Sii Netherlands Software Engineer Interview Guide

The 60-second brief

As a Software Engineer at Sii Netherlands, you will operate at the critical intersection of software engineering and physical systems. Sii is a leading global engineering and IT consulting partner, and in the Netherlands, our teams drive high-impact projects across advanced industries. In this role, you will specifically contribute to the development of next-generation aerospace and mission-critical systems. Your work will directly power embedded flight code, ground control applications, and complex hardware-in-the-loop (HIL) simulation environments. This position is vital to our engineering delivery because the systems you design demand absolute reliability, precision, and performance. A single software bug in an aerospace environment can have catastrophic consequences.

This guide is scoped to a Software Engineer candidate at Sii Netherlands.

Focus areas

• Testing Methodologies
• Writing Test Cases
• Bug Reporting / Defect Identification

The Sii Netherlands interview process

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

Initial Screening

The process begins with an initial screening to assess your qualifications and fit for the role.

What to demonstrate

• The process begins with an initial screening to assess your qualifications and fit for the role
• Depth in Testing Methodologies

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 Assessments

Candidates undergo technical assessments to evaluate their programming skills and technical knowledge.

What to demonstrate

• Candidates undergo technical assessments to evaluate their programming skills and technical knowledge
• Depth in Testing Methodologies

How to prepare

• Answer aloud and timed: Describe the purpose of the volatile keyword in C/C++. Can you provide a practical example where omitting it would cause a bug?
• Answer aloud and timed: How do interrupt service routines (ISRs) work, and what are the best practices for writing them?

Deep-Dive Discussions

In-depth discussions focus on technical depth and consulting capabilities relevant to client projects.

What to demonstrate

• In-depth discussions focus on technical depth and consulting capabilities relevant to client projects
• Depth in Testing Methodologies

How to prepare

• Answer aloud and timed: What are the primary differences between SPI, I2C, and CAN bus protocols in terms of speed, topology, and error handling?
• Answer aloud and timed: What is Hardware-in-the-Loop (HIL) testing, and how would you design a test suite for an onboard flight controller?

Client Alignment

Final discussions to ensure alignment with client expectations and project requirements.

What to demonstrate

• Final discussions to ensure alignment with client expectations and project requirements
• Depth in Testing Methodologies

How to prepare

• Answer aloud and timed: How do you design an automated test case to identify a race condition in a multi-threaded application?
• Answer aloud and timed: Describe your approach to debugging a system that randomly crashes due to memory corruption. What tools would you use?

Offer Stage

Candidates who successfully complete the previous steps will receive an offer.

What to demonstrate

• Candidates who successfully complete the previous steps will receive an offer
• Depth in Testing Methodologies

How to prepare

• Answer aloud and timed: How do you approach writing testable code for a legacy system that lacks documentation?
• Answer aloud and timed: Design a basic database schema for an aerospace tracking system containing tables for locations, aircraft, and scheduled flights.

Where candidates lose points

Going into the loop without having done this.

To ensure you perform at your best, keep these practical tips in mind during your preparation:

Going into the loop without having done this.

Prepare for the client-facing aspect: Remember that Sii is a consultancy. Be ready to demonstrate professionalism, active listening, and the ability to adapt to different client cultures and technical standards.

Going into the loop without having done this.

Brush up on your testing fundamentals: Do not focus solely on writing code. Be prepared to discuss how you would test your code, design test cases, and handle edge cases.

Going into the loop without having done this.

Ensure your CV is up to date and clearly highlights your experience with embedded systems, low-level programming, or aerospace projects. Our technical interviewers will dive deep into your past projects, so be prepared to explain your architectural choices and technical contributions in detail.

Questions & practice

Explain the difference between stack and heap memory allocation. When is heap allocation unsafe in an embedded

Medium · Embedded Systems & Low-Level Programming

Explain the difference between stack and heap memory allocation. When is heap allocation unsafe in an embedded system?

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 implement a circular buffer in C, and how do you ensure it is thread-safe?

Medium · Embedded Systems & Low-Level Programming

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 the purpose of the volatile keyword in C/C++. Can you provide a practical example where omitting it

Medium · Embedded Systems & Low-Level Programming

Describe the purpose of the volatile keyword in C/C++. Can you provide a practical example where omitting it would cause a bug?

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 interrupt service routines (ISRs) work, and what are the best practices for writing them?

Medium · Embedded Systems & Low-Level Programming

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 the primary differences between SPI, I2C, and CAN bus protocols in terms of speed, topology, and erro

Medium · Embedded Systems & Low-Level Programming

What are the primary differences between SPI, I2C, and CAN bus protocols in terms of speed, topology, and error handling?

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 design an automated test case to identify a race condition in a multi-threaded application?

Medium · System Integration & Testing

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?

Write the update path that detects a concurrent edit

Medium · optimistic concurrency · transactional outbox · audit history

resource carries version INT NOT NULL DEFAULT 1. resource_revision holds revision_id, resource_id, version, actor_user_id, change_kind, patch JSONB, request_id, created_at with UNIQUE (resource_id, version). outbox_event holds aggregate_type, aggregate_id, aggregate_version, event_type, payload, status. A PUT carries the version the client read. Write the exact statements for the single transaction that applies the edit, records the revision and enqueues 'resource.updated', and give the handler's branch on zero affected rows. Then say what PostgreSQL 16 does under READ COMMITTED when two of these updates hit one row at once.

