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Status

Current state: Under discussion

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

JIRA: KAFKA-19742 - Getting issue details... STATUS

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

Motivation

KIP-932 introduced queueing semantics in Kafka with new group and consumer types: share groups and share consumers. Multiple share consumers in a share group can subscribe to user topics and consume data cooperatively without partition limits.

A share consumer can operate in implicit or explicit mode. In implicit mode, the acknowledgement type is fixed to ACCEPT. In explicit mode, each record must be acknowledged explicitly. The acknowledgement types are:

  • ACCEPT - indicates the record was consumed successfully, causing it to be acknowledged on the server and never re-delivered to other consumers.

  • REJECT - indicates the record was not consumed successfully, causing it to be archived on the server and never re-delivered to other consumers.

  • RELEASE - indicates the record was not consumed successfully, causing it to be moved to available state, making it eligible for re-delivery.

When a share consumer polls a batch of records, the server attaches an acquisition lock timeout task to the batch. If the batch is not acknowledged before the acquisition lock expires, it transitions to available state and becomes eligible for re-delivery (unless delivery count limit is exceeded). The broker cancels and clears the acquisition lock timeout task on ACCEPT or REJECT. On RELEASE state is moved to available and acquisition task is restarted on next delivery attempt.

If a user application using a share consumer in explicit mode processes a record for a long time, it cannot use any acknowledgement type to affect the server acquisition lock timeout without changing the record state. If processing exceeds the timeout duration, the acquisition lock expires and the record is re-delivered, which may be undesirable.

This KIP proposes a new acknowledgement type, RENEW, for explicit mode. It lets a share consumer request the broker to renew the acquisition lock timeout, thereby extending the current delivery attempt without changing the server's record state.

Public Interfaces

  • There will be new addition in the org/apache/kafka/clients/consumer/AcknowledgeType.java  enum where we will add a new entry RENEW with id 4. 

    public enum AcknowledgeType {
    /** The record was consumed successfully. */
    ACCEPT((byte) 1),
    /** The record was not consumed successfully. Release it for another delivery attempt. */
    RELEASE((byte) 2),
    /** The record was not consumed successfully. Reject it and do not release it for another delivery attempt. */
    REJECT((byte) 3),
    /** Consumer needs more time to process the record. Renew the lease. */
    RENEW((byte) 4);	// New entry per KIP-1222
    ...
}
  • The acknowledgement type will be used in the existing ShareFetch and ShareAcknowledge RPCs. Hence, we must add new versions for them.

    • clients/src/main/resources/common/message/ShareFetchRequest.json 

      // ShareFetch
{
  "apiKey": 78,
  "type": "request",
  "listeners": ["broker"],
  "name": "ShareFetchRequest",
  // Version 0 was used for early access of KIP-932 in Apache Kafka 4.0 but removed in Apacke Kafka 4.1.
  //
  // Version 1 is the initial stable version (KIP-932).
  //
  // Version 2 supports RENEW ack type
  "validVersions": "1-2",
  "flexibleVersions": "0+",
  "fields": [
    { "name": "GroupId", "type": "string", "versions": "0+", "nullableVersions": "0+", "default": "null", "entityType": "groupId",
      "about": "The group identifier." },
    { "name": "MemberId", "type": "string", "versions": "0+", "nullableVersions": "0+",
      "about": "The member ID." },
    { "name": "ShareSessionEpoch", "type": "int32", "versions": "0+",
      "about": "The current share session epoch: 0 to open a share session; -1 to close it; otherwise increments for consecutive requests." },
    { "name": "MaxWaitMs", "type": "int32", "versions": "0+",
      "about": "The maximum time in milliseconds to wait for the response." },
    { "name": "MinBytes", "type": "int32", "versions": "0+",
      "about": "The minimum bytes to accumulate in the response." },
    { "name": "MaxBytes", "type": "int32", "versions": "0+", "default": "0x7fffffff",
      "about": "The maximum bytes to fetch. See KIP-74 for cases where this limit may not be honored." },
    { "name": "MaxRecords", "type": "int32", "versions": "1+",
      "about": "The maximum number of records to fetch. This limit can be exceeded for alignment of batch boundaries." },
    { "name": "BatchSize", "type": "int32", "versions": "1+",
      "about": "The optimal number of records for batches of acquired records and acknowledgements." },
    { "name": "Topics", "type": "[]FetchTopic", "versions": "0+",
      "about": "The topics to fetch.", "fields": [
      { "name": "TopicId", "type": "uuid", "versions": "0+", "about": "The unique topic ID.", "mapKey":  true },
      { "name": "Partitions", "type": "[]FetchPartition", "versions": "0+",
        "about": "The partitions to fetch.", "fields": [
        { "name": "PartitionIndex", "type": "int32", "versions": "0+", "mapKey":  true,
          "about": "The partition index." },
        { "name": "PartitionMaxBytes", "type": "int32", "versions": "0",
          "about": "The maximum bytes to fetch from this partition. 0 when only acknowledgement with no fetching is required. See KIP-74 for cases where this limit may not be honored." },
        { "name": "AcknowledgementBatches", "type": "[]AcknowledgementBatch", "versions": "0+",
          "about": "Record batches to acknowledge.", "fields": [
          { "name": "FirstOffset", "type": "int64", "versions": "0+",
            "about": "First offset of batch of records to acknowledge."},
          { "name": "LastOffset", "type": "int64", "versions": "0+",
            "about": "Last offset (inclusive) of batch of records to acknowledge."},
          { "name": "AcknowledgeTypes", "type": "[]int8", "versions": "0+",
            "about": "Array of acknowledge types - 0:Gap,1:Accept,2:Release,3:Reject,4:Renew."} // Version 2 supports RENEW ack type (KIP-1222)
         ]}
      ]}
    ]},
    { "name": "ForgottenTopicsData", "type": "[]ForgottenTopic", "versions": "0+",
      "about": "The partitions to remove from this share session.", "fields": [
      { "name": "TopicId", "type": "uuid", "versions": "0+", "about": "The unique topic ID."},
      { "name": "Partitions", "type": "[]int32", "versions": "0+",
        "about": "The partitions indexes to forget." }
    ]}
  ]
}

