As a Software Engineer at Epsilon Systems, you play a vital role in designing, developing, and maintaining sophisticated software solutions that support defense, maritime, and advanced technology programs. You will contribute directly to mission-critical systems, working alongside multidisciplinary engineering teams to deliver robust, reliable, and secure applications. Your work directly impacts complex defense products, operational software, and mission-support infrastructure that users rely on in high-stakes environments. This position is particularly interesting due to the intersection of modern software development methodologies and traditional systems engineering domains. You will frequently interact with specialized spaces such as DevSecOps pipelines, containerized environments, signal intelligence (SIGINT) systems, and cooperative engagement capability (CEC) frameworks. Whether you are optimizing core codebases or integrating complex distributed systems, your technical decisions carry significant strategic influence over project success. Expect a fast-paced yet structured environment where technical precision, security awareness, and cross-functional collaboration are paramount. While the scope of work can be demanding, it offers immense opportunities for professional growth as you tackle unique engineering challenges.
Phone Screen
reportedInitial screening conducted by a recruiter or HR representative, focusing on your resume and qualifications.
What to demonstrate
- Initial screening conducted by a recruiter or HR representative
- Focusing on your resume and qualifications
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
reportedOne or more interviews where you demonstrate your coding skills and problem-solving abilities.
What to demonstrate
- One or more interviews where you demonstrate your coding skills and problem-solving abilities
- Depth in Coding Exercises
How to prepare
- Answer aloud and timed: Explain the difference between TCP and UDP
- Answer aloud and timed: Internal implementation of hashmap
Behavioral Interviews
reportedInterviews to assess cultural fit and teamwork capabilities.
What to demonstrate
- Interviews to assess cultural fit and teamwork capabilities
- Depth in Coding Exercises
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.
PracHub editorial advice for the preparation topics above.
Brush up on core fundamentals
Do not rely solely on advanced frameworks; ensure your foundational knowledge of data structures, sorting algorithms, and object-oriented principles is sharp.
Communicate your thought process
During live coding exercises, never code in silence. Talk through your assumptions, explain alternative approaches, and invite collaboration from your interviewer.
Tailor your resume review
Be prepared to walk through every bullet point on your resume in detail. Interviewers often use your past projects as the baseline for technical questioning.
Choose a category, try a prompt, then open its approach, worked solution or follow-up when you need it.
Explain what given code does and how you would add a feature to a class, considering performance and multithre
Explain what given code does and how you would add a feature to a class, considering performance and multithreading
Approach
- Say what the runtime actually does before reasoning about the code.
- Name what is shared across threads and what owns each piece of state.
- Identify the window where an invariant is briefly untrue.
- 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?
Fizz Buzz and Array Questions
Fizz Buzz and Array Questions
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?
My life story and sort of how I got there
My life story and sort of how I got there
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?
Denormalise tenant onto revisions and backfill it live
resource_revision (revision_id, resource_id, version, actor_user_id, change_kind, patch, request_id, created_at) has 400M rows and no tenant column; tenant_id lives only on resource. Two reads need it: a tenant-scoped audit feed ordered by created_at DESC, and an offboarding purge. Both join back to resource today. Justify adding tenant_id to resource_revision against those two reads, name the anomaly the copy introduces and the constraint that prevents it, then give the ordered migration for a live table taking 1.2k writes/second — the lock each step takes, how the backfill is batched, and where each step stops being reversible. PostgreSQL 16.
Approach
- Justify from the access path rather than from taste. Without the column, the audit feed either scans resource_revision by created_at and discards other tenants' rows, or resolves the tenant's resource_ids first and probes with them — both proportional to the tenant's whole history rather than to one page. With (tenant_id, created_at DESC, revision_id DESC) it is a seek that stops at 50 rows, and the purge becomes a ranged delete instead of a join.
- Name the cost exactly: a second copy of a fact can disagree with the first. Make the disagreement unwritable rather than documented — add UNIQUE (resource_id, tenant_id) on resource so it can serve as a foreign-key target, then FOREIGN KEY (resource_id, tenant_id) REFERENCES resource (resource_id, tenant_id) on the revision table. A revision can then only ever carry its parent's tenant.
- Step one, expand: ALTER TABLE resource_revision ADD COLUMN tenant_id BIGINT NULL, with no default, so it is a catalogue change and no rewrite. It still needs ACCESS EXCLUSIVE for an instant, and that instant queues behind the longest open transaction on the table while every later query queues behind it — set lock_timeout to 2s and retry rather than wait.
