume PostgreSQL 16.

Approach

1. Name the two failures separately. OFFSET 20000 makes the server produce and discard 20,000 rows, so page cost grows with depth rather than with page size. Independently, any write that changes how many rows sort above the offset moves the window between two fetches, and the direction decides which anomaly you get: an insert lands at the head of updated_at DESC and pushes already-returned rows down past the boundary, so they are returned a second time; a delete above the offset, or a row whose updated_at is bumped above the cursor, pulls rows up and one is never returned at all. Nothing in the response reveals either.
2. Write the seek: WHERE tenant_id = $1 AND status = 'active' AND (updated_at, resource_id) < ($2, $3) ORDER BY updated_at DESC, resource_id DESC LIMIT 50. The row-value comparison is one index range rather than a disjunction, and both columns are NOT NULL, which is what makes that comparison well defined.
3. Map each predicate onto the index: tenant_id and status are equality on the leading columns, (updated_at, resource_id) is the range, and the ORDER BY matches the index order so no Sort node appears and the scan stops after 50 rows. The DESC in the definition only matters for mixed directions — a plain ascending btree on the same columns is read backwards for this query.
4. Put both sort columns in the cursor and nothing the client can tamper with into another tenant: base64 of (updated_at, resource_id), validated server-side, with tenant_id taken from the principal.

Follow-up

• The client asks for 'jump to page 400'. What do you offer instead, and what does the honest version cost?
• Sort order becomes user-selectable across four columns. How many indexes is that, and which would you refuse to add?

Explain the process of serialization in Java and when you would implement it.

Medium · Core Java,.NET & Full-Stack Fundamentals

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?

What are Java 8 Streams and Lambda expressions? Write an example filtering a collection.

Medium · Core Java,.NET & Full-Stack Fundamentals

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?

What are the key differences between C# and ASP.NET core architecture when building web APIs?

Medium · Core Java,.NET & Full-Stack Fundamentals

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?

Describe how you would build a scalable RESTful API using Spring Boot or ASP.NET Core.

Medium · System Architecture, Cloud & API Design

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 AWS services such as EC2, S3, API Gateway, and CDK integrate to support enterprise microservices?

Medium · System Architecture, Cloud & API Design

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?

How would you design a real-time system to monitor flight delays and push updates to passenger mobile applicat

Medium · System Architecture, Cloud & API Design

How would you design a real-time system to monitor flight delays and push updates to passenger mobile applications?

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?

Describe a situation where you had to debug a critical issue in a production environment under strict time lim

Medium · Behavioral & Operational Scenarios

Describe a situation where you had to debug a critical issue in a production environment under strict time limits.

Approach

1. Establish what changed and when, before forming any theory.
2. Pick a bisection that eliminates candidates whichever way it turns out.
3. Check the instrumentation before believing the symptom.
4. 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 Delta Air Lines candidates report.

Day 1 — Map the Delta Air Lines loop

• Write out the reported sequence: Application Review, Initial Screening, Deeper Interview Sessions, Final Interview, Offer Discussion.
• 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 5 reported rounds, with the weakest marked.

Day 2 — Work Java

• Spend the session on Java, which Delta Air Lines candidates report being tested on.
• Write one worked example in Java and time yourself on it.

Deliverable: One timed worked example in Java.

Day 3 — Work Spring Boot

• Spend the session on Spring Boot, which Delta Air Lines candidates report being tested on.
• Write one worked example in Spring Boot and time yourself on it.

Deliverable: One timed worked example in Spring Boot.

Day 4 — Work REST APIs

• Spend the session on REST APIs, which Delta Air Lines candidates report being tested on.
• Write one worked example in REST APIs and time yourself on it.

Deliverable: One timed worked example in REST APIs.

Day 5 — Answer out loud: Core Java,.NET & Full-Stack Fundamentals

• Answer aloud, timed: Explain the process of serialization in Java and when you would implement it.
• Answer aloud, timed: How do thread pools work in Java, and how do you prevent resource exhaustion in concurrent execution?

Deliverable: Spoken answers to 2 reported Core Java,.NET & Full-Stack Fundamentals question(s), under time.

Day 6 — Answer out loud: Data Structures, Algorithms & Live Coding

• Answer aloud, timed: Implement code to reverse or manipulate strings and arrays efficiently.
• Answer aloud, timed: How would you construct and traverse a custom Linked List structure?

Deliverable: Spoken answers to 2 reported Data Structures, Algorithms & Live Coding question(s), under time.

Day 7 — Answer out loud: System Architecture, Cloud & API Design

• Answer aloud, timed: Describe how you would build a scalable RESTful API using Spring Boot or ASP.NET Core.
• Answer aloud, timed: How do AWS services such as EC2, S3, API Gateway, and CDK integrate to support enterprise microservices?

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

Behavioural preparation

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

How do you handle failure recovery and circuit breaking in a high-traffic microservices ecosystem?

Medium · System Architecture, Cloud & API Design

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 data validation tools like Pydantic and static code analysis tools like Pylint.

Medium · System Architecture, Cloud & API Design

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 you used your technical skills to solve a major operational efficiency problem.

Medium · Behavioral & Operational Scenarios

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 manage competing priorities when working with cross-functional business stakeholders?

Medium · Behavioral & Operational Scenarios

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 approach working with teammates who have differing technical opinions?

Medium · Behavioral & Operational Scenarios

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 as a Software Engineer at Delta Air Lines, and what interests you about the aviation i

Medium · Behavioral & Operational Scenarios

Why do you want to work as a Software Engineer at Delta Air Lines, and what interests you about the aviation industry?

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?