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Dia&Co Software Engineer Interview Guide

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

The 60-second brief

The role of a Software Engineer at Dia&Co is pivotal in driving the company's mission to empower women through personalized fashion solutions. As a Software Engineer, you will contribute to the development of innovative applications and systems that enhance user experience and streamline operations. This position is not just about coding; it involves collaborating with cross-functional teams to create scalable solutions that directly impact our customers and the business's bottom line. In this role, you will engage with various teams, including product management and design, to build software that supports our diverse product offerings. Your work will influence critical areas such as fashion recommendation algorithms, website performance optimization, and inventory management systems.

This guide is scoped to a Software Engineer candidate at Dia&Co.

Focus areas

  • Take-Home Coding Challenges
  • Technical Interviewing
  • Behavioral Interviewing

The Dia&Co interview process

Dia&Co candidates report 3 rounds over 3-5 weeks. The stages below are what candidates describe, not a published process.

Phone Screen

Initial phone screen to assess candidate's background and fit for the role.

What to demonstrate

  • Initial phone screen to assess candidate's background and fit for the role
  • Depth in Take-Home Coding Challenges

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.

Take-Home Coding Challenge

Candidates complete a coding challenge to demonstrate technical skills.

What to demonstrate

  • Candidates complete a coding challenge to demonstrate technical skills
  • Depth in Take-Home Coding Challenges

How to prepare

  • Answer aloud and timed: Explain the difference between REST and GraphQL.
  • Answer aloud and timed: What considerations do you take into account when optimizing a database query?

Onsite Interview

In-person interview combining technical and behavioral assessments with team members.

What to demonstrate

  • In-person interview combining technical and behavioral assessments with team members
  • Depth in Take-Home Coding Challenges

How to prepare

  • Answer aloud and timed: Can you discuss your experience with cloud-based services?
  • Answer aloud and timed: Tell me about a time when you had to navigate a conflict within your team.

Where candidates lose points

Know the Mission

Familiarize yourself with Dia&Co’s mission and values. Demonstrating alignment can significantly impact your interview success.

Be Ready to Collaborate

Expect collaborative questions that assess how you work in teams. Highlight your teamwork experiences.

Practice Coding

Regularly practice coding problems, especially focusing on algorithms and data structures, as these are common in technical assessments.

Ask Questions

Prepare insightful questions for your interviewers. This shows your interest and helps you gauge the company fit.

Questions & practice

Describe a project where you implemented a complex algorithm.

Medium · Technical / Domain Questions

Approach

  1. Restate the input: its shape, its size, and what is guaranteed about it.
  2. Name the brute-force solution and its complexity before improving on it.
  3. Choose the data structure from the access pattern, not from familiarity.
  4. State the target complexity and say which constraint rules the naive version out.

Follow-up

  • How does this change if the input no longer fits in memory?
  • What is the worst case, and how likely is it on real data?

Write a function to determine if a string is a palindrome.

Medium · Coding / Algorithms

Approach

  1. Restate the input: its shape, its size, and what is guaranteed about it.
  2. Name the brute-force solution and its complexity before improving on it.
  3. Choose the data structure from the access pattern, not from familiarity.
  4. State the target complexity and say which constraint rules the naive version out.

Follow-up

  • How does this change if the input no longer fits in memory?
  • What is the worst case, and how likely is it on real data?

How would you implement a binary search algorithm?

Medium · Coding / Algorithms

Approach

  1. Restate the input: its shape, its size, and what is guaranteed about it.
  2. Name the brute-force solution and its complexity before improving on it.
  3. Choose the data structure from the access pattern, not from familiarity.
  4. State the target complexity and say which constraint rules the naive version out.

Follow-up

  • How does this change if the input no longer fits in memory?
  • What is the worst case, and how likely is it on real data?

Solve a problem involving data structures, such as linked lists or trees.

Medium · Coding / Algorithms

Approach

  1. Restate the input: its shape, its size, and what is guaranteed about it.
  2. Name the brute-force solution and its complexity before improving on it.
  3. Choose the data structure from the access pattern, not from familiarity.
  4. State the target complexity and say which constraint rules the naive version out.

Follow-up

  • How does this change if the input no longer fits in memory?
  • What is the worst case, and how likely is it on real data?

What considerations do you take into account when optimizing a database query?

