Wonga Software Engineer Interview Guide

The 60-second brief

A Software Engineer at Wonga is responsible for designing, building, and maintaining the highly scalable systems that power automated financial decision-making. Operating in a fast-paced fintech environment, engineers here work on complex, low-latency transaction engines, real-time risk assessment platforms, and payment integrations. The software you build directly impacts the speed and reliability of financial services delivered to millions of customers. Engineers at Wonga do not work in isolation. They collaborate closely with product owners, data scientists, and risk teams to translate complex business logic into clean, testable, and maintainable code. Because the platform processes highly sensitive financial data, there is a strong emphasis on security, system availability, and strict adherence to software craftsmanship standards.

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

Focus areas

• Programming Tests / Coding Challenges
• Architectural and Design Questions
• Testing (Test Coverage / Testability)

The Wonga interview process

Wonga candidates report 4 rounds over 3-5 weeks. The stages below are what candidates describe, not a published process.

Recruiter Call

Brief introductory call to discuss your background, career goals, and team alignment.

What to demonstrate

• Brief introductory call to discuss your background, career goals, and team alignment
• Depth in Programming Tests / 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.

Technical Phone Screen

30 to 60-minute call with senior engineers covering software engineering concepts and project experiences.

What to demonstrate

• 30 to 60-minute call with senior engineers covering software engineering concepts and project experiences
• Depth in Programming Tests / 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 Programming Test

Complete a programming test with up to a week to submit your code.

What to demonstrate

• Complete a programming test with up to a week to submit your code
• Depth in Programming Tests / Coding Challenges

How to prepare

• Answer aloud and timed: How do you ensure your code remains maintainable and clean when working with legacy systems?
• Answer aloud and timed: How would you design a real-time transaction processing system that guarantees exactly-once processing?

Intensive Interviews

Includes code review, pair-programming session, system architecture discussion, and behavioral interviews.

What to demonstrate

• Includes code review, pair-programming session, system architecture discussion, and behavioral interviews
• Depth in Programming Tests / Coding Challenges

How to prepare

• Answer aloud and timed: Describe how you would build a highly available integration with a third-party payment gateway or credit bureau.
• Answer aloud and timed: How do you handle failure states and retries in a microservices architecture without overloading downstream services?

Where candidates lose points

Prioritize testing in your take-home test

Do not treat tests as an afterthought. Write comprehensive unit tests and, if applicable, integration tests. Candidates who submit code with high test coverage and a clear testing strategy immediately stand out.

Do not over-engineer your solution

Keep your code simple and readable. Avoid implementing complex design patterns where a simpler, clean solution is more appropriate. Be ready to explain the trade-offs of your choices.

Not asking what the system looks like if it dies halfway through

For any multi-step write, say what state remains if the process stops between step two and step three, and what brings it back: a single transaction, a saga with compensating actions, an outbox, or a reconciliation job. Partial failure is routine at any real call volume, so 'that shouldn't happen' is an answer with nothing behind it.

Questions & practice

How do you manage database concurrency and prevent race conditions when processing transactions?

Medium · Technical & Software Craftsmanship

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?

Track a rolling failure rate per destination for circuit decisions

Easy · sliding window · ring buffer · circuit breaker

The egress service delivers about 1,500 webhooks per second across roughly 40,000 destinations, each call bounded by a 10 second timeout. Maintain, per destination, the failure rate over the trailing 60 seconds so a caller can ask before dispatch whether the circuit should open. Attempts arrive as (destination_id, finished_at_ms, outcome). Requirement: amortised O(1) per attempt, with total memory bounded by the destination count rather than by traffic. Give the structure, its exact memory, and the rule that stops a destination with three attempts from opening a circuit.

Approach

1. Name the exact-deque version and then reject it as the default. Holding timestamps and advancing a tail pointer past anything older than now minus 60 seconds is a correct two-pointer window at amortised O(1) per attempt, but its memory tracks in-window traffic, so one destination in a retry storm holds hundreds of thousands of entries while thousands of quiet destinations hold none.
2. Use a ring of 60 one-second buckets per destination, each bucket a pair of counters for attempts and failures. On an attempt, advance the ring by the elapsed whole seconds, zeroing at most min(elapsed, 60) buckets, then increment the head. That is amortised O(1) with a fixed footprint per destination.
3. State the footprint: 60 buckets times two 4-byte counters is 480 bytes of payload per destination, so 40,000 destinations is roughly 20 to 25 MB with per-entry overhead, bounded by the catalogue rather than by the rate. The cost is granularity, since the oldest bucket ages out in whole seconds, which is far tighter than the decision needs.
4. Require a minimum sample before the circuit may open. A destination with three attempts and three failures reads as 100 percent and is not evidence; a floor of roughly 20 attempts in the window makes the ratio meaningful, and below that floor use a run of consecutive failures as the trigger instead.

