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Mercari Software Engineer Interview Guide

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

The 60-second brief

As a Software Engineer at Mercari, you are responsible for building, scaling, and optimizing the backend architecture and user-facing systems that power one of the world's premier C2C marketplace platforms. Mercari operates across global engineering centers—including Tokyo, Palo Alto, and Bangalore—processing millions of concurrent marketplace listings, micro-transactions, and mobile payment integrations through Merpay. Your code directly impacts how sellers list items effortlessly and how buyers safely discover and purchase goods across diverse international markets. In this role, you will work primarily with technologies such as Golang, Java, PHP, and modern frontend/mobile stacks like React and Flutter. You will operate within a microservices ecosystem hosted on cloud infrastructure like GCP and AWS, managed via Docker and Kubernetes.

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

Focus areas

  • Golang (Go)
  • Machine Coding
  • Coding Tests / Online Assessments

The Mercari interview process

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

Recruiter Screen

Initial discussion about your background, location preferences, and basic technical alignment.

What to demonstrate

  • Initial discussion about your background, location preferences, and basic technical alignment
  • Depth in Golang (Go)

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 Screening

Focus on coding fundamentals, algorithmic problem-solving, and discussion of past projects.

What to demonstrate

  • Focus on coding fundamentals, algorithmic problem-solving, and discussion of past projects
  • Depth in Golang (Go)

How to prepare

  • Answer aloud and timed: Implement an algorithm to validate nested data structures or item listings with edge-case constraints.
  • Answer aloud and timed: Solve an array manipulation problem involving sliding windows or multi-pointer approaches.

Virtual Onsite

Comprehensive evaluation across several rounds focusing on system design, advanced coding, and domain-specific knowledge.

What to demonstrate

  • Comprehensive evaluation across several rounds focusing on system design, advanced coding, and domain-specific knowledge
  • Depth in Golang (Go)

How to prepare

  • Answer aloud and timed: Given a series of timestamps and user actions, compute rate-limiting thresholds or rolling statistics.
  • Answer aloud and timed: How do you handle concurrency, error management, and channels when writing scalable APIs in Golang?

Behavioral Interviews

Meet with engineering managers and cross-functional partners to discuss past experiences and cultural fit.

What to demonstrate

  • Meet with engineering managers and cross-functional partners to discuss past experiences and cultural fit
  • Depth in Golang (Go)

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 interviews above and write down what you would ask to confirm before it.

Where candidates lose points

Test the App Before Your Interview

Download and use the Mercari mobile app or web platform. Buy or sell an item, note user friction points, and come prepared with concrete engineering suggestions for improvement.

Be Explicit About System Scale

Avoid generic statements like "we handled high traffic." Quantify your impact using exact numbers (e.g., "managed 15,000 QPS on a MySQL database with 99.95% uptime").

Structure Your Code for Production

During live coding or take-home tests, do not write throwaway scripts. Treat every exercise as if it were being merged into a production microservices repository.

Emphasize Language Flexibility

While you can interview in your strongest backend language, show willingness to write Golang, Java, or PHP based on microservice ownership.

Questions & practice

Given an array of transactional records, find the maximum profit or optimal grouping using dynamic programming

Medium · Algorithms & Problem Solving

Given an array of transactional records, find the maximum profit or optimal grouping using dynamic programming techniques.

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 string manipulation problem requiring the use of hash sets and maps to maintain low space-time complex

Medium · Algorithms & Problem Solving

Solve a string manipulation problem requiring the use of hash sets and maps to maintain low space-time complexity.

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?

Implement an algorithm to validate nested data structures or item listings with edge-case constraints.

Medium · Algorithms & Problem Solving

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 an array manipulation problem involving sliding windows or multi-pointer approaches.

Medium · Algorithms & Problem Solving

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?

Given a series of timestamps and user actions, compute rate-limiting thresholds or rolling statistics.

Medium · Algorithms & Problem Solving

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 do you quantify and explain the system scale (requests per second, user concurrency, dataset size) of your

Medium · Behavioral & Product Focus

How do you quantify and explain the system scale (requests per second, user concurrency, dataset size) of your past projects?

