o612 — notes on distributed systems, Java, Kotlin and other JVM based technologies

git-pro — an Electron GUI that shares its state with a bash CLI

Thu, 01 Oct 2026 00:00:00 GMT

How git-pro, a small Electron app, manages multiple git identities by sharing one on-disk file format with a bash CLI — no database, no daemon.

Building a lightweight method tracer with Byte Buddy — from runtime subclassing to a Java agent

Sun, 27 Sep 2026 00:00:00 GMT

Trace Java methods without touching their source, using Byte Buddy — runtime subclasses, method delegation, inlined advice, and a premain Java agent, all verified.

Part 10: Turning the DSL into a maintainable workflow authoring library

Sun, 27 Sep 2026 00:00:00 GMT

Package the pipeline as a library — module boundaries, DSL versioning, extension seams, and the honest list of what was deliberately left out.

Part 9: Deploying the generated buyer process to Flowable

Fri, 25 Sep 2026 00:00:00 GMT

Keep the core model engine-neutral — map DSL metadata to flowable:* via a serializer seam, and deploy only the executable buyer pool to Flowable.

Part 8: Testing BPMN semantics, DI, and visual output

Thu, 24 Sep 2026 00:00:00 GMT

Assert the real contract — every node shaped, every edge resolved, containment, determinism — then prove bpmn-js renders it without warnings.

Part 7: Generating BPMN diagrams that actually render

Tue, 22 Sep 2026 00:00:00 GMT

Build a deterministic layered layout engine — flow-graph columns, lane rows, pinned boundary events, orthogonal routing — and emit BPMN DI XML.

Part 6: Generating BPMN 2.0 XML safely from the AST

Sun, 20 Sep 2026 00:00:00 GMT

Serialize the validated AST to BPMN 2.0 XML with StAX — namespaces, event definitions, condition expressions, and the engine-extension seam.

Part 5: Validation as a first-class feature

Sat, 19 Sep 2026 00:00:00 GMT

Build BpmnValidator — structured diagnostics for identifiers, flow semantics, gateways, lanes, collaboration, boundary events, and diagrams.

Part 4: Modeling the procurement collaboration end to end

Thu, 17 Sep 2026 00:00:00 GMT

The full running example — two pools, six lanes, inclusive-gateway conditional approvals, SLA timers, renegotiation, and an exception subprocess.

Part 3: Designing a type-safe Kotlin DSL for BPMN

Tue, 15 Sep 2026 00:00:00 GMT

Turn the AST into a fluent internal DSL — lambdas with receivers, @DslMarker scope control, and builders that compile into immutable definitions.

Part 2: Modeling BPMN collaborations, pools, lanes, and flow nodes

Mon, 14 Sep 2026 00:00:00 GMT

Define the immutable BPMN AST — collaborations vs processes, participants vs pools, lanes as references, and a sealed flow-node type hierarchy.

Part 1: Why build a Kotlin DSL for BPMN 2.0?

Sat, 12 Sep 2026 00:00:00 GMT

Why handwritten BPMN XML and visual modelers fall short for generated workflows — and the AST-first architecture this series builds around them.

Production hardening and the final exercise: prove it, then ship it

Thu, 10 Sep 2026 00:00:00 GMT

The final architecture review, threat-model update, failure drills, secret rotation, upgrade strategy, and the incident scenario that exercises every mechanism at once — v1.0.0.

Containerize and deploy: Compose for real, Kubernetes without lies

Wed, 09 Sep 2026 00:00:00 GMT

Production-shaped container images, the full Compose stack, Kustomize overlays, probes, NetworkPolicies, resource limits, and rollout guidance that matches what we actually built.

Test performance and resilience: measuring the framework, not the model

Mon, 07 Sep 2026 00:00:00 GMT

Gatling scenarios against the stub model isolate application overhead; controlled fault modes separate framework latency from downstream sickness; a results template keeps numbers honest.

Build the evaluation suite: testing what the model actually does

Sat, 05 Sep 2026 00:00:00 GMT

Versioned eval datasets, deterministic graders for citations and tool choice, pinned LLM-as-judge where needed, and a CI gate that separates hard failures from probabilistic drift.

End-to-end observability: one trace from HTTP to tool call

Thu, 03 Sep 2026 00:00:00 GMT

OpenTelemetry across agent, MCP server, and simulator; Prometheus metrics with low-cardinality labels; Loki logs; Tempo traces; Grafana dashboards — with redaction built in.