- Step two, dual-write: deploy the writer that populates tenant_id on every new revision while reads still use the join. Reversible by redeploying the previous build, because nothing reads the column yet.
Follow-up
- The backfill is half finished and a rollback is required. What state is the table in, and what does the previous build do with a half-populated column?
- How do you verify the backfill actually finished, given rows are still being inserted while it runs?
Keep soft-deleted accounts from blocking re-registration
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?
This category evaluates your fundamental computer science knowledge, language proficiency, and ability to writ
This category evaluates your fundamental computer science knowledge, language proficiency, and ability to write clean, working code.
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?
FizzBuzz, find first non-repeated char in a string, and String manipulation
FizzBuzz, find first non-repeated char in a string, and String manipulation
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 difference between TCP and UDP
Explain the difference between TCP and UDP
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?
Internal implementation of hashmap
Internal implementation of hashmap
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?
This category tests your ability to conceptualize software architecture and apply design principles to complex
This category tests your ability to conceptualize software architecture and apply design principles to complex scenarios.
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 design a certain game using OO techniques?
How would you design a certain game using OO techniques?
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 do you think in your own words a system engineer does?
What do you think in your own words a system engineer does?
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?
This category assesses your familiarity with deployment pipelines, modern infrastructure tools, and specialize
This category assesses your familiarity with deployment pipelines, modern infrastructure tools, and specialized technical scopes listed in job descriptions.
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?
DevOps related questions like containers, Kubernetes, and stuff on the job description
DevOps related questions like containers, Kubernetes, and stuff on the job description
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?
Listing latency scales with page size, not with filters
The tenant listing endpoint reads resource filtered by tenant_id and status, ordered by updated_at DESC, and returns each row plus the owner's display name from app_user and the actor of that resource's latest resource_revision. p99 is 55 ms at 10 rows per page and 1.4 s at 200. Database telemetry shows 401 statements per request, each under 1 ms, and nothing in the slow-query log. Diagnose the cause and give the fix, stating the statement count per request and the p99 you expect afterwards.
Approach
- Read the counters before forming a theory. 401 statements for 200 rows is one driver query plus two per row, and sub-millisecond execution with an empty slow-query log rules out a bad plan. The time is round trips, which is why it is invisible in every per-query metric and scales with rows returned rather than with filter selectivity.
- Name the two per-row statements from their normalised text: a single-row app_user lookup by user_id, and a resource_revision lookup by resource_id ordered by version DESC LIMIT 1. Confirm by dropping those two response fields and watching the statement count fall to one. That locates the calls in the serialisation layer, not the repository.
- Check that the arithmetic accounts for the whole gap. Measure one round trip to the replica in isolation; 400 trips at roughly 3 ms of network plus 0.2 ms of execution is about 1.3 s on top of a 55 ms baseline, which matches. If the multiplication had fallen short, the N+1 would only be part of the story and you would keep looking.
- Batch both lookups. Collect owner_user_ids and resource_ids from the driver query, then issue WHERE tenant_id = $1 AND user_id = ANY($2) for the users, and PostgreSQL's SELECT DISTINCT ON (resource_id) ... WHERE resource_id = ANY($2) ORDER BY resource_id, version DESC for the latest revision, which the UNIQUE (resource_id, version) index serves directly. On an engine without DISTINCT ON, use a lateral join or a row_number window. Three statements per request at any page size.
Follow-up
- The page size is capped at 200 today. What breaks first if it is raised to 2,000, and is it still this bug?
- How do you stop the next N+1 from reaching production, given that no individual query is slow and the endpoint's tests pass?
Built from the rounds and topics Epsilon Systems candidates report.
Prepare, practise & reflect
One practical outcome each day. Spend longer where you need it.
0 / 7 done01Map the Epsilon Systems loop
- Write out the reported sequence: Phone Screen, Technical Interviews, Behavioral 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.
02Work Coding Exercises
- Spend the session on Coding Exercises, which Epsilon Systems candidates report being tested on.
- Write one worked example in Coding Exercises and time yourself on it.
Deliverable: One timed worked example in Coding Exercises.
03Work Behavioral Interviews
- Spend the session on Behavioral Interviews, which Epsilon Systems candidates report being tested on.
- Write one worked example in Behavioral Interviews and time yourself on it.
Deliverable: One timed worked example in Behavioral Interviews.
04Work Object-Oriented Design (OOP)
- Spend the session on Object-Oriented Design (OOP), which Epsilon Systems candidates report being tested on.
- Write one worked example in Object-Oriented Design (OOP) and time yourself on it.
Deliverable: One timed worked example in Object-Oriented Design (OOP).