Medium · Technical / Domain Questions

Approach

  1. Name the grain you start from and join outward from it.
  2. Check whether any join is one-to-many before aggregating, or the sums inflate.
  3. Say which index the query would use, and what makes it unusable.
  4. Handle the rows that do not match: that is usually the actual question.

Follow-up

  • How does the query change if that join becomes one-to-many?
  • What happens to this when the table is ten times larger?

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?

Explain the difference between REST and GraphQL.

Medium · Technical / Domain Questions

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?

You have a dataset of user interactions. How would you design a feature to enhance user engagement?

Medium · Problem-Solving / Case Studies

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?

If tasked with improving the performance of an existing application, what steps would you take?

Medium · Problem-Solving / Case Studies

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?

Describe how you would approach scaling an application for increased traffic.

Medium · Problem-Solving / Case Studies

Approach

  1. Clarify what is being asked and what a complete answer contains.
  2. State your assumptions explicitly before working the problem.
  3. Say what you would check first and why it is the highest-information step.
  4. Work from the requirement backwards to the design.

Follow-up

  • What assumption would you test first?
  • How would you know your answer was wrong?

How do you approach debugging an application?

Medium · Technical / Domain Questions

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 Dia&Co candidates report.

Day 1 — Map the Dia&Co loop

  • Write out the reported sequence: Phone Screen, Take-Home Coding Challenge, Onsite 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 3 reported rounds, with the weakest marked.

Day 2 — Work Take-Home Coding Challenges

  • Spend the session on Take-Home Coding Challenges, which Dia&Co candidates report being tested on.
  • Write one worked example in Take-Home Coding Challenges and time yourself on it.

Deliverable: One timed worked example in Take-Home Coding Challenges.

Day 3 — Work Technical Interviewing

  • Spend the session on Technical Interviewing, which Dia&Co candidates report being tested on.
  • Write one worked example in Technical Interviewing and time yourself on it.

Deliverable: One timed worked example in Technical Interviewing.

Day 4 — Work Behavioral Interviewing

  • Spend the session on Behavioral Interviewing, which Dia&Co candidates report being tested on.
  • Write one worked example in Behavioral Interviewing and time yourself on it.

Deliverable: One timed worked example in Behavioral Interviewing.

Day 5 — Answer out loud: Technical / Domain Questions

  • Answer aloud, timed: Describe a project where you implemented a complex algorithm.
  • Answer aloud, timed: How do you approach debugging an application?

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

Day 6 — Answer out loud: Behavioral / Leadership

  • Answer aloud, timed: Tell me about a time when you had to navigate a conflict within your team.
  • Answer aloud, timed: How do you prioritize tasks when faced with tight deadlines?

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

Day 7 — Answer out loud: Problem-Solving / Case Studies

  • Answer aloud, timed: You have a dataset of user interactions. How would you design a feature to enhance user engagement?
  • Answer aloud, timed: If tasked with improving the performance of an existing application, what steps would you take?

Deliverable: Spoken answers to 2 reported Problem-Solving / Case Studies question(s), under time.

Behavioural preparation

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

Can you discuss your experience with cloud-based services?

Medium · Technical / Domain Questions

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 when you had to navigate a conflict within your team.

Medium · Behavioral / Leadership

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 prioritize tasks when faced with tight deadlines?

Medium · Behavioral / Leadership

Approach

  1. Pick a story where you made the decision, not one where you watched it.
  2. State the situation in two sentences and spend the rest on the reasoning.
  3. Give the blast radius: what could have broken, and what you measured.
  4. Name the disagreement and how you resolved it with evidence.

Follow-up

  • What would you do differently if you ran that again?
  • How did you know your change caused the improvement?

Describe a situation where you took the initiative on a project.

Medium · Behavioral / Leadership

Approach

  1. Pick a story where you made the decision, not one where you watched it.
  2. State the situation in two sentences and spend the rest on the reasoning.
  3. Give the blast radius: what could have broken, and what you measured.
  4. Name the disagreement and how you resolved it with evidence.

Follow-up

  • What would you do differently if you ran that again?
  • How did you know your change caused the improvement?

How do you handle feedback and criticism?

Medium · Behavioral / Leadership

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 motivates you to work in the technology sector?

Medium · Behavioral / Leadership

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?