Add guardrails and bounded orchestration: budgets, timeouts, and degraded modes

Wed, 02 Sep 2026 00:00:00 GMT

Prompt size limits, retry classification, deadline propagation, bulkheads, and graceful degradation — the layer that keeps a sick dependency from becoming an incident.

Add write tools and human approval: the gate the model cannot cross

Mon, 31 Aug 2026 00:00:00 GMT

Arm create_incident and append_incident_note behind scope checks, idempotency keys, and a single-use approval token bound to user, tenant, and a hash of the exact arguments.

Secure both boundaries: Keycloak, JWT validation, and tenant authorization

Sat, 29 Aug 2026 00:00:00 GMT

Replace trusted headers with real OIDC: Keycloak realm import, issuer/audience/scope validation on agent-api and the MCP server, tenant claims, and the negative security matrix.

Connect the agent to remote MCP tools: policy registry and bounded loop

Fri, 28 Aug 2026 00:00:00 GMT

Configure the Spring AI MCP client, wrap tool execution in a per-tool policy registry, inject tenant context the model cannot choose, and bound the agent loop.

Build the Kotlin MCP operations server: tools as a secured boundary

Wed, 26 Aug 2026 00:00:00 GMT

Implement the mcp-operations-server in Kotlin: Streamable HTTP transport, @McpTool read tools, a typed simulator client, structured errors, and contract tests.

Add typed, self-correcting outputs: the IncidentAssessment schema

Mon, 24 Aug 2026 00:00:00 GMT

Turn model prose into a validated IncidentAssessment record: JSON schema generation, provider-native output where available, bounded retries, and semantic checks beyond syntax.

Add access-controlled RAG and citations

Sun, 23 Aug 2026 00:00:00 GMT

Tenant-filtered pgvector retrieval, delimited context construction, deterministic citation mapping, and a real abstention path when the evidence is not there.

Build the runbook ingestion pipeline: Markdown to pgvector

Fri, 21 Aug 2026 00:00:00 GMT

Parse versioned Markdown runbooks into semantic chunks, embed them with Ollama, store them in pgvector with tenant metadata, and re-ingest only what changed.

Create the agent API: ChatClient, provider profiles, and a deterministic stub model

Wed, 19 Aug 2026 00:00:00 GMT

Stand up agent-api with Spring AI ChatClient, Ollama and hosted-provider profiles, REST and SSE endpoints, and a stub ChatModel that keeps CI fully offline.

Build the deterministic operations simulator

Tue, 18 Aug 2026 00:00:00 GMT

Implement the fictional ops backend: Flyway migrations, JdbcClient persistence, idempotent incident creation, and controllable latency, flaky, malformed, and outage fault modes.

Scaffold the multi-module build: Gradle 9, convention plugins, and architecture tests

Sun, 16 Aug 2026 00:00:00 GMT

Build the ops-agent-platform skeleton: Gradle wrapper, version catalog, four convention plugins, Java 25 toolchain, and ArchUnit rules that run from day one.

What we are building: an enterprise operations agent

Fri, 14 Aug 2026 00:00:00 GMT

The architecture, scope, and threat model for a JVM AI agent that answers runbook questions, calls secured MCP tools, and never writes without approval.

Processing millions of records in Java: chunking, streams, and memory-efficient pipelines

Thu, 13 Aug 2026 00:00:00 GMT

Process huge datasets in Java without OOM — combine keyset-paginated batches or true database streams with Stream pipelines, lambdas, and checkpoints.

Analyzing Java heap dumps in production — a practical guide to memory leaks and retained objects

Tue, 11 Aug 2026 00:00:00 GMT

Capture and analyze Java heap dumps from Docker and Kubernetes workloads with a repeatable Eclipse MAT workflow that finds the object retaining the memory.

The production checklist

Sun, 09 Aug 2026 00:00:00 GMT

The full measurement-to-optimization workflow condensed, the mistakes that invalidate results, and a capstone exercise that walks the whole loop on a bottleneck you induce yourself.

Performance tests in CI and Kubernetes

Sat, 08 Aug 2026 00:00:00 GMT

A GitHub Actions regression job, which tests belong at which pipeline stage, Kubernetes resource settings for honest benchmarks, and why CPU throttling lies about latency.

Stress, spike, and soak testing

Thu, 06 Aug 2026 00:00:00 GMT

Three boundary-finding test profiles — stepped stress, sudden spike, multi-hour soak — plus what healthy overload behavior looks like and how to find the breaking point safely.