Approach

1. One transaction, three writes, no network call inside it: UPDATE resource SET title = $3, version = version + 1, updated_at = now() WHERE resource_id = $1 AND tenant_id = $4 AND version = $2; then INSERT the resource_revision row at version $2 + 1; then INSERT the outbox_event row at the same aggregate_version. The event goes to a table rather than a broker because no transaction spans both.
2. Branch on the affected-row count before doing anything else. Zero has three causes — stale version, wrong tenant, row gone — so re-read once and map to 409 carrying the current version, or 404 for an id outside the caller's tenant, which also stops the endpoint confirming that another tenant's id exists.
3. State the engine behaviour instead of assuming it. Under READ COMMITTED the second UPDATE blocks on the row lock, and when the first commits PostgreSQL re-evaluates the WHERE clause against the newly committed row, so the version predicate now fails and the statement reports zero rows. Under REPEATABLE READ the identical collision raises SQLSTATE 40001 instead, so the handler must fold both shapes into one conflict response.
4. Keep UNIQUE (resource_id, version) even though the predicate already serialises writers. It is what makes a lost update unwritable if any other path ever reaches the revision table, and it converts a logic bug into 23505 rather than into a silently missing history row.

Follow-up

• A client sends the version it read ten minutes ago and the resource has moved three versions. What is in your 409 so it can resolve the conflict without a full re-fetch?
• Two editors, two disjoint fields, no overlap. Does your answer still refuse the second write, and should it?

Denormalise tenant onto revisions and backfill it live

Hard · denormalisation · online migration · backfill

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

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

What is Hardware-in-the-Loop (HIL) testing, and how would you design a test suite for an onboard flight contro

Medium · System Integration & Testing

What is Hardware-in-the-Loop (HIL) testing, and how would you design a test suite for an onboard flight controller?

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 approach writing testable code for a legacy system that lacks documentation?

Medium · System Integration & Testing

Approach

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

Follow-up

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

Design a basic database schema for an aerospace tracking system containing tables for locations, aircraft, and

Medium · Software Architecture & Data Modeling

Design a basic database schema for an aerospace tracking system containing tables for locations, aircraft, and scheduled flights.

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 structure a RESTful API to handle real-time telemetry data streaming from a ground station?

Medium · Software Architecture & Data Modeling

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?

What design patterns would you use to build a modular telemetry dashboard that needs to support multiple commu

Medium · Software Architecture & Data Modeling

What design patterns would you use to build a modular telemetry dashboard that needs to support multiple communication protocols?

Approach

1. Fix the scope first: who calls this, how often, and what they do when it fails.
2. Name the read and write paths separately; they rarely have the same bottleneck.
3. Choose a partition key and say what query it makes expensive.
4. State the consistency you need, and where you are willing to be stale.

Follow-up

• What breaks first when traffic grows ten times?
• How does this behave when that dependency is down for an hour?

Explain how you would manage state synchronization between a ground control interface and an embedded flight s

Medium · Software Architecture & Data Modeling

Explain how you would manage state synchronization between a ground control interface and an embedded flight 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?

Describe your approach to debugging a system that randomly crashes due to memory corruption. What tools would

Medium · System Integration & Testing

Describe your approach to debugging a system that randomly crashes due to memory corruption. What tools would you use?

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 Sii Netherlands candidates report.

Day 1 — Map the Sii Netherlands loop

• Write out the reported sequence: Initial Screening, Technical Assessments, Deep-Dive Discussions, Client Alignment, Offer Stage.
• 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 Testing Methodologies

• Spend the session on Testing Methodologies, which Sii Netherlands candidates report being tested on.
• Write one worked example in Testing Methodologies and time yourself on it.

Deliverable: One timed worked example in Testing Methodologies.

Day 3 — Work Writing Test Cases

• Spend the session on Writing Test Cases, which Sii Netherlands candidates report being tested on.
• Write one worked example in Writing Test Cases and time yourself on it.

Deliverable: One timed worked example in Writing Test Cases.

Day 4 — Work Bug Reporting / Defect Identification

• Spend the session on Bug Reporting / Defect Identification, which Sii Netherlands candidates report being tested on.
• Write one worked example in Bug Reporting / Defect Identification and time yourself on it.

Deliverable: One timed worked example in Bug Reporting / Defect Identification.

Day 5 — Answer out loud: Embedded Systems & Low-Level Programming

• Answer aloud, timed: Explain the difference between stack and heap memory allocation. When is heap allocation unsafe in an embedded system?
• Answer aloud, timed: How do you implement a circular buffer in C, and how do you ensure it is thread-safe?

Deliverable: Spoken answers to 2 reported Embedded Systems & Low-Level Programming question(s), under time.

Day 6 — Answer out loud: System Integration & Testing

• Answer aloud, timed: What is Hardware-in-the-Loop (HIL) testing, and how would you design a test suite for an onboard flight controller?
• Answer aloud, timed: How do you design an automated test case to identify a race condition in a multi-threaded application?

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

Day 7 — Answer out loud: Software Architecture & Data Modeling

• Answer aloud, timed: Design a basic database schema for an aerospace tracking system containing tables for locations, aircraft, and scheduled flights.
• Answer aloud, timed: How would you structure a RESTful API to handle real-time telemetry data streaming from a ground station?

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

Behavioural preparation

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

Describe a time when you had to explain a complex technical issue to a non-technical stakeholder or client. Ho

Medium · Behavioral & Consultancy Alignment

Describe a time when you had to explain a complex technical issue to a non-technical stakeholder or client. How did you structure your explanation?

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 client requests a feature that compromises the system's safety or archit

Medium · Behavioral & Consultancy Alignment

How do you handle a situation where a client requests a feature that compromises the system's safety or architectural integrity?

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 challenging technical project where you had to learn a new tool or language under a tight dead

Medium · Behavioral & Consultancy Alignment

Tell me about a challenging technical project where you had to learn a new tool or language under a tight 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?

What is your approach to resolving technical disagreements within a multidisciplinary engineering team?

Medium · Behavioral & Consultancy Alignment

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?