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Preligens Software Engineer Interview Guide

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

The 60-second brief

At Preligens (a Safran company), a Software Engineer sits at the absolute intersection of cutting-edge artificial intelligence, aerospace, and high-performance systems engineering. You will design, optimize, and scale software systems that process critical data, monitor complex physical systems, and deliver actionable intelligence to defense and national security stakeholders. The systems you build are not just standard enterprise applications; they are high-availability, low-latency platforms that handle massive data streams under highly demanding operational constraints. The impact of this position is immense, directly influencing the safety, reliability, and efficiency of aerospace infrastructure and defense intelligence.

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

Focus areas

  • Data acquisition systems
  • C
  • C++

The Preligens interview process

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

Recruiter Call

Initial screening call with a recruiter to discuss your background and high-level alignment with the role.

What to demonstrate

  • Initial screening call with a recruiter to discuss your background and high-level alignment with the role
  • Depth in Data acquisition systems

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 Discussion

Technical and behavioral discussion with the hiring manager or a senior team member.

What to demonstrate

  • Technical and behavioral discussion with the hiring manager or a senior team member
  • Depth in Data acquisition systems

How to prepare

  • Answer aloud and timed: Describe a scenario where you would choose manual memory management over automatic garbage collection.
  • Answer aloud and timed: What are smart pointers in C++, and how do they help prevent memory leaks?

Take-Home Assignment

Practical take-home assignment simulating a real-world engineering challenge.

What to demonstrate

  • Practical take-home assignment simulating a real-world engineering challenge
  • Depth in Data acquisition systems

How to prepare

  • Answer aloud and timed: How does CPU cache locality impact the performance of data structure traversal?
  • Answer aloud and timed: What are the primary differences between TCP and UDP, and in what scenarios would you choose one over the other for real-time telemetry?

Technical Restitution Interview

Interview with the team to present your design choices from the take-home assignment.

What to demonstrate

  • Interview with the team to present your design choices from the take-home assignment
  • Depth in Data acquisition systems

How to prepare

  • Answer aloud and timed: How do you handle packet loss or out-of-order delivery in a custom UDP-based protocol?
  • Answer aloud and timed: Describe your experience writing drivers or interfaces for LAN-based instrumentation devices.

Final Strategic Interview

Final interview with a VP or executive to assess overall fit and strategy.

What to demonstrate

  • Final interview with a VP or executive to assess overall fit and strategy
  • Depth in Data acquisition systems

How to prepare

  • Answer aloud and timed: How would you design a software buffer to handle high-frequency incoming data streams without blocking the main execution thread?
  • Answer aloud and timed: Explain how the HTTP/2 protocol improves upon HTTP/1.1 in terms of multiplexing and latency.

Where candidates lose points

Prepare for the restitution session

Treat the presentation of your take-home technical test like a professional system review. Explain your architectural choices clearly, acknowledge any limitations or trade-offs, and explain how you would scale the system in a production environment.

Focus on performance and reliability

In all your technical discussions, emphasize your commitment to writing highly optimized, resource-efficient, and reliable code. Mentioning experience with profiling tools and memory leak detection is highly beneficial.

Highlight cross-functional experience

Show that you know how to collaborate with QA, hardware teams, and product managers. Providing examples of how you facilitated thorough testing or helped define product requirements will set you apart.

Be clear and structured

When answering behavioral or situational questions, use the STAR method (Situation, Task, Action, Result) to keep your responses concise, focused, and impactful.

Questions & practice

Explain the difference between a process and a thread, and describe how they manage memory.

Medium · Systems Programming & Concurrency

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?

How do you prevent race conditions and deadlocks in a multi-threaded C++ or C# application?

Medium · Systems Programming & Concurrency

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?

Describe a scenario where you would choose manual memory management over automatic garbage collection.

Medium · Systems Programming & Concurrency

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?

What are smart pointers in C++, and how do they help prevent memory leaks?

Medium · Systems Programming & Concurrency

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?

How does CPU cache locality impact the performance of data structure traversal?

