hgraph-kafka

C++-first Kafka services for hgraph, implementing the contract in hgraph RFC 0015. The extension uses librdkafka's C API and exposes the same service model to native C++ and Python graphs.

One path-bound, multi-interface service_impl owns the Kafka clients for a configuration. Subscriptions, publish requests, explicit commits, and events all bind to that service instance. Graph output reaches Kafka through the service's sink inputs; records, delivery reports, and events re-enter the root graph through bounded push sources. Kafka clients and worker threads are created on graph start and stopped with the graph.

The public record and configuration shapes are hgraph compound scalars. Kafka headers preserve order, duplicates, null values, and empty byte strings.

Native C++

The installed package exports hgraph::kafka:

#include <hgraph/kafka/service.h>
#include <hgraph/kafka/value_builders.h>
#include <hgraph/lib/std/operators/conversion.h>
#include <hgraph/lib/std/operators/registration.h>

using namespace hgraph;
using namespace hgraph::kafka;

struct KafkaGraph {
    static constexpr auto name = "kafka_graph";

    static void compose(Wiring &w) {
        const auto path = service::path("primary");
        register_service(
            w, path,
            service_config().bootstrap_servers({Str{"localhost:9092"}}).build());

        auto key = wire<stdlib::const_, TS<KafkaSubscriptionKey>>(
            w, subscription_key()
                   .topics({Str{"orders"}})
                   .group_id(Str{"orders-worker"})
                   .build());
        auto subscription = subscribe(w, path, key);

        auto record = wire<stdlib::const_, TS<KafkaProduceRecord>>(
            w, make_produce_record(Bytes{"ready"}));
        auto delivery = publish(
            w, path, publish_request(w, Str{"status"}, record));

        auto cursor = wire<stdlib::getattr_, TS<KafkaCursor>>(
            w, subscription, Str{"cursor"});
        commit(w, path, cursor);
        auto event = events(w, path);
    }
};

KafkaSubscriptionOutput provides the record and its matching next-offset cursor on the same graph tick, plus subscription state. A cursor is accepted only while its subscription identity, assignment generation, and partition remain live. Commits are monotonic per assigned partition.

Python

The Python authoring surface lowers to the same native service:

import hgraph as hg
import hgraph_kafka as kafka

@hg.graph
def app():
    kafka.register_kafka_service(
        kafka.KafkaServiceConfig.from_bootstrap_servers(
            ["localhost:9092"], client_id="orders-worker"
        ),
        path="primary",
    )
    key = kafka.KafkaSubscriptionKey(
        topics=("orders",),
        group_id="orders-worker",
        start_position=kafka.KafkaStartPosition.committed(),
    )
    subscription = kafka.kafka_subscribe(
        hg.const(key, tp=hg.TS[kafka.KafkaSubscriptionKey]),
        path="primary",
    )
    kafka.kafka_commit(subscription["cursor"], path="primary")

The core hgraph wheel owns a guarded compatibility shim at the released hgraph.adaptors.kafka import path. Existing message_publisher, message_subscriber, KafkaMessage, and register_kafka_adaptor imports continue to work when hgraph-kafka is installed. The extension wheel installs only hgraph_kafka; it never contributes files to the core hgraph package.

Recovery and simulation

Subscriptions support explicit topic, pattern, or partition selection; group or independent assignment; earliest, latest, committed, timestamp, explicit, and graph-start positions; snapshot, timestamp, and explicit stop boundaries; key filters; deterministic timestamp/topic/partition/offset replay; and explicit or graph-delivery commits.

Simulation is intentionally limited to bounded, record-time recovery. The consumer preloads the finite replay and schedules records at deterministic graph times. Publish, commit, unbounded asynchronous input, and OnGraphDelivery commit mode are rejected in simulation rather than silently changing their semantics.

Build and test

This is a first-party extension in the hgraph monorepo. It remains a separate CMake package and Python distribution: the top-level core package does not link librdkafka or install these modules.

For an in-tree native development build from the repository root:

cmake -S . -B build-kafka \
  -DHGRAPH_BUILD_KAFKA_EXTENSION=ON \
  -DBUILD_TESTING=ON
cmake --build build-kafka --parallel
ctest --test-dir build-kafka --output-on-failure

The extension can still be configured independently against an installed hgraph SDK:

cmake -S . -B build -DCMAKE_PREFIX_PATH=/path/to/hgraph/install
cmake --build build --parallel
ctest --test-dir build --output-on-failure

Build its separately deployable ABI3 wheel from the repository root after making the matching hgraph SDK discoverable through CMAKE_PREFIX_PATH:

CMAKE_PREFIX_PATH=/path/to/hgraph/sdk \
  uv build --wheel --package hgraph-kafka --python 3.12

The deterministic suite uses librdkafka's mock cluster and the extension fake transport. To include a real broker round trip, provide a clean topic:

HGRAPH_KAFKA_INTEGRATION_BOOTSTRAP=localhost:9092 \
HGRAPH_KAFKA_INTEGRATION_TOPIC=hgraph-kafka-integration \
ctest --test-dir build --output-on-failure

Wheel builds require the SDK installed by a stable-ABI hgraph wheel. The extension rejects an SDK that links Python::Python, because that would pin the nominal ABI3 module to the build interpreter.