Follow-up

• The fleet is 30 instances and each sees roughly a thirtieth of a destination's traffic. Where does the rate actually live, and what does a per-instance answer get wrong?
• A destination answers in 9.5 seconds and succeeds. It is not failing but it is consuming your per-destination concurrency. What signal should open the circuit here?

Find overlapping job attempts and peak concurrency from lease records

Medium · sweep line · intervals · leases

A day of job_run history yields about 50,000,000 attempt records: (job_run_id, job_type, attempt, started_at, finished_at which is NULL when the worker died, lease_expires_at). Leases expire on a clock, so a job that outran its lease ran twice. Produce (a) every job_run_id whose attempts overlapped in wall-clock time and (b) the peak number of simultaneously running attempts per job_type with the minute it occurred. Target O(n log n). State how you treat a NULL finished_at and what clock skew does to your answer.

Approach

1. Define the interval before sorting anything: an attempt occupies [started_at, COALESCE(finished_at, lease_expires_at)). finished_at is observed and lease_expires_at is only a promise, so every attempt without a finish contributes an estimate and the whole result is a lower bound on overlap rather than an exact count.
2. For peak concurrency, sweep: emit 2n endpoints, sort by (timestamp, kind) with ends ordered before starts at equal timestamps, then walk the sequence maintaining a counter per job_type and record each type's maximum with its timestamp. O(n log n) dominated by the sort, O(n) space, or O(1) extra if the sort is external and the walk streams.
3. For overlap detection, do not compare attempts pairwise. A single global sort by (job_run_id, started_at) gives both the grouping and the order; within a group, keep the maximum end seen so far and report an overlap exactly when the next start is less than that running maximum, which is one linear pass after the sort.
4. Half-open intervals matter and are easy to get wrong: with closed intervals an attempt ending at the same millisecond another begins reads as concurrency two, and across 50,000,000 records that artefact swamps the real signal.

Follow-up

• A handler is not idempotent and you have found 400 overlapping jobs. Which of them actually caused damage, and what would you query to find out?
• Peak concurrency for one job_type is 4 against a configured cap of 4. Is the cap working, or is the data hiding attempts that never started?

Explain why the owner filter ignores the listing index

Medium · composite index · query plans · sort elimination

The only index on resource is (tenant_id, status, updated_at DESC, resource_id DESC). A new endpoint returns one user's resources across all statuses, newest created first: WHERE tenant_id = $1 AND owner_user_id = $2 ORDER BY created_at DESC LIMIT 20. On a tenant with 2M rows it takes 900 ms and EXPLAIN shows a sort above a large scan. Explain precisely why the existing index cannot serve it, give the index that can, and state which of these the new index still will not help: owner_user_id alone across tenants; the same query ordered by updated_at. PostgreSQL 16.

Approach

1. Separate the two jobs an index does. For filtering, a composite btree is seekable only on a left prefix, so with no predicate on status the scan can at best range over tenant_id and test owner_user_id per row; PostgreSQL 16 has no btree skip scan to jump the unconstrained column.
2. For ordering, the index is sorted by (status, updated_at) within a tenant and not by created_at, so the LIMIT cannot stop early: every matching row is read and then sorted. That is the 'Sort Method: top-N heapsort' line, and it is why the plan reads 2M rows to answer with 20.
3. Derive the replacement from the access path — equality, equality, then the ordering column: CREATE INDEX CONCURRENTLY ON resource (tenant_id, owner_user_id, created_at DESC). The scan seeks to the (tenant, owner) range and walks 20 entries in order, so the Sort node disappears along with the row-read.
4. Treat INCLUDE (title, status) as conditional, not free. An index-only scan still visits the heap for any row whose page is not marked all-visible, so on a table taking 1.2k writes/second the win depends on autovacuum keeping the visibility map current, and the wider index costs more on every insert.