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?

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?

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?

Design a high-throughput banner service that controls which promotional banners are displayed to specific user

Medium · Backend & Microservices Engineering

Design a high-throughput banner service that controls which promotional banners are displayed to specific users at designated time windows.

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 strategies do you use for performance tuning, monitoring, and setting SLO/SLA targets in a microservices

Medium · Backend & Microservices Engineering

What strategies do you use for performance tuning, monitoring, and setting SLO/SLA targets in a microservices deployment?

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 manage containerized workloads using Docker and Kubernetes under high traffic spikes.

Medium · Backend & Microservices Engineering

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 choose between an RDBMS like MySQL and a NoSQL data store for a marketplace inventory module?

Medium · Backend & Microservices Engineering

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?

Design a high-level architecture for a C2C marketplace item listing and search service.

Medium · System Design & Scalability

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 design a scalable notification system capable of sending push notifications to millions of activ

Medium · System Design & Scalability

How would you design a scalable notification system capable of sending push notifications to millions of active buyers and sellers?

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?

Walk through the design of an asynchronous payment processing pipeline with retry mechanisms and idempotency g

Medium · System Design & Scalability

Walk through the design of an asynchronous payment processing pipeline with retry mechanisms and idempotency guarantees.

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 isolate read and write workloads for an e-commerce catalog experiencing heavy read traffic?

Medium · System Design & Scalability

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?

Exports duplicate a row range about once a week

Hard · leases · clock skew · idempotency

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

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

Suggested practice checklist

Built from the rounds and topics Mercari candidates report.

Day 1 — Map the Mercari loop

  • Write out the reported sequence: Recruiter Screen, Technical Screening, Virtual Onsite, Behavioral 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 Golang (Go)

  • Spend the session on Golang (Go), which Mercari candidates report being tested on.
  • Write one worked example in Golang (Go) and time yourself on it.

Deliverable: One timed worked example in Golang (Go).

Day 3 — Work Machine Coding

  • Spend the session on Machine Coding, which Mercari candidates report being tested on.
  • Write one worked example in Machine Coding and time yourself on it.

Deliverable: One timed worked example in Machine Coding.

Day 4 — Work Coding Tests / Online Assessments

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

Deliverable: One timed worked example in Coding Tests / Online Assessments.

Day 5 — Answer out loud: Algorithms & Problem Solving

  • Answer aloud, timed: Given an array of transactional records, find the maximum profit or optimal grouping using dynamic programming techniques.
  • Answer aloud, timed: Solve a string manipulation problem requiring the use of hash sets and maps to maintain low space-time complexity.

Deliverable: Spoken answers to 2 reported Algorithms & Problem Solving question(s), under time.

Day 6 — Answer out loud: Backend & Microservices Engineering

  • Answer aloud, timed: How do you handle concurrency, error management, and channels when writing scalable APIs in Golang?
  • Answer aloud, timed: Design a high-throughput banner service that controls which promotional banners are displayed to specific users at designated time windows.

Deliverable: Spoken answers to 2 reported Backend & Microservices Engineering question(s), under time.

Day 7 — Answer out loud: System Design & Scalability

  • Answer aloud, timed: Design a high-level architecture for a C2C marketplace item listing and search service.
  • Answer aloud, timed: How would you design a scalable notification system capable of sending push notifications to millions of active buyers and sellers?

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

Behavioural preparation

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

How do you handle concurrency, error management, and channels when writing scalable APIs in Golang?

Medium · Backend & Microservices Engineering

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?

Why do you want to work at Mercari, and what specific features of our marketplace application would you improv

Medium · Behavioral & Product Focus

Why do you want to work at Mercari, and what specific features of our marketplace application would you improve?

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 time when you took a bold technical risk that failed. What did you learn, and how did you rebound?

Medium · Behavioral & Product Focus

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 situation where you had to collaborate across cross-border teams with varying language profici

Medium · Behavioral & Product Focus

Tell me about a situation where you had to collaborate across cross-border teams with varying language proficiencies or work cultures.

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 engineering backlog items when business deadlines conflict with technical debt cleanup?

Medium · Behavioral & Product Focus

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?