Status

Current state: Under discussion

Discussion thread: thread-1 thread-2, 

Voting thread: 

JIRA: Related

Summary: Extract transaction identity and lifecycle management from `KafkaProducer` into a first-class
`TransactionSession` client abstraction, enabling any Kafka client (producer, share consumer, external coordinator, or any other entity) 
to participate in transactions without producer coupling or reflection hacks.

1. Motivation

1.1 The Problem: Multi-Participant Transactions Need a Shared Identity Abstraction

Kafka's transaction coordinator is a general-purpose distributed transaction manager. Its metadata (TransactionMetadata) stores:

```
transactionalId   -- session identity
producerId        -- session token
producerEpoch     -- fencing token
state             -- lifecycle state (ONGOING, PREPARE_COMMIT, ...)
topicPartitions   -- participating partitions
txnTimeoutMs      -- correctness bound
```

None of these fields are producer-specific. The naming is historical -- producerId and producerEpoch could equally be called sessionId and sessionEpoch. The wire protocol RPCs (InitProducerId, AddPartitionsToTxn, EndTxn, TxnHeartbeat) carry only these identity fields. They do not carry a "client type" discriminator.

What KIP-939 Solved

KIP-939 enabled Kafka to act as a formal participant in Two-Phase Commit (2PC) transactions by:

• Adding Recovery APIs: Introduced prepareTransaction() and completeTransaction(), allowing external systems to coordinate Kafka's commit/abort status.

• Resilient Resumption: Allowed new producers to resume a "prepared" transaction after a crash without automatically aborting it.

• Removing Timeouts: Provided an enable2Pc mode that sets transaction timeouts to infinity, preventing premature aborts during external coordination.

The Unsolved Problem

While KIP-939 added the right tools, it put them in the wrong place:

• Wrong Abstraction: Transaction methods (like preparing and completing) are forced into the KafkaProducer class, even though they don't involve producing records.

• Heavyweight Bloat: To simply commit a transaction, you are forced to instantiate a full KafkaProducer with its entire infrastructure (buffer pools, sender threads, serializers).

• Inaccessible Logic: The core "brain" of transactions—the state machine and coordinator logic—is buried in internal producer packages, making it impossible for other clients (like consumers or custom coordinators) to use transactions independently.

1.2 Entities Already Need Transaction Participation

The Kafka ecosystem already has six distinct entity types that interact with the transaction coordinator, each working around the producer-centric API:

2. Public Interfaces

2.1 New Class: TransactionSession

Package: org.apache.kafka.clients.transaction

TransactionSession is a lightweight, thread-safe client focused strictly on transaction coordinator interaction.

It decouples lifecycle management from record production.

```

public class TransactionSession implements Closeable {

    public TransactionSession(Map<String, Object> configs);

    public static TransactionSession resume(
        String transactionalId,
        long producerId,
        short producerEpoch,
        Map<String, Object> configs
    );

    // --- Lifecycle ---
    public void initialize();
    public void beginTransaction();
    public PreparedTxnState prepareTransaction();
    public void completeTransaction(PreparedTxnState preparedTxnState);
    public void commitTransaction();
    public void abortTransaction();

    // --- Identity ---
    public String transactionalId();

    // --- Participation ---
    public void addPartitionsToTransaction(Collection<TopicPartition> partitions);
    
    public void sendOffsetsToTransaction(
        Map<TopicPartition, OffsetAndMetadata> offsets,
        ConsumerGroupMetadata groupMetadata
    );

    public void addShareAcksToTransaction(
        String groupId,
        Collection<ShareAcknowledgment> acknowledgments
    );

    // --- Liveness ---
    public void heartbeat();

    @Override
    public void close();
}

```

2.2 Configuration

TransactionSession accepts a subset of existing producer configs plus one new config:

No new broker-side configs. No new wire protocol RPCs. TransactionSession sends the same RPCs that KafkaProducer sends today:

 FindCoordinator, InitProducerId, AddPartitionsToTxn, EndTxn, AddOffsetsToTxn, TxnOffsetCommit, TxnHeartbeat (KIP 1309).

