Why

A Reddit thread (r/apachekafka) asked whether Kafka can replace Redis for broadcasting cache updates to ~25 Spring Boot pods. I have run exactly this pattern (compacted topic + per-pod consumer group + readiness gating) in production for years and want to share the answer as a full article with a runnable example, matching the depth of the site’s existing tutorials (_articles/outboxes.md, _articles/enums.md).

What Changes

• New article _articles/local-cache-via-kafka.md (Jekyll post) covering: the Reddit question, the broadcast-via-per-pod-consumer-group pattern, consistency tradeoffs vs Redis, readiness gating before serving traffic, tombstone-based deletes, orphaned consumer groups, startup-time/batch-consume concerns, and partitioning for concurrent consumption. Includes TL;DR block, mermaid diagrams, FAQ front matter, SEO front matter (description, excerpt, tags, tech_icon) per CLAUDE.md conventions.
• New runnable code example built on top of the existing scaffold at /home/vampir/Documents/ssi/articles/kafka-backed-local-read-replica-article (separate git repo, currently a generic untouched Spring Boot template):
  • A standalone library module implementing “not ready until fully consumed” (annotation-driven tracking of last-consume-time/errors, Spring readiness integration).
  • The main app module: consumes a compacted topic into a thread-safe local cache, exposes a scheduled job and an HTTP endpoint reading from that cache, consumer group name templated with a per-pod UUID sourced from a k8s ConfigMap.
  • Kafka in KRaft mode + a Kafka UI, a topic-init step that creates the compacted topic and pre-populates it (not from the app), k8s manifests or Helm chart for minikube, and Makefile automation so a reader can go from clone to running demo with a small number of commands.
• Article code snippets are copied from the finished, working example repo (not invented), so the example is built and verified before the article is written.

Capabilities

New Capabilities

• local-cache-kafka-article: the Jekyll blog post content, front matter, diagrams, and SEO/AEO conventions for the new article.
• kafka-local-cache-example: the companion runnable Spring Boot + Kafka (KRaft) example living in the external repo — library module, app module, containerization, and minikube deployment automation used as the source of truth for the article’s code snippets.

Modified Capabilities

(none — no existing spec-level behavior changes on this site)

Impact

• Affected repo (this repo): adds one new file under _articles/; no changes to layouts, includes, or existing posts. Follows existing _articles/*.md conventions (verify exact front matter fields against outboxes.md before writing).
• Affected repo (external): /home/vampir/Documents/ssi/articles/kafka-backed-local-read-replica-article — adds a new Gradle module (library), extends the existing app module, adds Docker/k8s/Helm assets, extends the Makefile, updates the README. Has its own independent git history; changes there are committed separately from this repo’s OpenSpec-tracked change.
• No production systems affected — the example targets local minikube only; no real Redis migration performed anywhere.