Evidence-based optimizations

Tue, 04 Aug 2026 00:00:00 GMT

Five optimizations done the way they should be done — hypothesis first, one change, measured re-test — covering indexes, N+1, projections, HikariCP sizing, thread pools, and caching.

Diagnosing common Spring Boot bottlenecks

Mon, 03 Aug 2026 00:00:00 GMT

The recurring bottleneck shapes — database, application, JVM runtime, infrastructure — each with its metric signature and the cheapest experiment that proves or kills the hypothesis.

Reading the results

Sat, 01 Aug 2026 00:00:00 GMT

How to read a Gatling report, the PromQL queries that explain what it shows, and a symptom-to-bottleneck table that turns a latency graph into a hypothesis.

Running a load test correctly

Thu, 30 Jul 2026 00:00:00 GMT

Warm-up, steady state, and cool-down; the one-variable-at-a-time experiment loop; coordinated omission; and how to tell client-side latency from server-side latency.

Gatling simulations

Wed, 29 Jul 2026 00:00:00 GMT

Four complete Gatling simulations — read-heavy, mixed write, paginated search, and a stepped stress test — plus feeders, open vs closed models, and assertions that fail the run.

Realistic test data

Mon, 27 Jul 2026 00:00:00 GMT

Generate a million orders with a deterministic SQL script, extract feeder files for Gatling, and pick a data-reset strategy that keeps test runs isolated.

The Order Management API

Sat, 25 Jul 2026 00:00:00 GMT

Build the baseline Spring Data JPA service — schema, indexes, entities, repository, service, controller — designed to be measured, not to be perfect.

Instrument before you measure

Fri, 24 Jul 2026 00:00:00 GMT

Wire Actuator and Micrometer into the order API, expose Prometheus metrics, and learn which metric names answer which diagnosis questions — before the first Gatling run.

A reproducible test environment

Wed, 22 Jul 2026 00:00:00 GMT

Stand up PostgreSQL, Prometheus, and Grafana with Docker Compose, and pin down the variables — CPU, memory, data volume, warm-up — that make two test runs comparable.

A vocabulary for measuring

Mon, 20 Jul 2026 00:00:00 GMT

Seven kinds of performance test, what each one can and cannot tell you, and the SLI/SLO vocabulary that turns "is it fast?" into a question you can answer.

Workflow orchestration with BPMN versus distributed sagas

Sat, 18 Jul 2026 00:00:00 GMT

Northwind's returns-and-refunds process needs a human approval step and a two-week waiting period — deciding whether that still fits the saga orchestrator from Chapter 11, or needs a BPMN engine instead.

Multi-tenancy patterns

Fri, 17 Jul 2026 00:00:00 GMT

Northwind Platform's white-label expansion: what happens to database-per-service, the JWT claims from Chapter 26, and every downstream call once 'tenant' becomes a first-class concept.

Security patterns: OAuth2/OIDC, JWT validation, RBAC/ABAC, and token propagation

Wed, 15 Jul 2026 00:00:00 GMT

Implementing the JWT validation this series deferred since Chapter 4, plus deciding what happens to a customer's identity as their request crosses order-service, inventory-service, and the legacy ambassador.

Kubernetes-native microservice design

Mon, 13 Jul 2026 00:00:00 GMT

Pulling together every Kubernetes primitive this series has used piecemeal since Chapter 1 into one coherent picture of what makes a service actually Kubernetes-native, not just deployed on it.

Blue-green, canary, and feature-flag-based deployment

Sun, 12 Jul 2026 00:00:00 GMT

Rolling out inventory-service's rewritten reservation algorithm to 5% of flash-sale traffic first, with a fast, one-command rollback if it misbehaves.

Contract testing and consumer-driven contracts

Fri, 10 Jul 2026 00:00:00 GMT

Replacing order-service's hand-written WireMock stub of inventory-service with a Pact contract that inventory-service's own CI verifies against its real implementation.

Observability: logs, metrics, traces, and correlation IDs

Wed, 08 Jul 2026 00:00:00 GMT

Wiring OpenTelemetry through Northwind's gateway, BFF, and services so a slow checkout can actually be traced to its cause, instead of guessed at from six separate log streams.

Service mesh and zero-trust service communication

Tue, 07 Jul 2026 00:00:00 GMT

When Northwind's six hand-configured Envoy sidecars from Chapter 15 finally justify replacing manual configuration with a mesh control plane.