2.3 Integration with KafkaProducer

KafkaProducer gains a new constructor and method to accept an external TransactionSession:

```

public class KafkaProducer<K, V> {
    /**
     * Bind producer to an external session. 
     * Lifecycle methods (e.g., beginTransaction) throw IllegalStateException.
     */
    public KafkaProducer(Map<String, Object> configs, TransactionSession session);

    /**
     * Access the session (internal or external). Returns null if non-transactional.
     */
    public TransactionSession transactionSession();
}

```

Backward Compatibility: Constructing a producer with transactional.id in the config works as it does today.

It will internally create a TransactionSession and use its existing methods (initTransactions, beginTransaction, etc.) as convenience wrappers.

No code changes are required for existing users.

2.4 Integration with KafkaShareConsumer

For KIP-1289 transactional acknowledgments, the share consumer accepts a TransactionSession:

```

public class KafkaShareConsumer<K, V> {

    /**

     * Acknowledge records within the provided session.

     */

    public void acknowledgeTransactionally(

        TransactionSession session,

        Map<TopicPartition, Set<Long>> acknowledgments

    );

}
```

3. Proposed Changes

3.1 Architecture: Before and After

Before (current):

After (this KIP):

3.2 Extracting TransactionSession from TransactionManager

State mapping:

3.3 No Wire Protocol Changes

This KIP reuses existing RPCs and schemas. TransactionSession becomes the new sender for all transaction-related requests:

3.4 Internal Network Client

TransactionSession uses a lightweight NetworkClient for coordinator communication, similar to the consumer's HeartbeatThread.

Dedicated Connection - Maintains its own connection to the coordinator broker.

4. Use Cases

4.1 Apache Flink: Lightweight Transaction Completion

Flink's exactly-once KafkaSink separates the write path (writer subtask) from the commit path (committer). 

```java
// Before (Flink -- reflection, fragile)
FlinkKafkaInternalProducer producer = new FlinkKafkaInternalProducer(configs);
producer.resumeTransaction(producerId, epoch);  // reflection on TransactionManager internals
producer.commitTransaction();

// After (this KIP -- public API, stable)
TransactionSession session = TransactionSession.resume(
    transactionalId, producerId, epoch, configs
);
session.commitTransaction();
session.close();
```

4.2 Share Group Consumer: Transactional Acknowledgments (KIP-1289)

```

// 1. Open shared session
TransactionSession session = new TransactionSession(configs);
session.initialize();
session.beginTransaction();

// 2. Producer writes records using session identity
KafkaProducer<K, V> producer = new KafkaProducer<>(producerConfigs, session);
producer.send(outputRecord);

// 3. Share consumer acknowledges within the SAME transaction
shareConsumer.acknowledgeTransactionally(session, acknowledgments);

// 4. Atomic commit for both entities
session.commitTransaction();

```

4.4 Kafka Connect: Exactly-Once Source Tasks

```java

class ExactlyOnceWorkerSourceTask {
    private TransactionSession session;
    private KafkaProducer<byte[], byte[]> producer;

    void maybeBeginTransaction() { session.beginTransaction(); }
    void commitTransaction()      { session.commitTransaction(); }
}

```

4.5 Custom 2PC Coordinators

```java

// Phase 1: Prepare
kafkaSession.beginTransaction();
producer.send(records);
database.prepareTransaction(dbTxnId); 
kafkaSession.prepareTransaction();    

// Phase 2: Commit (Recovery-friendly)
TransactionSession resumed = TransactionSession.resume(txnId, pid, epoch, configs);
resumed.commitTransaction();          
database.commitTransaction(dbTxnId);

```

4.6 Kafka Connect Sink with Share Groups: Exactly-Once Kafka-to-Kafka (KIP-1302)

// KIP-1302 exactly-once pattern with TransactionSession
void iterationExactlyOnce() {
    ConsumerRecords<byte[], byte[]> records = shareConsumer.poll(pollTimeout);
    
    session.beginTransaction();
    try {
        // 1. Producer writes output records
        for (SinkRecord record : convertMessages(records)) {
            producer.send(new ProducerRecord<>(outputTopic, record.key(), record.value()));
        }

        // 2. Share consumer acknowledges input within the SAME session
        session.addShareAcksToTransaction(
            shareConsumer.groupMetadata().groupId(),
            ShareAcknowledgements.fromRecords(records, AcknowledgeType.ACCEPT)
        );

        session.commitTransaction();
    } catch (Exception e) {
        session.abortTransaction();
    }
}

Architectural comparison:

5. Compatibility, Deprecation, and Migration Plan

5.1 Full Backward Compatibility

KafkaProducer constructed with transactional.id in the config continues to work exactly as today.

