Field Notes
Notes on event-driven Java, Spring Boot, Kafka, and secure systems for regulated banking.
I grew a resilience audit from a single agent into a scatter-gather graph with typed edges, an adversarial verification gate and injected failures. Recall barely moved; trust did.
Aircraft maintenance manuals solved the problem of readers who can't ask what you meant. I took ASD-STE100's rules and applied them to tool descriptions, error messages and prompts for AI agents.
A walkthrough of harness-stats, a Claude Code mod that shows cost, context, cache and every model request live. It covers the ($, e, next) hook chain, turn.step streaming, $.state and testing.
Cats Core · Part 6
What the Nec in ValidatedNec guarantees, why errors.toList failed to compile in my Play controller, and when NonEmptyList is the better container.
Cats Core · Part 5
A price and a delivery cost are both optional, and nested map gives me Some(Some(15)). Apply combines independent values into one result, and Applicative adds the starting value a whole cart needs.
Scala and the JVM · Part 18
I replaced this site's Spring Boot backend with Scala 2.13, Play 3, Anorm and Twirl in one pull request. The rewrite was the easy part: this is how I fixed the contracts, kept the outbox wire format, and rehearsed the rollback on a restored production backup.
Security and privacy, in practice
For a month my Keycloak admin console was one HTTPS request from anyone. How I closed it at the Traefik edge, opened a loopback-only door over SSH, and the details that decide whether the tunnel works or locks you out.
Cats Core · Part 4
List and Option already know how to map. I give my own Quote a Functor instance, reuse the same discount function, and trace why identity and composition matter.
I tried the Visualizations preview with a dinner request, then changed the guest count, dietary split, and budget to see what really updated. The result was useful precisely because its assumptions were visible.
Cats Core · Part 3
I knew how to combine two error reports. Monoid gives an empty batch a useful result, and foldMap takes the same idea to a shopping cart: turn each line into a contribution, then combine the totals.
The multi-part series and running themes, rather than one long pile.
Learning Typelevel Cats Core in Scala 2 through runnable examples: combining optional values, collecting validation errors, and understanding the type classes behind practical operations.
Learning Scala 2 and Play from a Java and Spring background, step by step: language fundamentals, sbt workflows, domain modelling, asynchronous services, with runnable examples and practical comparisons.
Working through large language models from the ground up — turning text into tokens, embeddings and the geometry of meaning, the transformer architecture, attention, how these models are trained, and what separates one generation from the next.
Learning Embabel by building with it, in order — planning, durable sessions, testing without a model, tools, human-in-the-loop, tracing, exposing it over MCP.
Build it, attack it, keep extending it — the Ask Zakaria concierge from the first prompt through prompt-injection red-teaming to reading a job description and taking questions by voice.
Instrument it, learn to query it, then point it at something that thinks.
An agent as a collaborator rather than an autocomplete — design systems, durable knowledge, automation ladders, subagent fleets on my own source, and now in CI.
Using Codex and ChatGPT as engineering collaborators — repository workflows, security reviews, automation, and the practical boundaries that keep the work controlled.
Attacking my own features and reading the protocols underneath them — secrets delivery, response headers, a published address for bad news, a cookie banner that sets no cookies, and what an auth flow is actually protecting you from.
Shipping and operating my own product in public — one jar serving two hosts, making it findable, dropping experiments into it, shipping a free CV builder off the side.
The design and architecture patterns every backend engineer should have at hand — each built as a small, runnable showcase you can watch work, with the trade-offs and the framework gotchas spelled out rather than hand-waved.