API composition and aggregator services

Sun, 05 Jul 2026 00:00:00 GMT

Formalizing the fan-out-and-merge technique mobile-bff used back in Chapter 4, with a principled partial-failure policy instead of failing the whole request on one slow dependency.

Dead-letter queues and retry topics

Fri, 03 Jul 2026 00:00:00 GMT

What notification-service should do with a confirmation email that keeps failing to send — without blocking every order behind it or retrying forever.

Competing consumers and work queues

Thu, 02 Jul 2026 00:00:00 GMT

Scaling notification-service's email sending horizontally by having multiple instances compete for work from one Kafka consumer group, instead of each getting its own copy.

Cache-aside and distributed caching

Tue, 30 Jun 2026 00:00:00 GMT

Caching inventory-service's hottest stock lookups in Redis without ever serving a customer a phantom sold-out or in-stock result during a flash sale.

Circuit breaker, retry, timeout, fallback, and bulkhead

Sun, 28 Jun 2026 00:00:00 GMT

Using Resilience4j to stop a slow inventory-service from taking order-service's whole thread pool down with it, the exact failure this series predicted back in Chapter 7.

Sidecar and ambassador patterns

Sat, 27 Jun 2026 00:00:00 GMT

Pulling TLS, mTLS, and the legacy-monolith proxy logic out of order-service's own code and into deployment-level companion containers instead.

Anti-corruption layer

Thu, 25 Jun 2026 00:00:00 GMT

Formalizing the translation boundary between order-service's clean domain model and the legacy monolith's tangled customer schema, so the legacy model never leaks in.

Strangler Fig migration pattern

Tue, 23 Jun 2026 00:00:00 GMT

Using the API gateway from Chapter 4 to incrementally route traffic away from Northwind's legacy PHP monolith, one capability at a time, without a big-bang cutover.

Transactional outbox and inbox / idempotent consumer

Mon, 22 Jun 2026 00:00:00 GMT

Closing the gap between committing a local transaction and actually publishing its event, and the consumer-side discipline that makes redelivery safe.

Saga: choreography and orchestration

Sat, 20 Jun 2026 00:00:00 GMT

Closing the stock-reservation/payment consistency gap this series has carried since Chapter 7, comparing an event-driven choreographed saga against an explicit orchestrator.

CQRS: command query responsibility segregation

Thu, 18 Jun 2026 00:00:00 GMT

Splitting order-service's write model from its read model so a slow reporting query stops competing with checkout traffic for the same tables.

Event sourcing

Wed, 17 Jun 2026 00:00:00 GMT

Storing an order's history as an append-only event log instead of a mutable row, and why Northwind adopts it for one service, not the whole platform.

Publish-subscribe and event notification

Mon, 15 Jun 2026 00:00:00 GMT

The difference between a thin event notification and a fat event-carried payload, and how that one design choice determines every consumer's coupling to the publisher's schema.

Event-driven architecture

Sat, 13 Jun 2026 00:00:00 GMT

Replacing Northwind's synchronous order-placement chain with Kafka events, and why that trades immediate consistency for availability and looser coupling.

Synchronous communication: REST and gRPC

Fri, 12 Jun 2026 00:00:00 GMT

When to keep REST for service-to-service calls and when gRPC's typed contracts and binary framing actually earn their added build-tooling complexity.

Service discovery and centralized configuration

Wed, 10 Jun 2026 00:00:00 GMT

Why hardcoded service URLs and per-service config files stop working as Northwind scales past a handful of instances, and how discovery and config servers solve it.

API gateway and backend for frontend

Mon, 08 Jun 2026 00:00:00 GMT

How to stop every client from calling three services directly, using a shared API gateway for cross-cutting concerns and a backend-for-frontend where clients genuinely differ.

Database per service

Sun, 07 Jun 2026 00:00:00 GMT

Why splitting deployables without splitting databases only gets you halfway, and how to move from schema-per-service on one Postgres instance to a real database per service.

Microservices decomposition by business capability

Fri, 05 Jun 2026 00:00:00 GMT

How to split a modular monolith into real services along business-capability boundaries instead of technical layers, and why bounded contexts beat entity-based splits.

Modular monolith as a starting architecture

Wed, 03 Jun 2026 00:00:00 GMT

Why most teams should start microservices projects as a modular monolith, and how to enforce module boundaries in Kotlin and Gradle so the split later is a refactor, not a rewrite.

