The role of a Software Engineer at Core One is pivotal in shaping innovative digital solutions that meet the evolving needs of our clients and the market. As a Software Engineer, you will be responsible for developing, testing, and maintaining software applications that are integral to our operations and service delivery. This position not only contributes to the technical backbone of our products but also plays a critical role in enhancing user experience, driving business efficiency, and ensuring the scalability of our solutions. At Core One, you will engage with complex systems and work on challenging projects that demand both creativity and technical expertise. You may find yourself involved in developing applications that support government operations, financial services, or cloud-based solutions, each requiring a keen understanding of both the technical and strategic aspects of software development. The impact of your work will resonate throughout the organization, as you collaborate with cross-functional teams to deliver high-quality software that meets user expectations and business goals.
Phone Screens
reportedInitial screenings to evaluate candidates' basic qualifications and fit.
What to demonstrate
- Initial screenings to evaluate candidates' basic qualifications and fit
- Depth in Cloud Architecture
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
reportedIn-depth technical assessments to evaluate problem-solving capabilities.
What to demonstrate
- In-depth technical assessments to evaluate problem-solving capabilities
- Depth in Cloud Architecture
How to prepare
- Answer aloud and timed: Discuss a challenging bug you encountered and how you resolved it.
- Answer aloud and timed: How do you ensure code quality and maintainability?
Onsite or Virtual Interview
reportedFinal stage where candidates engage directly with team members.
What to demonstrate
- Final stage where candidates engage directly with team members
- Depth in Cloud Architecture
How to prepare
- Answer aloud and timed: What tools and technologies are you proficient in, and how have you applied them?
- Answer aloud and timed: Describe a time when you had to solve a difficult technical challenge. What steps did you take?
PracHub editorial advice for the preparation topics above.
Practice coding
Regularly solve coding challenges on platforms like LeetCode or HackerRank to sharpen your programming skills and prepare for technical assessments.
Engage with peers
Network with current or former employees of Core One to gain insights into the interview process and company culture.
Tailor your responses
Use the STAR method to structure your answers, ensuring you clearly articulate your experiences and the impact of your contributions.
Research the company
Understand Core One's mission, values, and recent projects to demonstrate your interest and alignment during interviews.
Choose a category, try a prompt, then open its approach, worked solution or follow-up when you need it.
Write a function to reverse a linked list.
Write a function to reverse a linked list.
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?
Given a binary tree, how would you traverse it in order?
Given a binary tree, how would you traverse it in order?
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?
How would you implement a search algorithm, and what considerations would you take into account?
How would you implement a search algorithm, and what considerations would you take into account?
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?
Find version gaps and relay lag with window functions
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
- 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.
- 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.
- 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.
- 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?
Explain the difference between procedural and object-oriented programming.
Explain the difference between procedural and object-oriented programming.
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?
What are design patterns, and can you give examples of some you have used?
What are design patterns, and can you give examples of some you have used?
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?
Discuss a challenging bug you encountered and how you resolved it.
Discuss a challenging bug you encountered and how you resolved it.
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?
How do you ensure code quality and maintainability?
How do you ensure code quality and maintainability?
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?
What tools and technologies are you proficient in, and how have you applied them?
What tools and technologies are you proficient in, and how have you applied them?
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?
Imagine you need to optimize a slow-running application. What strategies would you consider?
Imagine you need to optimize a slow-running application. What strategies would you consider?
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?
How would you approach designing a new feature for an existing application?
How would you approach designing a new feature for an existing application?
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?
Design a scalable web application for a growing user base. What components would you include?
Design a scalable web application for a growing user base. What components would you include?
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 approach database design for a new application?
How would you approach database design for a new application?
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?
Discuss the trade-offs of using microservices versus a monolithic architecture.
Discuss the trade-offs of using microservices versus a monolithic architecture.
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?
Exports duplicate a row range about once a week
Roughly once a week an export writes a file containing a duplicated range of rows. The affected job_run rows show attempt = 1, status = succeeded, one started_at, and a lease_owner naming a different host from the one whose logs show the job starting. Leases last 30 seconds and are heartbeated every 10 from inside the handler; lease_expires_at is computed on the worker and compared against the database's now(). Find the mechanism, and give a fix that holds even if you cannot fix the clocks.
Approach
- Start from the fact that eliminates the obvious answer. attempt = 1 means no retry was recorded, so this is not a re-run after failure; two workers ran the same row concurrently and the takeover path never touched the counter. lease_owner naming a host other than the one that started the job is the same statement from the other side.
- Enumerate the mechanisms that cause a premature takeover, then find the signal that separates them. Either the lease genuinely expired because the heartbeat did not fire, which is what happens when the heartbeat runs on the handler's own thread and the handler makes a long blocking call, or it only appeared expired because two clocks disagree, since lease_expires_at is written from the worker's clock and evaluated against the database's. The discriminator is the distribution: incidents clustered on the longest exports indict the heartbeat, incidents clustered on one host indict skew. Measure both, and measure each host's offset against the database directly.
