Software Consultancy · Zagreb, Croatia
I'm Hrvoje Grdic. Through Backslash, I design, build and operate the systems companies move money through: payment platforms, billing and commerce integrations, and the AWS infrastructure underneath them. You work with me directly: the person who scopes the work is the one who writes it, deploys it and carries the pager.
What I do
Most of my work sits where money, third-party APIs and uptime meet, the places where a quiet bug becomes an incorrect payment.
Payment processing platforms, billing and invoicing, reconciliation and transaction handling built to be correct under retries, partial failures and replays.
Ruby on Rails · idempotency · reconciliation · audit trails
Keeping data consistent across systems that were never designed to agree: rate limits, webhook-and-polling hybrids, backfills and constant upstream API drift.
REST & webhooks · background jobs · retries · queues
The AWS estate that runs it all (networking, compute, databases, caching and monitoring), with zero-downtime deploys and rollback paths that have actually been rehearsed.
AWS · Docker · Lambda · Postgres · Redis
Merchant-facing software where the web app meets real hardware: ordering, receipt printing, loyalty, and native bridges into point-of-sale device ecosystems.
React · Android · Clover SDK · React Native
Selected work
A few engagements where the requirement was specific enough to be measured, and the failure mode was expensive.
A Rails sync engine holding items, orders, customers and payments consistent across BigCommerce, Square and Clover in real time. Three systems with different data models, different rate limits and no shared source of truth.
The interesting part was the background-job architecture: ordering guarantees so events could not be applied out of sequence, retries that were safe to run twice, and idempotency throughout so a replayed webhook never double-charged or duplicated an order.
Ruby on Rails · background jobs · idempotency · third-party APIs
An advertising company running screens across hotels and venues had a contractual obligation: any playlist, however many pages, had to be published to its displays within seven minutes. Large playlists were missing that window.
I sharded the rendering workload and fanned it out across AWS Lambda, scaling invocations with the size of the job and caching aggressively in Redis. Lambda code shipped as Docker images, with live traffic split across EC2 and Lambda targets behind an ALB using weighted routing. The deadline held.
Ruby on Rails · AWS Lambda · Redis · Docker · ALB
A customer ordering application integrated deeply with Clover: orders arriving from the app had to print correctly on two different targets: a kitchen ticket for staff and a customer-facing receipt, with a loyalty programme running alongside.
Work of this kind is why I ended up building several native bridges between Android and the Clover SDK, plus hybrid Ionic and Cordova integrations, across multiple engagements in that ecosystem.
Android · Clover SDK · Ionic / Cordova · Rails
A founder with an idea and no engineering team. I built the entire fintech product from zero (architecture, backend, front end, infrastructure and deployment) as the sole engineer, working directly with the CEO for eighteen months.
The product carried the company through a $2M Series A. An in-house team was hired after the raise, and the product is still operating today.
Ruby on Rails · React · sole engineer
Clients
Current and past engagements through Backslash since 2014, from early-stage startups to platforms you have already used today.
Plus 20+ further engagements across payments, e-commerce and SaaS. Some clients are covered by NDA and are not listed.
How I work
Backslash is deliberately small. That is the product, not a limitation.
The person who scopes your project is the person who builds it. Nothing gets sold by one team and delivered by another, and there is no ramp-up period paid for at your expense.
Schema, application, front end, AWS infrastructure, deploys, monitoring and on-call. On several engagements I have been the single point of technical accountability for the entire platform.
Where payments are involved I design for correctness first: idempotent operations, reconciliation, auditable state transitions, and backwards-compatible migrations so releases don't require downtime.
Most of my client relationships have run for years rather than weeks, which means I am usually the one maintaining what I built, a strong incentive to build it properly the first time.
I work through the Toptal network, whose screening process admits roughly the top 3% of applicants, and hold an MSc in Computing from FER, University of Zagreb.
Work is contracted and invoiced through Backslash d.o.o., a registered Croatian company. Remote, with working hours that overlap both European and US time zones.
Open source
I wrote the official Treblle SDK for Ruby: Rails middleware that reports API requests, responses and errors to Treblle's monitoring platform while keeping sensitive fields out of what leaves the application. MIT licensed. View on GitHub →
Get in touch
For project enquiries, questions about any of the work above, or anything else Backslash-related, email is the best way to reach me. I read and answer everything myself.
hrvoje.grdic1@gmail.com