Željko Kozina
Technical Architect, REST Api Designer, Security Enthusiast
Dublin, Ireland
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Željko has spent over 15 years in IT, mostly consulting for banks and insurance companies, with a focus on modernising legacy systems. Along the way, he developed deep expertise in JVM-based technologies and has become a go-to voice on topics such as API design. When he's not untangling someone else's technical debt, he's happily creating his own through a growing collection of personal projects he enthusiastically starts and occasionally finishes. Željko is passionate about sharing knowledge in a way that actually sticks, which usually means keeping things practical, a little entertaining, and free of unnecessary jargon.
Area of Expertise
Topics
GraphQL on the JVM: A Query of Options
The JVM ecosystem offers several mature GraphQL frameworks, each with distinct philosophies and trade-offs. This talk surveys the landscape, including Spring for GraphQL, Netflix DGS, Airbnb's Viaduct, and graphql-java as the underlying engine. We will compare their approach to schema definition (code-first vs schema-first), data fetching, DataLoader integration, federation support, and developer ergonomics. Through live code demos, we'll see how frameworks address common challenges such as N+1 queries, error handling, and testing, highlighting practical differences not documented. We'll also discuss how to choose the right tool for your team's stack, scale, and Spring (or non-Spring) commitments. You'll leave with a practical framework for evaluating these options on your next project.
One Schema to Rule Them All: Building a Federated Supergraph Without Losing Your Mind
As organisations scale their GraphQL adoption beyond a single team, schema federation becomes essential and complex. This talk explores the practical realities of building and operating a federated supergraph, highlighting the distinct strengths of Apollo Federation v2, Netflix DGS Federation, and GraphQL Mesh. We'll compare how each approach tackles entity ownership and boundary design, schema composition and breaking-change detection in CI, cross-subgraph authentication and authorisation patterns, and observability across a distributed graph. Drawing on real-world war stories, we'll address what documentation omits: how each solution manages schema governance across autonomous teams, handles subgraph versioning, and approaches debugging production issues spanning multiple services. You'll leave with concrete patterns for scaling GraphQL federation and a clearer understanding of where each federation approach excels, so you can avoid turning your supergraph into a distributed monolith.
JSON: The Lingua Franca of APIs (and Everything Else)
Every time you fire an API query, your beautifully crafted response comes back wrapped in JSON. But how well do you actually *know* JSON? Not "I can read it" know, I am talking about "I know exactly where the spec draws the line" know. Buckle up. We're going deep on the most successful data format the web has ever seen.
From Zero to Query: Build, Secure, and Deploy a Production-Ready GraphQL API in One Afternoon
In this hands-on workshop, you'll go from an empty project to a deployed, production-grade GraphQL API, no prior GraphQL experience required. Basic Spring Boot knowledge is desired. Starting with schema design fundamentals, we'll incrementally build a fully functional API using Spring for GraphQL (or your preferred JVM framework). Each module builds on the last: defining your schema and first resolvers, modelling relationships and solving the N+1 problem with DataLoaders, adding field-level authentication and input validation, writing integration tests with GraphQL-specific assertions, and deploying to a live environment. By the end, you'll have a running service you built yourself, a clear understanding of GraphQL's core concepts, and, just as importantly, the judgment to know when GraphQL is the right choice and when it isn't. Bring your laptop and your curiosity.
GraphQL in the Age of Agents: Why Your Next AI Client Speaks in Queries
LLM-powered agents require structured, discoverable, self-documenting APIs, and GraphQL fits perfectly with its introspectable schema and precise data fetching. This talk explores the emerging intersection of GraphQL and AI agents. We'll cover why GraphQL's type system gives agents a major advantage over REST for tool use, how to expose a GraphQL schema as an LLM tool with automatic argument construction from the schema, practical patterns for letting agents compose queries dynamically while keeping cost and complexity bounded, and real-world examples using function calling with OpenAI, Anthropic, and open-source models. We'll build a working agent live that discovers, queries, and mutates a GraphQL API, and discuss guardrails like query allow-lists, mutation scoping, and human-in-the-loop approval for destructive operations.
Bring Your REST API: A Hands-On Workshop for Adding a GraphQL Layer to What You Already Have
You don't need a greenfield project to start with GraphQL. In this workshop, you'll build a GraphQL API that wraps existing REST endpoints, the most common real-world adoption path that tutorials almost never cover. We'll work through exposing REST resources as GraphQL types without 1:1 mapping, batching and deduplicating upstream REST calls using DataLoaders, handling authentication passthrough from GraphQL to downstream services, adding fields that aggregate data from multiple REST endpoints into a single query, and implementing pagination that translates between REST offset patterns and GraphQL cursor conventions. Bring a public REST API you work with (or use the one we provide) and leave with a working GraphQL layer sitting in front of it. You'll also walk away with a decision framework for which endpoints benefit most from a GraphQL wrapper, and which ones to leave alone.
Trash Talk - Optimising Java Garbage Collection
Understanding how Java organizes memory is important for every Java professional, but this particular topic is a common knowledge gap for many software professionals. This talk will help you understand the different parts of Java memory and the various garbage collectors so you can select your preferred one.
Lets explore how memory management can help you effectively improve performance.
Netflix dropped REST, but should you? REST developers' Assessment of Spring GraphQL
GraphQL is undoubtedly one of the popular kids on the Development playground right now. Last year, Netflix announced that it was dropping REST completely in favour of GraphQL. GraphQL is a new way to build APIs, one that's easier, faster, better, and more flexible than the old RESTful API system, or at least that's the promise. In this session, we will cover the basics of Spring GraphQL, explore its strengths and weaknesses, and determine whether it is a better option for your company than a conventional REST API.
MakeIT 2026 / JCON OpenBlend Slovenia 2026 Sessionize Event Upcoming
Dublin Java User Group Upcoming
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