- Read the reclaim query precisely. In PostgreSQL now() is transaction start time, not statement time, so a reclaimer holding a long transaction compares against an older timestamp than expected; clock_timestamp() is the statement-time function. This is worth ruling in or out before you redesign anything, because it changes which rows look expired.
- Remove the second clock rather than trying to synchronise it. Issue and extend the lease in the database, with lease_expires_at = now() + interval '30 seconds' in both the claim and the heartbeat, so exactly one clock is ever compared and worker skew stops mattering to this predicate.
Follow-up
- The displaced worker has already streamed half the file to object storage. What makes that side effect safe to repeat?
- You now count takeovers. What alert fires on that counter, and at what threshold?
Built from the rounds and topics Core One candidates report.
Prepare, practise & reflect
One practical outcome each day. Spend longer where you need it.
0 / 7 done01Map the Core One loop
- Write out the reported sequence: Phone Screens, Technical Interviews, Onsite or Virtual 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.
02Work Cloud Architecture
- Spend the session on Cloud Architecture, which Core One candidates report being tested on.
- Write one worked example in Cloud Architecture and time yourself on it.
Deliverable: One timed worked example in Cloud Architecture.
03Work System Design
- Spend the session on System Design, which Core One candidates report being tested on.
- Write one worked example in System Design and time yourself on it.
Deliverable: One timed worked example in System Design.
04Work Programming
- Spend the session on Programming, which Core One candidates report being tested on.
- Write one worked example in Programming and time yourself on it.
Deliverable: One timed worked example in Programming.
05Answer out loud: Technical / Domain Questions
- Answer aloud, timed: Explain the difference between procedural and object-oriented programming.
- Answer aloud, timed: What are design patterns, and can you give examples of some you have used?
Deliverable: Spoken answers to 2 reported Technical / Domain Questions question(s), under time.
06Answer out loud: Problem-Solving / Case Studies
- Answer aloud, timed: Describe a time when you had to solve a difficult technical challenge. What steps did you take?
- Answer aloud, timed: Imagine you need to optimize a slow-running application. What strategies would you consider?
Deliverable: Spoken answers to 2 reported Problem-Solving / Case Studies question(s), under time.
07Answer out loud: Behavioral / Leadership
- Answer aloud, timed: Tell me about a time you worked on a team project. What was your role, and how did you contribute?
- Answer aloud, timed: Describe a situation where you had to handle a conflict with a coworker. How did you resolve it?
Deliverable: Spoken answers to 2 reported Behavioral / Leadership 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.
Describe a time when you had to solve a difficult technical challenge. What steps did you take?
Describe a time when you had to solve a difficult technical challenge. What steps did you take?
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 a time you worked on a team project. What was your role, and how did you contribute?
Tell me about a time you worked on a team project. What was your role, and how did you contribute?
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 a situation where you had to handle a conflict with a coworker. How did you resolve it?
Describe a situation where you had to handle a conflict with a coworker. How did you resolve it?
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 motivates you to excel in your work, and how do you keep yourself accountable?
What motivates you to excel in your work, and how do you keep yourself accountable?
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
Describe a time when you had to solve a difficult technical challenge. What steps did you take?
- 02
Tell me about a time you worked on a team project. What was your role, and how did you contribute?
- 03
Describe a situation where you had to handle a conflict with a coworker. How did you resolve it?
- 04
What motivates you to excel in your work, and how do you keep yourself accountable?
How difficult are the interviews at Core One?
The interviews at Core One are rigorous, focusing heavily on technical skills and problem-solving abilities. However, with adequate preparation in the relevant areas, candidates find that they can navigate the process successfully.
Core One Software Engineer candidate reports ↗How long does the interview process typically take?
The timeline from initial screening to offer can vary, but candidates should expect the entire process to take several weeks. Be prepared for multiple rounds of interviews, including both technical assessments and behavioral interviews.
Core One Software Engineer candidate reports ↗What differentiates successful candidates at Core One?
Successful candidates often demonstrate a blend of technical expertise, problem-solving skills, and cultural fit. They effectively communicate their thought processes and collaborate well with others.
Core One Software Engineer candidate reports ↗What is the company culture like at Core One?
Core One fosters a collaborative and innovative culture, valuing teamwork and a commitment to excellence. Employees are encouraged to share ideas and contribute to continuous improvement.
Core One Software Engineer candidate reports ↗Are remote work options available?
Core One offers flexibility in work arrangements, including remote and hybrid options, depending on the role and team dynamics.
Core One Software Engineer candidate reports ↗What topics does Core One test in interviews?
Core One interviews most often cover Risk Management, Program Management, IT Program Management, Stakeholder Management, and Project Planning. The exact emphasis depends on the specific role you apply for.
Core One 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.
- 01Core One 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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