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. Engineering at emphasizes high autonomy, rapid release cycles, and quantitative performance tuning based on user behavior logs, performance telemetry, and transactional traffic. Mercari Joining Mercari means tackling complex scalability challenges, such as optimizing database queries on MySQL or NoSQL platforms, designing resilient event-driven architectures, and building low-latency RESTful and gRPC APIs. Expect an engineering culture driven by the company's core values: "Go Bold," "All for One," and "Be a Pro."
Recruiter Screen
reportedInitial 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
reportedFocus 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
reportedComprehensive 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
reportedMeet 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.
PracHub editorial advice for the preparation topics above.
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.
Choose a category, try a prompt, then open its approach, worked solution or follow-up when you need it.
Given an array of transactional records, find the maximum profit or optimal grouping using dynamic programming
Given an array of transactional records, find the maximum profit or optimal grouping using dynamic programming techniques.
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?
Solve a string manipulation problem requiring the use of hash sets and maps to maintain low space-time complex
Solve a string manipulation problem requiring the use of hash sets and maps to maintain low space-time complexity.
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?
Implement an algorithm to validate nested data structures or item listings with edge-case constraints.
Implement an algorithm to validate nested data structures or item listings with edge-case constraints.
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?
Solve an array manipulation problem involving sliding windows or multi-pointer approaches.
Solve an array manipulation problem involving sliding windows or multi-pointer approaches.
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 series of timestamps and user actions, compute rate-limiting thresholds or rolling statistics.
Given a series of timestamps and user actions, compute rate-limiting thresholds or rolling statistics.
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 do you quantify and explain the system scale (requests per second, user concurrency, dataset size) of your
How do you quantify and explain the system scale (requests per second, user concurrency, dataset size) of your past projects?
Approach
- Say what the runtime actually does before reasoning about the code.
- Name what is shared across threads and what owns each piece of state.
- Identify the window where an invariant is briefly untrue.
- 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
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?
Explain why the owner filter ignores the listing index
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
- 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.
- 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.
- 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.
- 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
Design a high-throughput banner service that controls which promotional banners are displayed to specific users at designated time windows.
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?
What strategies do you use for performance tuning, monitoring, and setting SLO/SLA targets in a microservices
What strategies do you use for performance tuning, monitoring, and setting SLO/SLA targets in a microservices deployment?
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?
Explain how you manage containerized workloads using Docker and Kubernetes under high traffic spikes.
Explain how you manage containerized workloads using Docker and Kubernetes under high traffic spikes.
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 choose between an RDBMS like MySQL and a NoSQL data store for a marketplace inventory module?
How do you choose between an RDBMS like MySQL and a NoSQL data store for a marketplace inventory module?
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 high-level architecture for a C2C marketplace item listing and search service.
Design a high-level architecture for a C2C marketplace item listing and search service.
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 design a scalable notification system capable of sending push notifications to millions of activ
How would you design a scalable notification system capable of sending push notifications to millions of active buyers and sellers?
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?
Walk through the design of an asynchronous payment processing pipeline with retry mechanisms and idempotency g
Walk through the design of an asynchronous payment processing pipeline with retry mechanisms and idempotency guarantees.
Approach
- Say who the caller is and what they do when the call fails halfway.
- Define the identity of a request so a retry cannot double-apply it.
- Separate accepted, pending, failed and confirmed; they are different facts.
- 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?
How do you isolate read and write workloads for an e-commerce catalog experiencing heavy read traffic?
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 Mercari candidates report.
Prepare, practise & reflect
One practical outcome each day. Spend longer where you need it.
0 / 7 done01Map 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.
02Work 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).
03Work 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.
04Work 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.
05Answer 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.
06Answer 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.
07Answer 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.
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.
How do you handle concurrency, error management, and channels when writing scalable APIs in Golang?
How do you handle concurrency, error management, and channels when writing scalable APIs in Golang?
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?
Why do you want to work at Mercari, and what specific features of our marketplace application would you improv
Why do you want to work at Mercari, and what specific features of our marketplace application would you improve?
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 time when you took a bold technical risk that failed. What did you learn, and how did you rebound?
Describe a time when you took a bold technical risk that failed. What did you learn, and how did you rebound?
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 situation where you had to collaborate across cross-border teams with varying language profici
Tell me about a situation where you had to collaborate across cross-border teams with varying language proficiencies or work cultures.
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?
How do you prioritize engineering backlog items when business deadlines conflict with technical debt cleanup?
How do you prioritize engineering backlog items when business deadlines conflict with technical debt cleanup?
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
How do you handle concurrency, error management, and channels when writing scalable APIs in Golang?
- 02
Why do you want to work at Mercari, and what specific features of our marketplace application would you improve?
- 03
Describe a time when you took a bold technical risk that failed. What did you learn, and how did you rebound?
- 04
Tell me about a situation where you had to collaborate across cross-border teams with varying language proficiencies or work cultures.
How difficult are the technical assessments at Mercari?
The technical assessment usually consists of medium-to-hard LeetCode-style algorithmic problems or a week-long take-home assignment. Focus on writing production-ready code with modular structure, robust error handling, and comprehensive edge-case handling.
Mercari Software Engineer candidate reports ↗What differentiates successful candidates in the Mercari process?
Successful candidates demonstrate a blend of solid technical execution and authentic enthusiasm for Mercari's marketplace app. Expressing genuine insights on product improvement and showcasing alignment with company values like "Go Bold" are key differentiators.
Mercari Software Engineer candidate reports ↗How important is past experience with high-scale systems?
Very important. Interviewers frequently query candidates on exact scale parameters—such as requests per second, database volumes, and latency thresholds—from their previous engineering roles.
Mercari Software Engineer candidate reports ↗What is the typical timeframe from initial screening to offer?
The full process generally takes 3 to 5 weeks. Feedback after each interview round is typically provided within 1 to 3 business days, though take-home assignment reviews can take slightly longer.
Mercari Software Engineer candidate reports ↗Does Mercari support remote or flexible work arrangements?
Mercari typically operates under a hybrid workstyle, allowing team members flexibility to balance remote work with scheduled in-office days at engineering hubs like Palo Alto, Tokyo, or Bangalore.
Mercari Software Engineer candidate reports ↗What topics does Mercari test in interviews?
Mercari interviews most often cover Design Thinking, Java, Algorithmic Problem Solving, Communication, and PHP. The exact emphasis depends on the specific role you apply for.
Mercari 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.
- 01Mercari 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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