05Answer out loud: Technical & Core Programming
- Answer aloud, timed: This category evaluates your fundamental computer science knowledge, language proficiency, and ability to write clean, working code.
- Answer aloud, timed: FizzBuzz, find first non-repeated char in a string, and String manipulation
Deliverable: Spoken answers to 2 reported Technical & Core Programming question(s), under time.
06Answer out loud: System Design & Object-Oriented Programming
- Answer aloud, timed: This category tests your ability to conceptualize software architecture and apply design principles to complex scenarios.
- Answer aloud, timed: How would you design a certain game using OO techniques?
Deliverable: Spoken answers to 2 reported System Design & Object-Oriented Programming question(s), under time.
07Answer out loud: DevSecOps & Domain-Specific Knowledge
- Answer aloud, timed: This category assesses your familiarity with deployment pipelines, modern infrastructure tools, and specialized technical scopes listed in job descriptions.
- Answer aloud, timed: DevOps related questions like containers, Kubernetes, and stuff on the job description
Deliverable: Spoken answers to 2 reported DevSecOps & Domain-Specific Knowledge question(s), under time.
Expand any day for tasks and deliverables. Your progress is saved on this device.
Behavioural rounds judge the decision you made and what it cost.
This category explores your past professional journey, teamwork, and alignment with the company environment.
This category explores your past professional journey, teamwork, and alignment with the company 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?
Tell me a little about yourself
Tell me a little about yourself
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 your previous work experience
Tell me about your previous work experience
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?
What is your previous professional experience (if any), and how do the skills that you have relate to this job
What is your previous professional experience (if any), and how do the skills that you have relate to this job position?
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 one time where you had to manage a conflict
Describe one time where you had to manage a conflict
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 will you report military members doing a job incorrectly?
How will you report military members doing a job incorrectly?
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?
What size company do you want to work for?
What size company do you want to work for?
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?
- 01
This category explores your past professional journey, teamwork, and alignment with the company environment.
- 02
Tell me a little about yourself
- 03
Tell me about your previous work experience
- 04
What is your previous professional experience (if any), and how do the skills that you have relate to this job position?
How difficult is the interview process, and how much preparation time should I plan for?
The overall difficulty is generally considered moderate, though individual rounds can vary significantly based on the interviewing team. Plan for at least two to three weeks of dedicated preparation, focusing heavily on core coding fundamentals, object-oriented design principles, and reviewing your past resume projects.
Epsilon Systems Software Engineer candidate reports ↗What differentiates successful candidates from those who are not passed along?
Successful candidates combine solid technical execution with clear, concise communication and a positive attitude. Interviewers look for engineers who can explain their problem-solving methodology out loud, accept feedback gracefully during live coding sessions, and demonstrate genuine interest in the company's mission.
Epsilon Systems Software Engineer candidate reports ↗What is the typical timeline from initial screen to receiving an offer?
The timeline can vary widely, ranging from a few weeks to a couple of months depending on contract scheduling and internal approval processes. Maintaining proactive communication with your recruiter is important, but be prepared for occasional pauses in the pipeline.
Epsilon Systems Software Engineer candidate reports ↗Are remote work options available for this role?
Work arrangements depend entirely on the specific project and team location. While many engineering roles require on-site presence due to the defense and classified nature of the work, certain senior positions or specific contracts offer hybrid or fully remote flexibility.
Epsilon Systems Software Engineer candidate reports ↗How should I prepare for the behavioral portions of the interview?
Focus on structuring your responses around your resume experiences, academic projects, or past internships. Be ready to discuss how you handle technical disagreements, manage project conflicts, and collaborate within cross-functional engineering teams.
Epsilon Systems Software Engineer candidate reports ↗What topics does Epsilon Systems test in interviews?
Epsilon Systems interviews most often cover Communication, Coding Exercises, Quality Assurance (QA), Project Management, and Behavioral Interviewing. The exact emphasis depends on the specific role you apply for.
Epsilon Systems Software Engineer candidate reports ↗Sources & methodology 3 sources ↗
No official company page is cited. Rounds and questions come from candidate reports and PracHub editorial material; each source shows the date it was read.
- 01Epsilon Systems Software Engineer candidate reports ↗
Company-reported rounds, questions and FAQ.
Candidate reports · Accessed 2026-09-22 - 02PracHub Software Engineer practice ↗
PracHub practice material, not company-reported.
PracHub page · Accessed 2026-09-22 - 03PracHub preparation framework ↗
PracHub preparation guidance.
PracHub page · Accessed 2026-09-22
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