Follow-up

• 90% of rows are status='active'. Would a partial index WHERE status = 'active' change your answer, and for which of the three queries?
• A dashboard runs this for 40 owners in one page load. What changes about the design?

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 principles of SOLID design and how you apply them in your daily development.

Medium · Technical & Software Craftsmanship

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 Test-Driven Development (TDD), and what are the trade-offs of using it in a fast-moving startup enviro

Medium · Technical & Software Craftsmanship

What is Test-Driven Development (TDD), and what are the trade-offs of using it in a fast-moving startup environment?

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?

Explain the difference between optimistic and pessimistic locking, and when you would use each.

Medium · Technical & Software Craftsmanship

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 ensure your code remains maintainable and clean when working with legacy systems?

Medium · Technical & Software Craftsmanship

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 real-time transaction processing system that guarantees exactly-once processing?

Medium · System Design & Architecture

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 build a highly available integration with a third-party payment gateway or credit burea

Medium · System Design & Architecture

Describe how you would build a highly available integration with a third-party payment gateway or credit bureau.

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?

How do you handle failure states and retries in a microservices architecture without overloading downstream se

Medium · System Design & Architecture

How do you handle failure states and retries in a microservices architecture without overloading downstream services?

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 database technologies would you choose for a ledger system, and how would you structure the schema for au

Medium · System Design & Architecture

What database technologies would you choose for a ledger system, and how would you structure the schema for auditability?

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 a situation where you had to debug a critical issue in production under intense time pressure.

Medium · Behavioral & Team Collaboration

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 Wonga candidates report.

Day 1 — Map the Wonga loop

• Write out the reported sequence: Recruiter Call, Technical Phone Screen, Take-Home Programming Test, Intensive Interviews.
• 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 4 reported rounds, with the weakest marked.

Day 2 — Work Programming Tests / Coding Challenges

• Spend the session on Programming Tests / Coding Challenges, which Wonga candidates report being tested on.
• Write one worked example in Programming Tests / Coding Challenges and time yourself on it.

Deliverable: One timed worked example in Programming Tests / Coding Challenges.

Day 3 — Work Architectural and Design Questions

• Spend the session on Architectural and Design Questions, which Wonga candidates report being tested on.
• Write one worked example in Architectural and Design Questions and time yourself on it.

Deliverable: One timed worked example in Architectural and Design Questions.

Day 4 — Work Testing (Test Coverage / Testability)

• Spend the session on Testing (Test Coverage / Testability), which Wonga candidates report being tested on.
• Write one worked example in Testing (Test Coverage / Testability) and time yourself on it.

Deliverable: One timed worked example in Testing (Test Coverage / Testability).

Day 5 — Answer out loud: Technical & Software Craftsmanship

• Answer aloud, timed: Explain the principles of SOLID design and how you apply them in your daily development.
• Answer aloud, timed: What is Test-Driven Development (TDD), and what are the trade-offs of using it in a fast-moving startup environment?

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

Day 6 — Answer out loud: System Design & Architecture

• Answer aloud, timed: How would you design a real-time transaction processing system that guarantees exactly-once processing?
• Answer aloud, timed: Describe how you would build a highly available integration with a third-party payment gateway or credit bureau.

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

Day 7 — Answer out loud: Behavioral & Team Collaboration

• Answer aloud, timed: Tell me about a time you had a strong disagreement with a Product Owner regarding a technical decision. How did you resolve it?
• Answer aloud, timed: Describe a situation where you had to debug a critical issue in production under intense time pressure.

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

Behavioural preparation

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

How do you approach caching in a distributed system, and how do you handle cache invalidation?

Medium · System Design & Architecture

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 a strong disagreement with a Product Owner regarding a technical decision. How di

Medium · Behavioral & Team Collaboration

Tell me about a time you had a strong disagreement with a Product Owner regarding a technical decision. How did you resolve it?

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 mentoring junior engineers and conducting constructive code reviews?

Medium · Behavioral & Team 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?

Tell me about a time you had to balance delivering a feature quickly versus taking the time to write perfect,

Medium · Behavioral & Team Collaboration

Tell me about a time you had to balance delivering a feature quickly versus taking the time to write perfect, refactored code.

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 ambiguity when requirements for a new feature are not fully defined?

Medium · Behavioral & Team 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?