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.