Arηs Group Software Engineer Interview Guide

The 60-second brief

As a Software Engineer at Arηs Group, you play a pivotal role in delivering high-impact technological solutions for a diverse portfolio of clients. Arηs Group operates as a consultancy and systems integrator, meaning your work will often be embedded within client projects or dedicated teams, focusing on building scalable, robust, and efficient software systems. You will be expected to bridge the gap between complex business requirements and high-quality technical implementation. This role is critical to the organization's mission of digital transformation. You will contribute to various domains, ranging from Java-based enterprise backends to modern cloud-native architectures.

This guide is scoped to a Software Engineer candidate at Arηs Group.

Focus areas

• Java
• Spring Boot
• IEnumerable vs IQueryable (LINQ)

The Arηs Group interview process

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

Initial Contact

The process begins with initial communication regarding the opportunity.

What to demonstrate

• The process begins with initial communication regarding the opportunity
• Depth in Java

How to prepare

• Answer aloud and timed: Differences between an Abstract class and an interface.
• Answer aloud and timed: How does the HashMap work internally?

Technical Screening

Candidates undergo a technical assessment to evaluate their hard skills.

What to demonstrate

• Candidates undergo a technical assessment to evaluate their hard skills
• Depth in Java

How to prepare

• Answer aloud and timed: Explain the exact behavior of Spring Boot annotations.
• Answer aloud and timed: What is the difference between IEnumerable and IQueryable?

Behavioral Assessment

An evaluation of soft skills and ability to integrate into a project team.

What to demonstrate

• An evaluation of soft skills and ability to integrate into a project team
• 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 assessment above and write down what you would ask to confirm before it.

Client Interview

A distinct interview stage where communication style is critically assessed.

What to demonstrate

• A distinct interview stage where communication style is critically assessed
• Depth in Java

How to prepare

• Answer aloud and timed: Explain design patterns you have used in past projects.
• Answer aloud and timed: How would you represent this problem using Object-Oriented Programming (OOP) principles?

Final Decision

The final stage where a decision is made regarding the candidate's application.

What to demonstrate

• The final stage where a decision is made regarding the candidate's application
• Depth in Java

How to prepare

• Answer aloud and timed: How does garbage collection work, and how would you implement it reliably?
• Answer aloud and timed: Unidirectional vs. bidirectional for OneToMany relationships.

Where candidates lose points

Structure your answers

When asked about past projects, use the STAR method (Situation, Task, Action, Result) to keep your answers concise and impactful.

Focus on the "Why

Don't just list technologies you've used. Explain the technical challenges you faced and why you chose specific design patterns or architectures to solve them.

Show curiosity

Ask the interviewers about the team culture, the specific challenges of the project, and how the team handles technical debt.

Be prepared for legacy systems

Since Arηs Group works with various clients, you may encounter legacy tech stacks. Show a willingness to learn and adapt rather than rejecting older technologies outright.

Questions & practice

Differences between an Abstract class and an interface.

Medium · Technical Core & Language Proficiency

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 the HashMap work internally?

Medium · Technical Core & Language Proficiency

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?

Explain the exact behavior of Spring Boot annotations.

Medium · Technical Core & Language Proficiency

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 is the difference between IEnumerable and IQueryable?

Medium · Technical Core & Language Proficiency

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 pass a value type parameter as a reference?

Medium · Technical Core & Language Proficiency

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 garbage collection work, and how would you implement it reliably?

Medium · Architecture & System Design

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?

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?

Keep soft-deleted accounts from blocking re-registration

Easy · soft delete · unique constraints · engine differences

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

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

Explain design patterns you have used in past projects.

Medium · Architecture & System 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 would you represent this problem using Object-Oriented Programming (OOP) principles?

Medium · Architecture & System 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?

Unidirectional vs. bidirectional for OneToMany relationships.

Medium · Architecture & System 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 identify a memory leak?

Medium · Architecture & System Design

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 Arηs Group candidates report.

Day 1 — Map the Arηs Group loop

• Write out the reported sequence: Initial Contact, Technical Screening, Behavioral Assessment, Client Interview, Final Decision.
• 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 Arηs Group candidates report being tested on.
• Write one worked example in Java and time yourself on it.

Deliverable: One timed worked example in Java.

Day 3 — Work Spring Boot

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

Deliverable: One timed worked example in Spring Boot.

Day 4 — Work IEnumerable vs IQueryable (LINQ)

• Spend the session on IEnumerable vs IQueryable (LINQ), which Arηs Group candidates report being tested on.
• Write one worked example in IEnumerable vs IQueryable (LINQ) and time yourself on it.

Deliverable: One timed worked example in IEnumerable vs IQueryable (LINQ).

Day 5 — Answer out loud: Technical Core & Language Proficiency

• Answer aloud, timed: Differences between an Abstract class and an interface.
• Answer aloud, timed: How does the HashMap work internally?

Deliverable: Spoken answers to 2 reported Technical Core & Language Proficiency question(s), under time.

Day 6 — Answer out loud: Architecture & System Design

• Answer aloud, timed: How do you identify a memory leak?
• Answer aloud, timed: Explain design patterns you have used in past projects.

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

Day 7 — Answer out loud: Behavioral & Professional Alignment

• Answer aloud, timed: Present your background and your motivations for joining.
• Answer aloud, timed: What are your ambitions regarding your evolution within the company?

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

Behavioural preparation

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

Present your background and your motivations for joining.

Medium · Behavioral & Professional 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?

What are your ambitions regarding your evolution within the company?

Medium · Behavioral & Professional 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?

How do you see yourself in 5 years?

Medium · Behavioral & Professional 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?

Describe your past jobs and the nature of your previous projects.

Medium · Behavioral & Professional 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?