From lab to production: rollout phases and checklists

Mon, 01 Jun 2026 00:00:00 GMT

A five-phase rollout with exit criteria, a relevance gate with _rank_eval, an automated readiness check, the questions to answer first, and what to build next.

Failure modes, and the tests that catch them

Sun, 31 May 2026 00:00:00 GMT

Every failure mode found while building this series, from resurrected profiles to leaking facets, and the unit and Testcontainers tests that catch them.

Operating it safely: monitoring, snapshots, security, and personal data

Fri, 29 May 2026 00:00:00 GMT

Monitoring signals, snapshots with retention, least-privilege API keys, and personal-data controls, including a facet leak that bypassed authorisation.

Performance optimisation, measured

Wed, 27 May 2026 00:00:00 GMT

Refresh, caches, index sorting, the profile API, and the slow log, each measured on the lab, including one standard tip that made indexing slower here.

Backfill, the outbox relay, and zero-downtime reindexing

Tue, 26 May 2026 00:00:00 GMT

A Kotlin indexer with keyset backfill, BulkIngester backpressure, per-item errors, an outbox relay, and a v1 to v2 reindex ending in an atomic alias swap.

Pagination for large result sets

Sun, 24 May 2026 00:00:00 GMT

Why deep from and size fails, search_after cursors, a point in time for stable pages, where scroll still fits, and an opaque cursor contract for the Search API.

Aggregations and facet counts

Fri, 22 May 2026 00:00:00 GMT

Terms, cardinality, composite, and date histogram aggregations checked against PostgreSQL, why top-N goes wrong across shards, and disjunctive facets in Kotlin.

Query design for profile search

Thu, 21 May 2026 00:00:00 GMT

Query and filter context, bool clauses, cross_fields and fuzzy name matching, highlighting, and the "active Prashants in Bihar" query in JSON and Kotlin.

Building the Spring Boot search service

Tue, 19 May 2026 00:00:00 GMT

A Kotlin Spring Boot 4 Search API over TLS: index bootstrap from reviewed JSON, request and response DTOs, exact lookups, and clean failure handling.

Choosing an Elasticsearch client for Kotlin and Spring Boot

Sun, 17 May 2026 00:00:00 GMT

The Java API Client, its async and low-level variants, and Spring Data Elasticsearch, run from Kotlin, with the JSON-mapper and 10,000-total traps explained.

Shards, settings, and capacity planning for 300 million profiles

Sat, 16 May 2026 00:00:00 GMT

Primaries, replicas, refresh, indexing buffer, and translog explained, then a shard-sizing worksheet built from lab measurements and a Rally track you can run.

Autocomplete, typos, synonyms, and name relevance

Thu, 14 May 2026 00:00:00 GMT

edge_ngram, search_as_you_type, and completion compared on a million names, plus fuzziness, live synonym updates, a request-cache trap, and BM25 via _explain.

Mapping and analysis in depth

Tue, 12 May 2026 00:00:00 GMT

How text and keyword fields, analysers, filters, and normalisers decide what matches, shown with _analyze, with fielddata and mapping explosion reproduced.

Designing the user-profile index

Mon, 11 May 2026 00:00:00 GMT

A versioned user-profile index behind read and write aliases, a field capability matrix, the full creation request, and a million profiles loaded and checked.

Search architecture and keeping Elasticsearch in sync

Sat, 09 May 2026 00:00:00 GMT

Why dual writes to PostgreSQL and Elasticsearch fail, how outbox and CDC compare, and an idempotent sync design with row versions, re-reads, and safe deletes.

The REST API surface, from documents to the cluster

Thu, 07 May 2026 00:00:00 GMT

Document, bulk, by-query, task, and inspection APIs in a live lab, with optimistic locking, external versions, and two version traps a sync pipeline must avoid.

Elasticsearch for relational developers

Wed, 06 May 2026 00:00:00 GMT

Clusters, shards, replicas, segments, refresh, and mappings, each shown in a live lab and compared with PostgreSQL, including where the table analogy breaks.

The search problem at 300 million profiles

Mon, 04 May 2026 00:00:00 GMT

Where PostgreSQL indexes stop fitting flexible user search at 300 million profiles, and where they are still enough, shown with EXPLAIN on a local lab.

JVM internals, performance tuning, and diagnostics in Java 21

Sat, 02 May 2026 00:00:00 GMT

How the JVM actually runs Java 21 — bytecode, class loading, JIT, GC — plus an evidence-first method for tuning and diagnosing production incidents.

