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Frontier Software Engineer Interview Guide

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

The 60-second brief

As a Software Engineer at Frontier, you play a pivotal role in maintaining and evolving the infrastructure that powers modern telecommunications. You are not just writing code; you are building the systems that ensure reliable connectivity for millions of users. Whether you are working on VOIP applications, network routing, or internal automation tools, your work directly impacts the stability and efficiency of our national network. This position requires a unique blend of software development expertise and a strong understanding of network engineering principles. You will often find yourself bridging the gap between high-level application logic and the underlying hardware, such as Cisco equipment or complex routing protocols. It is a challenging, high-impact environment where your ability to solve problems at scale will determine the success of our digital transformation initiatives.

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

Focus areas

  • Java
  • Routing
  • Switching

The Frontier interview process

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

Screening Call

Initial call from a recruiter or outsourced agency to gauge your background.

What to demonstrate

  • Initial call from a recruiter or outsourced agency to gauge your background
  • 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.

Technical Interviews

A series of technical interviews that may include one-on-one sessions and panel interviews.

What to demonstrate

  • A series of technical interviews that may include one-on-one sessions and panel interviews
  • Depth in Java

How to prepare

  • Answer aloud and timed: What is your experience with routing and switching protocols for Cisco equipment?
  • Answer aloud and timed: Explain the fundamentals of HTTP and how you would debug a connectivity issue.

Behavioral Interviews

Interviews focusing on your ability to thrive in a team and assess behavioral fit.

What to demonstrate

  • Interviews focusing on your ability to thrive in a team and assess behavioral fit
  • Depth in Java

How to prepare

  • Prepare three examples from your own work, each with a decision you made and an outcome you can quantify.
  • Re-read the description of the behavioral interviews above and write down what you would ask to confirm before it.

Practical Coding Assessments

Occasional assessments to evaluate your coding skills in a practical context.

What to demonstrate

  • Occasional assessments to evaluate your coding skills in a practical context
  • Depth in Java

How to prepare

  • Answer aloud and timed: Can you write a basic Java function to process a specific set of data inputs while I observe?
  • Answer aloud and timed: How do you handle a scenario where you encounter a bug in a system you are unfamiliar with?

Final Decision-Making

The final stage where decisions are made regarding the candidate's application.

What to demonstrate

  • The final stage where decisions are made regarding the candidate's application
  • Depth in Java

How to prepare

  • Answer aloud and timed: What testing methods do you employ to ensure your code is production-ready?
  • Answer aloud and timed: Describe a time you had to learn a new technology or system rapidly to complete a project.

Where candidates lose points

Use the STAR method

When answering behavioral questions, structure your responses by describing the Situation, Task, Action, and Result.

Know your resume

Be prepared to discuss any detail on your resume in depth, especially past projects or technical roles.

Ask meaningful questions

At the end of your interview, ask about the team’s current challenges or the company culture; this shows you are serious about the role.

Stay positive

Even if an interview feels difficult, maintain a professional and optimistic attitude throughout.

Questions & practice

Please walk us through a standard implementation of a hash map or a two-sum algorithm.

Medium · Coding & 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?

Can you write a basic Java function to process a specific set of data inputs while I observe?

Medium · Coding & 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?

What testing methods do you employ to ensure your code is production-ready?

Medium · Coding & 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?

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?

Find version gaps and relay lag with window functions

Medium · window functions · event tables · frame semantics

outbox_event holds event_id, aggregate_type, aggregate_id, aggregate_version, event_type, payload, status ('pending','published','dead'), attempts, created_at, published_at. A projection is missing rows and you must decide whether the relay skipped events or the consumer dropped them. Write three queries over the last seven days: one listing every aggregate_id whose published aggregate_version sequence has a hole, one giving per-day counts with a running total, and one returning the newest published event per aggregate. For each, say where the window function is evaluated relative to WHERE and LIMIT. PostgreSQL 16.