Medium · Systems Programming & Concurrency

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?

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?

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?

What are the primary differences between TCP and UDP, and in what scenarios would you choose one over the othe

Medium · Networking & Data Acquisition

What are the primary differences between TCP and UDP, and in what scenarios would you choose one over the other for real-time telemetry?

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 would you design a software buffer to handle high-frequency incoming data streams without blocking the mai

Medium · Networking & Data Acquisition

How would you design a software buffer to handle high-frequency incoming data streams without blocking the main execution thread?

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 the HTTP/2 protocol improves upon HTTP/1.1 in terms of multiplexing and latency.

Medium · Networking & Data Acquisition

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?

Walk through a complex software architecture you designed from scratch, explaining the trade-offs you made.

Medium · Software Architecture & 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 you apply SOLID design principles to ensure a codebase remains modular and extensible?

Medium · Software Architecture & 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?

Describe how you would design a plug-in architecture that allows third-party developers to add new data proces

Medium · Software Architecture & Design

Describe how you would design a plug-in architecture that allows third-party developers to add new data processing modules to an existing system.

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?

What is dependency injection, and how does it facilitate unit testing in large-scale applications?

Medium · Software Architecture & 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 you balance performance optimization with code readability and maintainability?

Medium · Software Architecture & 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?

Listing latency scales with page size, not with filters

Easy · n+1 · latency · query batching

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

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

Suggested practice checklist

Built from the rounds and topics Preligens candidates report.

Day 1 — Map the Preligens loop

  • Write out the reported sequence: Recruiter Call, Technical Discussion, Take-Home Assignment, Technical Restitution Interview, Final Strategic Interview.
  • 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 Data acquisition systems

  • Spend the session on Data acquisition systems, which Preligens candidates report being tested on.
  • Write one worked example in Data acquisition systems and time yourself on it.

Deliverable: One timed worked example in Data acquisition systems.

Day 3 — Work C

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

Deliverable: One timed worked example in C.

Day 4 — Work C++

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

Deliverable: One timed worked example in C++.

Day 5 — Answer out loud: Systems Programming & Concurrency

  • Answer aloud, timed: Explain the difference between a process and a thread, and describe how they manage memory.
  • Answer aloud, timed: How do you prevent race conditions and deadlocks in a multi-threaded C++ or C# application?

Deliverable: Spoken answers to 2 reported Systems Programming & Concurrency question(s), under time.

Day 6 — Answer out loud: Networking & Data Acquisition

  • Answer aloud, timed: What are the primary differences between TCP and UDP, and in what scenarios would you choose one over the other for real-time telemetry?
  • Answer aloud, timed: How do you handle packet loss or out-of-order delivery in a custom UDP-based protocol?

Deliverable: Spoken answers to 2 reported Networking & Data Acquisition question(s), under time.

Day 7 — Answer out loud: Software Architecture & Design

  • Answer aloud, timed: Walk through a complex software architecture you designed from scratch, explaining the trade-offs you made.
  • Answer aloud, timed: How do you apply SOLID design principles to ensure a codebase remains modular and extensible?

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

Behavioural preparation

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

How do you handle packet loss or out-of-order delivery in a custom UDP-based protocol?

Medium · Networking & Data Acquisition

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 writing drivers or interfaces for LAN-based instrumentation devices.

Medium · Networking & Data Acquisition

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 had to resolve a technical disagreement with a senior team member or architect.

Medium · Behavioral & Collaboration

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 situation where a critical software bug is discovered right before a major deployment dead

Medium · Behavioral & Collaboration

How do you handle a situation where a critical software bug is discovered right before a major deployment deadline?

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 mentor a junior engineer. How did you structure your feedback?

Medium · Behavioral & Collaboration

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 writing technical documentation, and why is it important in a defense or aerospace context

Medium · Behavioral & Collaboration

How do you approach writing technical documentation, and why is it important in a defense or aerospace context?

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?

Share an experience where you had to quickly learn a new technology or framework to deliver a project.

Medium · Behavioral & Collaboration

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?