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Scala Kafka Cursor Rules

Cursor rules for Scala Kafka.

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Save this as .cursor/rules/scala-kafka-cursorrules-prompt-file.mdc in your project, commit it, and Cursor will load it. Review it first: community files can be opinionated or out of date.

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---
description: "Cursor rules for Scala Kafka."
globs: **/*
alwaysApply: false
---
# general-scala-clean-code.mdc

- Declare vals/vars as close as possible to first use.
- Name length should be proportional to scope: 1-2 chars allowed only inside small lambdas.
- Avoid nested for-comprehensions deeper than two levels; factor out steps into helpers.
- Split a single source file by responsibility.
- Use *tail-rec* optimisation (`@tailrec`) where appropriate.
- Prefer *immutable* collections and avoid mutation during iteration.
- When interop with Java forces mutability, wrap it in a pure facade with the use of .asScala, to retain functional API.
- When something is nullable, wrap it into an Option, to retain functional API.
- Keep cyclomatic complexity below 10 for any method; IDE inspections should warn.

# general-scala-development-practices.mdc

# ========== GENERAL PRINCIPLES ==========
- You are an experienced Senior Scala Developer.
- You always adhere to SOLID, DRY, KISS and YAGNI principles.
- Prefer *pure* functions; minimise side-effects. Where effects are required, make them explicit (e.g. using scala.util.Try, Either, or cats-effect IO/Task if adopted).
- Use *val* over *var*; collections must be immutable unless mutability is proven cheaper & safe.
- Replace *null* with Option, Either or a domain-specific ADT.
- Use pattern matching exhaustively and handle the *default* case only when truly open-ended.
- Prefer for-comprehensions, map/flatMap/fold, and higher-order functions over imperative loops.
- Prefer *case classes* and *sealed traits* for algebraic data types.
- Extract common logic into private or package-private helpers; avoid long methods (> 30 LOC).
- Prefer extension methods or type classes over inheritance when adding behaviour.
- Keep public APIs small, surface only what the module owns.
- Break every task into the smallest composable pure functions before wiring them together.

# ========== NAMING & SYNTAX ==========
- Class / object / trait names are UpperCamelCase nouns (e.g. *NotificationStreamApp*).
- Methods & vals are lowerCamelCase verbs or nouns (e.g. *process*, *serde*, *productKey*).
- Constants use `SCREAMING_SNAKE_CASE`.
- Similar to Java’s static final members, if the member is final, immutable and it belongs to a package object or an object, it may be considered a constant.
- Symbolic names (`|>`) are allowed *only* when they align with widespread FP idioms.
- Match expressions use `match`/`case` over nested if/else chains; for simple two-branch logic prefer `if … then … else …` expressions.

# ========== ERROR HANDLING & LOGGING ==========
- Catch the most specific Exception first; convert checked Java exceptions to an ADT or `Try`.
- No empty `catch` blocks; log at *debug* or *error* level with a meaningful message.
- Leverage `scala.util.Using` (or cats-effect `Resource`) for auto-closing resources.
- Avoid “defensive” logging or `println`; use SLF4J (Logback) with the *scala-logging* wrapper.

# ========== TESTING ==========
- Use ScalaTest in a **Given-When-Then** layout with the use of AnyFlatSpec.
- Focus on critical paths and business invariants; do not over-test boilerplate.
- Property-based tests (ScalaCheck) for pure functions with non-trivial invariants.
- Set up integration tests as a subproject named “integration” and treat integration tests as standard tests

# ========== PERFORMANCE & SAFETY ==========
- Avoid blocking calls inside Kafka stream processing; if unavoidable, off-load to a dedicated thread-pool.
- Convert Java collections to Scala equivalents once at the boundary; never bounce back and forth.
- Use underscore-separated digits for large numeric literals (e.g. `val timeoutMs = 30_000`).

# ========== MODERN SCALA 3 FEATURES ==========
- Use *Enums* for finite alternatives instead of Java-style enums.
- Embrace *opaque types* to avoid accidental misuse of primitive wrappers.
- Use *context parameters* (`using`) for type-class evidence instead of classic implicit lists when convenient.
- Prefer `given`/`using` syntax over `implicit` where supported.

# ========== CLEAN BUILD ==========
- The sbt build uses **scalafmt** for formatting; treat any scalafmt violation as a build error.

# kafka-development-practices.mdc

- All topic names config values (Typesafe Config or pure-config).
- Use Format or Codec from the JSON or AVRO or another library that is being used in the project.
- Streams logic must be tested with `TopologyTestDriver` (unit-test) plus an integration test against local Kafka.

# linting-formatting.mdc

- **scalafmt:** Enforce Google-inspired scalafmt configuration with `maxColumn = 100`.