Internally, it creates a TransactionSession and delegates to it, but the public API (initTransactions(), beginTransaction(), commitTransaction(), abortTransaction(), sendOffsetsToTransaction()) is unchanged.

5.2 Deprecation Path

No deprecations. The KafkaProducer convenience methods remain the recommended API for simple produce-and-commit patterns. TransactionSession is for advanced use cases: 2PC, cross-entity transactions, external coordinators.

6. Security

6.1 Authorization

TransactionSession requires the same ACLs as the current producer transaction API:

6.2 Session Sharing

When a TransactionSession is shared between a producer and a share consumer (Use Case 4.2), both entities operate under the same transactionalId and require the same ACLs.

The session identity is not multiplied -- there is one session, one producerId, one epoch, regardless of how many clients use it.

7. Relationship to KIP-939 (Support Participation in 2PC)

KIP-939 (Accepted, authored by Artem Livshits) is the foundational KIP that enables Kafka to participate in externally-coordinated two-phase commit. This KIP  is explicitly designed as a complement to KIP-939, not a replacement. The following table clarifies the boundary:

What KIP-939 Got Right

1. Implicit prepare. KIP-939 rejected an explicit "prepare" RPC, keeping Kafka's existing "implicit prepare" (flush-as-prepare). The external coordinator tracks prepared state, not the broker. This avoids duplicating state and an extra synchronous operation on the transaction coordinator topic. TransactionSession.prepareTransaction() wraps this same implicit-prepare mechanism.

2. keepPreparedTxn flag. Separating "don't abort the ongoing transaction" from "enable 2PC" was correct. Flink needs keepPreparedTxn=true even without enable2Pc=true (for clusters that don't grant 2PC privileges). TransactionSession uses this flag transparently.

3. PreparedTxnState as serializable {producerId, epoch}. This is the portable transaction identity that can be stored in any database. TransactionSession.resume() accepts the same identity fields.

What KIP-939's Rejected Alternatives Reveal

KIP-939 explicitly rejected a HeartBeat RPC (page 11): "HeartBeat RPC definitely sounds like a 'good thing to do'. It is not clear, though, what would be the cases when we need to handle these situations differently." Since then, KIP-1309 has been proposed to add exactly this heartbeat, with strong community support. The evolution from KIP-939's rejection to KIP-1309's proposal demonstrates that the ecosystem's needs have grown beyond KIP-939's original scope. TransactionSession provides the natural home for the heartbeat thread (KIP 1309), which runs independently of the producer's Sender thread.

Why TransactionSession Is Not Redundant With KIP-939

The key question: "If KIP-939 provides initTransactions(true) + completeTransaction(), why do we need TransactionSession?"

Answer: KIP-939 solved the protocol problem. this KIP solves the abstraction problem.

KIP-939 added the correct primitives to the producer API. But the producer API is the wrong abstraction for three emerging use cases that KIP-939 did not anticipate:

1. KIP-1289 (Transactional Share Acks): A share consumer must send AddShareAcksToTxnRequest using the producer's (producerId, epoch). There is no method for this on KafkaProducer today, and adding one would pollute the record-producer interface with share-consumer semantics.

2. KIP-1302 (Share Groups in Connect Sink): The exactly-once Kafka-to-Kafka path requires three entities (producer, share consumer, transaction coordinator) to participate in one transaction. The shared identity must be accessible to both producer and consumer without one proxying through the other.

3. /KIP-1309 (Transaction Heartbeat): The heartbeat thread should belong to the transaction session, not the producer. The producer may be idle (no records to send) while the transaction is active during a long checkpoint. The heartbeat must continue independently.

TransactionSession is the missing abstraction that connects KIP-939's 2PC primitives, KIP-1289's multi-entity transactions, KIP-1302's Connect sink EOS, and KIP 1309's liveness detection into a coherent client-side architecture.

8. Test Plan

8.1 Unit Tests

8.2 Integration Tests

8.3 Compatibility Tests

9. Reference 

KIP-98 - Exactly Once Delivery and Transactional Messaging

KIP-939: Support Participation in 2PC

KIP-1289 Support Transactional Acknowledgments for Share Groups

KIP-1302: Support Share Groups (Queue Semantics) in Kafka Connect Sink Connectors

KIP-1309: Improve transaction liveness checking

10. Rejected Alternatives

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