Approach

  1. Gaps: compute lead(aggregate_version) OVER (PARTITION BY aggregate_id ORDER BY aggregate_version) in a subquery, then filter next_version <> aggregate_version + 1 in the outer query. Window functions are evaluated after WHERE, GROUP BY and HAVING and before the outer ORDER BY and LIMIT, so the predicate cannot sit in the same WHERE clause and PostgreSQL 16 has no QUALIFY.
  2. Say what the seven-day filter does to the answer: it truncates every partition, so the first row per aggregate has no predecessor inside the window and a hole spanning the boundary is invisible. Widen the window, or join to resource.version as the authority for the true maximum.
  3. Running total: SELECT date_trunc('day', created_at) AS d, count() AS n, sum(count()) OVER (ORDER BY date_trunc('day', created_at) ROWS UNBOUNDED PRECEDING). An aggregate inside a window call is legal because grouping runs before windowing. The grouping key is unique per row here so ROWS and RANGE agree, but write the frame anyway — over ungrouped rows with tied timestamps the default RANGE frame pulls in every peer row and the total jumps.
  4. Newest per aggregate: DISTINCT ON (aggregate_id) ... ORDER BY aggregate_id, aggregate_version DESC is the cheap PostgreSQL-only form when an index matches that order; row_number() OVER (PARTITION BY aggregate_id ORDER BY aggregate_version DESC) = 1 is the portable form and needs a subquery for the same evaluation-order reason as the gap query.

Follow-up

  • Relay failover redelivers events. Does a duplicate break the gap query, and how would you detect one from this table alone?
  • Turn the gap check into a continuous monitor rather than a query someone runs after an incident. What does it watch?

Can you describe the difference between trunking and access ports in a VLAN configuration?

Medium · Technical & Domain Expertise

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?

How do you approach database queries and data structure optimization in a production environment?

Medium · Technical & Domain Expertise

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?

How do you manage your productivity when working in a remote or distributed team environment?

Medium · Behavioral & Situational

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 explain the process of troubleshooting a VOIP application issue?

Medium · Technical & Domain Expertise

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?

Explain the fundamentals of HTTP and how you would debug a connectivity issue.

Medium · Technical & Domain Expertise

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 Frontier candidates report.

Day 1 — Map the Frontier loop

  • Write out the reported sequence: Screening Call, Technical Interviews, Behavioral Interviews, Practical Coding Assessments, Final Decision-Making.
  • 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 Frontier 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 Routing

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

Deliverable: One timed worked example in Routing.

Day 4 — Work Switching

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

Deliverable: One timed worked example in Switching.

Day 5 — Answer out loud: Technical & Domain Expertise

  • Answer aloud, timed: How would you explain the process of troubleshooting a VOIP application issue?
  • Answer aloud, timed: Can you describe the difference between trunking and access ports in a VLAN configuration?

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

Day 6 — Answer out loud: Coding & Problem Solving

  • Answer aloud, timed: Please walk us through a standard implementation of a hash map or a two-sum algorithm.
  • Answer aloud, timed: Can you write a basic Java function to process a specific set of data inputs while I observe?

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

Day 7 — Answer out loud: Behavioral & Situational

  • Answer aloud, timed: Describe a situation where you had to explain a complex technical concept to a non-technical stakeholder.
  • Answer aloud, timed: How do you manage your productivity when working in a remote or distributed team environment?

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

Behavioural preparation

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

What is your experience with routing and switching protocols for Cisco equipment?

Medium · Technical & Domain Expertise

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 scenario where you encounter a bug in a system you are unfamiliar with?

Medium · Coding & Problem Solving

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 time you had to learn a new technology or system rapidly to complete a project.

Medium · Coding & Problem Solving

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 explain a complex technical concept to a non-technical stakeholder.

Medium · Behavioral & Situational

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 us about a time you faced a significant obstacle in a project; how did you overcome it?

Medium · Behavioral & Situational

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 are you interested in working at Frontier, and how do you align with our current goals?

Medium · Behavioral & Situational

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 feedback, especially when it challenges your technical approach?

Medium · Behavioral & Situational

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?