Capstone: an auditable scheme decision pipeline

Fri, 01 May 2026 00:00:00 GMT

Wire Option, Result, Validated, Reader, State, and IO into one eligibility flow — pure policy core, typed failures, deferred effects, complete audit trail.

Java interoperability — when the JDK is already enough

Wed, 29 Apr 2026 00:00:00 GMT

Converters between jfp types and Optional, Stream, and CompletableFuture, plus the honest table of when to reach for the library versus plain Java.

Async boundaries

Mon, 27 Apr 2026 00:00:00 GMT

Interop with CompletableFuture and virtual threads instead of building a runtime — independent registry lookups run in parallel, dependent ones stay sequential.

State: transitions without mutable globals

Sun, 26 Apr 2026 00:00:00 GMT

State is a function S → (A, S) — a value plus a new state. Model application status transitions as composable steps instead of a mutable workflow field.

Reader: explicit environments

Fri, 24 Apr 2026 00:00:00 GMT

A Reader is a function waiting for its config — it threads policy versions, thresholds, and clocks through computations without globals.

IO: effects at the boundary

Wed, 22 Apr 2026 00:00:00 GMT

An educational IO that defers side effects until unsafeRun — the policy core stays pure while audit writes and event publication compose as values.

Lazy evaluation and memoization

Tue, 21 Apr 2026 00:00:00 GMT

Wrap expensive evidence lookups in Lazy so they run at most once and only when a rule actually needs them — call-by-need semantics in 30 lines.

Folds, traversals, and sequencing

Sun, 19 Apr 2026 00:00:00 GMT

foldLeft collapses a collection, traverse turns List> into Result,E>, and Validated.sequence gathers every document error at once.

Semigroups and monoids

Fri, 17 Apr 2026 00:00:00 GMT

An associative combine plus an identity element — the two smallest algebra laws — let Validated merge errors and folds combine results without order anxiety.

Kleisli composition for effectful functions

Wed, 15 Apr 2026 00:00:00 GMT

Ordinary compose stops working when every step returns Result — Kleisli composition reconnects A→Result functions so registry chains stay flat.

The monad laws, tested

Tue, 14 Apr 2026 00:00:00 GMT

Left identity, right identity, and associativity — the three laws that make flatMap chains safe to refactor, tested for real against Option and Result.

Monads: sequencing dependent steps

Sun, 12 Apr 2026 00:00:00 GMT

flatMap exists because map cannot flatten — when each step returns a context like Result, monadic bind threads the value through and short-circuits on failure.

Applicatives and validation

Fri, 10 Apr 2026 00:00:00 GMT

When checks are independent, chain-of-flatMap is wrong — it stops at the first failure. Validated accumulates every error through map2 and a Semigroup.

Functors: mapping inside a context

Thu, 09 Apr 2026 00:00:00 GMT

map is the whole idea of a functor — transform the value without touching the context — and its two laws are what make DTO-to-domain mapping safe to chain.

Result and Either: errors as values

Tue, 07 Apr 2026 00:00:00 GMT

Model expected failures — registry timeouts, invalid identities, missing citizens — as Result so the error channel is typed, composable, and visible.

Option: absence as data

Sun, 05 Apr 2026 00:00:00 GMT

Build Option as a sealed Some|None type with map, flatMap, and fold — and use it where a missing land record is a valid state, not an error, never a null.

Algebraic data types

Sat, 04 Apr 2026 00:00:00 GMT

Model eligibility outcomes as a sealed sum type — Eligible, Ineligible, ManualReview — and let pattern matching make alternatives explicit at the boundary.

Pure functions and immutable data

Thu, 02 Apr 2026 00:00:00 GMT

What purity buys — substitution, testability, auditability — and how Java 21 records plus defensive copies give you immutable domain values without ceremony.

Functions and composition

Tue, 31 Mar 2026 00:00:00 GMT

Give Fn1 its andThen, compose, and identity, then state the two laws — identity and associativity — that let you refactor pipelines without changing behavior.

Why functional programming in Java?

Mon, 30 Mar 2026 00:00:00 GMT

Set up the jfp Gradle project and meet the problem this series solves — mutable, exception-driven eligibility code — before writing a single abstraction.

Hexagonal architecture with Spring Boot and Kotlin — a certificate service built from swappable modules

Sat, 28 Mar 2026 00:00:00 GMT

Ports and adapters in Kotlin — a Spring Boot certificate issuance engine where the core is framework-free and every adapter, and every certificate type, is a swappable module.