Zacharia Kamau Ndung'u — Full-Stack Software Engineer

$cat about.md

I'm a full-stack engineer specialising in payment systems and fintech infrastructure. My day-to-day involves owning critical services end to end — from schema design and API contracts to the React dashboards that sit on top of them.

Most of my work lives in the hard, unsexy layer: idempotent transaction processing, distributed locking, ledger reconciliation, and making sure money moves exactly once even when the network doesn't cooperate.

Outside of work I write about payment systems on Medium, explore the Nairobi startup ecosystem, and occasionally try to understand why the NSE moves the way it does.

years in fintech

10+

microservices in prod

duplicate transactions shipped

$ls ~/stack

▸ languages/

— TypeScript
— Java
— Python
— SQL

▸ frontend/

— React
— Next.js
— TanStack Table
— Tailwind

▸ backend/

— NestJS
— Spring Boot
— Flask
— PostgreSQL
— Redis
— RabbitMQ

▸ infra/

— GCP
— AWS
— Docker
— GitHub Actions
— Keycloak
— APISIX

$ls -la ~/projects

Selected work

KiotaPay Ledgers

Zero reconciliation incidents

Microservice handling global ledger creation, audit, and real-time tracking across a B2B spend platform.

Spring BootPostgreSQLRedisRabbitMQ

Lead engineer2024↘

Built an async ledger regeneration system with a nightly rolling batch cycle, CompletableFuture pipelines, and Redis distributed locking to prevent concurrent runs. Resolved long-standing reconciliation gaps by enforcing strict idempotency at every processing stage.

open case study →

Payment Gateway

Idempotent at scale

Core gateway service routing M-Pesa Daraja and I&M Bank transactions across KiotaPay's microservices.

Spring BootRedisRabbitMQKeycloak

Backend engineer2024↘

Implemented Redis SET NX atomic locking with SHA-256 server-derived keys to prevent duplicate transaction processing during pod scaling and redeployments. Integrated APISIX for traffic routing and JWT validation via Keycloak.

open case study →

SpaceAndOccupy

Multi-tenant SaaS

Property management SaaS with async task processing, deployed on GCP Cloud Run.

FlaskReactCeleryRabbitMQGCP

Fullstack2023↘

Architected and deployed a multi-tenant rental management platform. Solved Cloud Run health check failures for Celery workers, configured GCP firewall rules for AMQP, and set up RabbitMQ on Compute Engine with Nginx and SSL.

open case study →

Invoicing & Quotation Service

End-to-end payment flow

Standalone microservice generating PDF invoices and processing M-Pesa payments for business clients.

Spring BootOpenHTMLtoPDFRabbitMQM-Pesa Daraja

Backend engineer2023↘

Built a Gradle-based microservice with RabbitMQ event publishing, PDF generation via OpenHTMLtoPDF, and full M-Pesa Daraja STK Push integration. Designed around an event-driven architecture so downstream services stay decoupled.

open case study →

$git log --oneline experience

Full-Stack Software Engineer

@ KiotaPay

Jun 2023 — present● now

› Own and maintain the payment gateway and ledger microservices, which process all payouts across the platform.
› Built a Redis-based distributed lock service ensuring idempotent transaction processing even during pod scaling or redeployments.
› Implemented secure authentication with JWT via Keycloak and APISIX for API traffic routing and token validation.
› Developed the Roles & Permissions microservice (Spring Boot) used for user management across all internal apps.
› Led NestJS backend work including server-side pagination, Hakikisha phone lookup integration, and TanStack Table frontend features.
› Built a nightly ledger regeneration batch system with Redis distributed locking, rolling over a 4-night cycle.

Software Engineer

@ Kelhel

Feb 2023 — Dec 2024

› Designed and built a POS system for a pharmaceutical company covering sales, inventory, and customer management.
› Integrated M-Pesa Daraja API for mobile payment processing and used PostgreSQL for structured data storage.
› Built the frontend in Next.js with Redux Toolkit for state management and ShadCN for UI components.

Front-End Engineer (Internship)

@ Dowell Research UK Limited

Jun 2022 — Oct 2023

› Developed and maintained client-side web applications with a focus on performance and responsiveness.
› Built custom WordPress plugins that improved accessibility and usability for end users.

Tech Content Writer

@ Upwork

Jan 2021 — Aug 2023

› Produced technical content on payment systems, software architecture, and fintech for clients across multiple industries.

$cat ~/writing/*.md

paymentsIdempotency in Payment Gateways: Why Your Money Moves Exactly Once2026 · 7 min→

LearningFrom Delivering Cakes to Writing Code: My Journey into Software Engineering (Part 1)2025 · 4 min→

JavascriptHow Javascript Works Behind the Scene2023 · 3 min→

JavascriptConfiguring Tailwind CSS in a React JS Project2023 · 4 min→

$./contact.sh

Let's build something.

Open to senior/staff fullstack roles, contract work, and interesting open-source collaborations. Fastest way to reach me is email.

ndunguzachary24@gmail.com →

githubgithub.com/Zackius↗linkedinlinkedin.com/in/zachariandungu↗emailndunguzachary24@gmail.com↗mediummedium.com/@ndunguzachary↗