This page is meant as a template for writing a KIP. To create a KIP choose Tools->Copy on this page and modify with your content and replace the heading with the next KIP number and a description of your issue. Replace anything in italics with your own description.

Status

Current state: Under Discussion

Discussion thread: here [Change the link from the KIP proposal email archive to your own email thread]

JIRA: [KAFKA-19883]()

Please keep the discussion on the mailing list rather than commenting on the wiki (wiki discussions get unwieldy fast).

Motivation

Share Groups currently support only immediate acknowledgement modes (IMPLICIT/EXPLICIT). This causes **data loss** in distributed streaming frameworks:

1. Worker acknowledges records → Records removed from Kafka
2. Checkpoint fails before sink write
3. Records lost (acknowledged but never persisted)

**Goal:** Enable exactly-once read semantics via transactional acknowledgements.

### Use Cases

- **Apache Flink:** Exactly-once checkpointing with Share Group sources
- **Apache Spark:** Structured Streaming with Share Group consumers
- **Any coordinator-worker streaming framework** requiring atomic acknowledgements

Public Interfaces

### New Consumer API Methods

```java
public interface ShareConsumer<K, V> {
    // Existing
    void acknowledge(ConsumerRecord<K, V> record, AcknowledgeType type);
    
    // New: Transactional acknowledgement
    void acknowledge(ConsumerRecord<K, V> record, AcknowledgeType type, 
                     String transactionId);
}
```

### New Coordinator API

```java
public interface ShareTransactionCoordinator {
    void beginTransaction(String transactionId, Duration timeout);
    void prepareTransaction(String transactionId) throws TransactionPrepareException;
    void commitTransaction(String transactionId) throws TransactionCommitException;
    void abortTransaction(String transactionId);
    List<PendingTransaction> listPendingTransactions(String groupId);
}
```

New Configuration

Config                           Default      Description
share.acknowledgement.mode       explicit     Values: implicit, explicit, transactional
share.transaction.timeout.ms     60000        Maximum transaction duration before auto-abort
share.transaction.max.pending    5            Maximum concurrent transactions per group

New Wire Protocol Requests

Request                          Description
ShareAcknowledgeTransactional    Acknowledge records within a transaction
ShareBeginTransaction            Start a new batch transaction
SharePrepareTransaction          Phase 1: Prepare transaction
ShareCommitTransaction           Phase 2: Commit transaction
ShareAbortTransaction            Abort transaction and release records

New Metrics

Metric                                Type        Description
share-transaction-active              Gauge       Active transactions
share-transaction-prepare-time-ms     Histogram   Prepare latency
share-transaction-commit-time-ms      Histogram   Commit latency
share-transaction-abort-total         Counter     Aborted transactions
share-transaction-timeout-total       Counter     Timed-out transactions

Proposed Changes

Describe the new thing you want to do in appropriate detail. This may be fairly extensive and have large subsections of its own. Or it may be a few sentences. Use judgement based on the scope of the change.

Compatibility, Deprecation, and Migration Plan

• What impact (if any) will there be on existing users?
• If we are changing behavior how will we phase out the older behavior?
• If we need special migration tools, describe them here.
• When will we remove the existing behavior?

Test Plan

Describe in few sentences how the KIP will be tested. We are mostly interested in system tests (since unit-tests are specific to implementation details). How will we know that the implementation works as expected? How will we know nothing broke?

Rejected Alternatives

If there are alternative ways of accomplishing the same thing, what were they? The purpose of this section is to motivate why the design is the way it is and not some other way.