# Yonatan Karp-Rudin

> Staff Software Engineer based in Berlin. Building payment, compliance, and platform systems at Billie. Open source contributor.

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## Key pages
- [Projects](https://yonatankarp.com/projects/): Libraries, tools, open-source contributions, and a few games.
- [Blog](https://yonatankarp.com/blog/): Articles on Kotlin, Spring Boot, backend systems, APIs, and engineering practice.
- [CV](https://yonatankarp.com/cv/): Background, experience, and skills.

## Blog posts
- [I Stopped Being My AI Agent's Memory: Building a Self-Compiling Second Brain](https://yonatankarp.com/blog/self-compiling-second-brain/): How I gave my AI coding agent a real long-term memory using Claude Code hooks: a self-compiling wiki it writes to itself, with capture, ingest, and recall wired into the session lifecycle.
- [XKCD: From Tile to Picture - Seeing the Full Image](https://yonatankarp.com/blog/xkcd-from-tile-to-picture/): A step-by-step walkthrough of reconstructing the massive XKCD #1110 Click and Drag comic from its individual tile images using Kotlin.
- [How to Fix Misconfigurations in RedisTemplate](https://yonatankarp.com/blog/fix-misconfigurations-redistemplate/): A practical guide to identifying and fixing common RedisTemplate misconfiguration pitfalls in Spring Boot, with a focus on serialization issues.
- [Observability in Action: How to Use OpenTelemetry](https://yonatankarp.com/blog/observability-in-action-opentelemetry/): A hands-on guide to adding distributed tracing and observability to a Kotlin/Spring Boot service using OpenTelemetry and HoneyComb.
- [[Interview Questions] Concurrency vs. Parallelism: Decoding the Simultaneous Illusion](https://yonatankarp.com/blog/interview-questions-concurrency-parallelism/): A clear explanation of the difference between concurrency and parallelism, illustrated with practical JVM-based examples commonly asked about in technical interviews.
- [Real-Life Problem Solving: Lessons from My Engineering Principles](https://yonatankarp.com/blog/real-life-problem-solving-engineering-principles/): How software engineering problem-solving principles, usually applied to code, turned out to be surprisingly effective for navigating real-life challenges during a trip to Asia.
- [How To Build Your Domain Gateway With OpenAPI](https://yonatankarp.com/blog/build-domain-gateway-openapi/): An introduction to the domain gateway pattern and a practical tutorial for building one in Kotlin using OpenAPI code generation.
- [How to Use OpenAPI to Build a Robust RESTful API](https://yonatankarp.com/blog/openapi-robust-restful-api/): How to adopt a spec-first approach with OpenAPI to design and generate a robust, well-typed REST API using Spring Boot, Kotlin, and Gradle.
- [Create a Custom Spring Boot App Banner](https://yonatankarp.com/blog/custom-spring-boot-banner/): A fun walkthrough of how to replace the default Spring Boot startup banner with custom text and ASCII art to give your application a personal touch.
- [How does HashMap work in Java?](https://yonatankarp.com/blog/how-hashmap-works-java/): A deep dive into the internal mechanics of Java's HashMap, covering the equals/hashCode contract, bucket distribution, and the binary tree optimization introduced in Java 8.
- [How to Break Your HashMap in Less Than a Minute](https://yonatankarp.com/blog/break-hashmap-less-than-a-minute/): A cautionary demonstration of how using mutable objects as HashMap keys silently corrupts lookups, and the simple rule that prevents it.
- [How to inject multiple implementations in the Spring Framework](https://yonatankarp.com/blog/inject-multiple-implementations-spring/): An exploration of the different strategies Spring Framework offers for injecting multiple implementations of the same interface into a single bean.
- [4 Reasons Why Amazon Keyspaces is Problematic!](https://yonatankarp.com/blog/amazon-keyspaces-problematic/): Four concrete production problems the author encountered with Amazon Keyspaces (managed Cassandra), and why the service may not be ready for every workload.
- [Build your own Twitter Bot with Kotlin](https://yonatankarp.com/blog/build-twitter-bot-kotlin/): A beginner-friendly tutorial for building and deploying a fully functional Twitter bot from scratch using Kotlin and the Twitter4J library.