Senior Backend Engineer (Kotlin / Java)
Senior backend engineer on Fourthwall's single Kotlin monolith — a creator-ecommerce platform and merchant of record handling payments, refunds, payouts, tax, manufacturing and shipping integrations. Remote in Poland (B2B), using Spring Boot, jOOQ and PostgreSQL on Google Cloud, with coding agents and backend on-call part of the role.
- Compensation
- PLN 22.5K–PLN 38K / mo
- Location
- Poznań
- Work setup
- Remote
- Seniority
- Senior
- Employment
- Full time
- Experience
- Not specified
- Category
- Backend
- Posted
Skills
Must have
- Kotlin or Java on a Spring backend in production
- Solid SQL; knowing what the ORM hides
- Judging when a failure belongs in the return type (Either, Result, sealed type) vs an exception
- One third-party API integrated end to end: contract, retries, idempotency, failure modes
- Understanding cost, latency and failure modes of queries, HTTP calls and background jobs
- A shipped feature and what it changed for users or revenue
- A request you did not build as asked: what was wanted, what you did instead, and why
- Experience with a piece of work outside your usual area (e.g. data pull, Terraform module, partner sandbox)
- Clear written English
- Directing coding agents on production code and fixing the harness (skills, hooks) rather than prompts
- Join the backend on-call rotation
Nice to have
- Learnable stack: jOOQ, JobRunr, Pub/Sub, Terraform, MCP tools with Spring AI
Full description
Wynagrodzenie
B2B: 22500 – 38000 PLN / mies. netto
About Fourthwall
Creatives, brands and startups run shops, memberships and donations on Fourthwall, on their own domain and with their own data. MKBHD, Y Combinator, ThePrimeagen, Coffeezilla, Acquired, Vox and the Smithsonian are among them. More than 500,000 creatives and companies use it. Investors: Lightspeed, Initialized Capital and Seven Seven Six. Offices in Los Angeles and Poznań.
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Company: Fourthwall, a product company. You contract with us directly.
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Level: Senior individual contributor. Kotlin or Java in production qualifies; the work is in Kotlin.
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Location: Remote in Poland. Meeting in Poznań from time to time is a plus, not a requirement.
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Hours: Your own, with agreed overlap with the team.
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On-call: Part of the role. You join the backend rotation.
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Rate: 22,500 to 38,000 PLN net per month on B2B (invoice amount excluding VAT), long-term.
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Stack: Kotlin, Spring Boot, jOOQ on PostgreSQL, Arrow, Google Cloud, Codex / Claude Code.
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Process: Intro call, paid exercise of about three hours, 75 minutes with the team, decision within five working days.
About the product and culture
Our promise is that we will make creating and selling products as easy as having a great idea. To deliver on that promise, our product must be comprehensive, including handling product design, manufacturing, shipping, ecommerce platform, customer support, and taxes.
The only way to do that is with a superstar team who want to work hard, own large areas of responsibility, and who care deeply about making customers happy. And that’s exactly the type of engineers we’re looking for. Smart, motivated, and looking for a chance to make a big impact on a product.
The service you would join has a first commit from 2019. It has run the business since and still goes to production several times a day.
Fourthwall is the merchant of record. We take the payment, handle sales tax and VAT, get the product made in one of 17 locations, ship it and pay the seller. All of that runs through this service: one Kotlin codebase, one deployable. It talks to payment, tax, manufacturing and shipping partners and exposes a public API with webhooks.
A lot of what it does is payments, refunds, payouts and tax. A bug there costs real money, so idempotency and reconciliation matter more here than in a typical CRUD service. The codebase is seven years old and has the usual leftovers. We clean up as we go, but that is not the main job. We are on JDK 26, Kotlin 2.4 and Spring Boot 4.