feat(wirings): Kafka Streams DSL tier wirings + capstone demo (stacks on #3; Kafka mirror of Flink #6→#10)

[estebanzimanyi]

Kafka-side mirror of the MobilityFlink wirings stack (PRs #6→#10). Single PR with all four tier wirings + runnable composite demo, since Kafka Streams' DSL is naturally lambda-driven and most tier wirings collapse to small static-factory classes returning serializable functional-interface implementations.

Stacks on PR #3 (the Kafka codegen mirror). Additive-only — no existing file touched.

What's in this PR (6 new files under kafka-streams-app/src/main/java/org/mobilitydb/kafka/meos/wirings/)

Tier	Wiring class	Pattern
stateless (804 methods)	MeosStatelessOps	static factory: predicate(...) / intPredicate(...) / mapper(...) → Predicate<K,V> / ValueMapper<V,R> for KStream.filter / .mapValues
bounded-state (797 methods)	MeosBoundedStateProcessor	full Processor<KIn,VIn,KOut,VOut> class with KeyValueStore<KIn,byte[]> for per-key MEOS-handle state that survives changelog replay / rebalance
windowed (161 methods)	MeosWindowedAggregator	static factory: initializer(...) / aggregator(...) for KStream.groupByKey().windowedBy(...).aggregate(...)
cross-stream (140 methods)	MeosCrossStreamJoiner	static factory: joiner(...) wrapping serializable ValueJoiner for KStream.join(other, joiner, JoinWindows), same-key pairing, time-bounded match window
io-meta (195 methods)	covered by MeosStatelessOps.mapper(...)	(no state, no window)
sequence-only (14 methods)	inherently non-streamable, no wiring	—

Plus:

• MeosOpsRuntime.java — wirings-package alias for the codegen package's MEOS_AVAILABLE flag
• demo/MeosWiringsDemoTopology.java — runnable Kafka Streams topology composing all 4 tier wirings; uses TopologyTestDriver (no broker required)
• README.md — full tier vocabulary, lambda-first design rationale, demo walkthrough, coexistence with berlinmod.MEOSBridge

Cumulative wirings coverage: 2,097 of 2,097 emitted methods (100%) wirable through 5 generic classes; zero per-method registration. Identical to Flink side.

Design choice — lambda-first

Kafka Streams' DSL accepts lambdas directly (Predicate, ValueMapper, Aggregator, ValueJoiner). Only bounded-state needs a real class for state-store binding via Processor.init(ProcessorContext). The wirings reflect that asymmetry — small static-factory classes for the four lambda-shaped tiers, one full Processor class for bounded-state.

Adopters wanting a class-shaped wiring (matching Flink for cross-binding parity) can subclass any of the helpers — the serializable functional interfaces are public.

State-store discipline (bounded-state)

Same as the Flink wirings: raw jnr.ffi.Pointer does not survive Kafka Streams' changelog-replay / rebalance / state-rebuild paths (the state-store changelog is a Kafka topic; state must be byte-serializable to be replayable). MeosBoundedStateProcessor stores state as byte[] (typically MEOS-WKB or MEOS-WKT) with three adopter-supplied lambdas mediating the round-trip:

byte[] state               -- per-key serialized MEOS value (in changelog-backed KeyValueStore)
    ↓ deserialize (bytes → Pointer)
Pointer prev               -- in-flight MEOS handle
    ↓ step(prev, record) → (newPointer, output record)
Pointer next, Record       -- new in-flight handle + optional forward
    ↓ serialize (Pointer → bytes)
byte[] newState            -- back to KeyValueStore

First record for a key sees prior == null (no prior state); wiring skips deserialize and lets step seed.

End-to-end demo

MeosWiringsDemoTopology composes all four tier wirings into one Kafka Streams topology:

1. vehicle-events source → 2. stateless filter → 3. bounded-state processor (per-region running tbox union) → 4. windowed aggregator (30s tumbling) → 5. cross-stream joiner (±1m bound, joined against region-queries topic) → 6. overlap-output sink.

Run via mvn exec:java. main() always prints Topology.describe(); when MEOS available, instantiates TopologyTestDriver (no broker required).

Compile verification

Locally green: 110 .class files total (94 from PR #3 base + 16 new — 4 wiring classes + 11 nested lambda interfaces + MeosOpsRuntime + 1 demo class).

Stacking

Stacks on codegen/kafka-meos-ops (PR #3). Whole 11-file diff is contained under kafka-streams-app/src/main/java/org/mobilitydb/kafka/meos/wirings/.

[estebanzimanyi]

Consolidated into #13 (consolidate/kafka-streaming-implementation).

[estebanzimanyi]

Superseded by the Path-B consolidation: the former 12-deep stack is collapsed into three reviewable topical PRs — scaffold #1 → MEOS integration #14 → benchmark #15 — each one clean squashed commit with the generated-facade bulk, dead family-flag profiles, and invented synthetic corpus removed. Closing as folded into #14/#15.