    • clients/src/main/resources/common/message/ShareAcknowledgeRequest.json  

      // ShareAcknowledge
{
  "apiKey": 79,
  "type": "request",
  "listeners": ["broker"],
  "name": "ShareAcknowledgeRequest",
  // Version 0 was used for early access of KIP-932 in Apache Kafka 4.0 but removed in Apacke Kafka 4.1.
  //
  // Version 1 is the initial stable version (KIP-932).
  //
  // Version 2 will have RENEW ack type
  "validVersions": "1-2",
  "flexibleVersions": "0+",
  "fields": [
    { "name": "GroupId", "type": "string", "versions": "0+", "nullableVersions": "0+", "default": "null", "entityType": "groupId",
      "about": "The group identifier." },
    { "name": "MemberId", "type": "string", "versions": "0+", "nullableVersions": "0+",
      "about": "The member ID." },
    { "name": "ShareSessionEpoch", "type": "int32", "versions": "0+",
      "about": "The current share session epoch: 0 to open a share session; -1 to close it; otherwise increments for consecutive requests." },
    { "name": "Topics", "type": "[]AcknowledgeTopic", "versions": "0+",
      "about": "The topics containing records to acknowledge.", "fields": [
      { "name": "TopicId", "type": "uuid", "versions": "0+", "about": "The unique topic ID.", "mapKey": true },
      { "name": "Partitions", "type": "[]AcknowledgePartition", "versions": "0+",
        "about": "The partitions containing records to acknowledge.", "fields": [
        { "name": "PartitionIndex", "type": "int32", "versions": "0+", "mapKey": true,
          "about": "The partition index." },
        { "name": "AcknowledgementBatches", "type": "[]AcknowledgementBatch", "versions": "0+",
          "about": "Record batches to acknowledge.", "fields": [
          { "name": "FirstOffset", "type": "int64", "versions": "0+",
            "about": "First offset of batch of records to acknowledge." },
          { "name": "LastOffset", "type": "int64", "versions": "0+",
            "about": "Last offset (inclusive) of batch of records to acknowledge." },
          { "name": "AcknowledgeTypes", "type": "[]int8", "versions": "0+",
            "about": "Array of acknowledge types - 0:Gap,1:Accept,2:Release,3:Reject,4:Renew" } // Version 2 supports RENEW ack type (KIP-1222)
         ]}
      ]}
    ]}
  ]
}
  • New method exposed in clients/src/main/java/org/apache/kafka/clients/consumer/ShareConsumer.java interface to help the application determine RENEW interval. 
    /**
 Returns the acquisition lock timeout value in milliseconds for the last set of records fetched from the brokers.
*/
public Optional<Integer> acquisitionLockTimeoutMs();

Proposed Changes

KIP-932 added two new RPCs for record fetching and acknowledging: ShareFetch and ShareAcknowledge. From the share consumer, these RPCs are called when an application invokes poll(Duration), acknowledge(ConsumerRecord, AcknowledgementType), or commitSync(Duration)/commitAsync().

In implicit mode, an application calls poll() on the share consumer and receives a batch of records. It can acknowledge these records on a subsequent call to poll() (piggybacking on ShareFetch). Calling commitSync()/commitAsync() sends a ShareAcknowledge but only the ACCEPT acknowledgement type is used. Calling acknowledge(ConsumerRecord, AcknowledgementType) is illegal in this mode. This KIP does not target implicit mode.

In explicit mode, polling works the same, but the application must acknowledge each record explicitly using one of the AcknowledgementType values. If record processing might take time, the application should use the new acknowledgement type RENEW as an argument to acknowledge(ConsumerRecord, AcknowledgementType) for that record. It can then call either commitSync()/commitAsync() or poll() to send the acknowledgement to the server. Furthermore, the application could also leverage the new ShareConsumer.acquisitionLockTimeoutMs() method to decide on the timeframe in which to make the RENEW call.

On the broker side, receiving a RENEW acknowledgement for a specific batch or offset will cancel the existing acquisition lock timeout task and start a new one with the same timeout value as group.share.record.lock.duration.ms. There is one caveat here. If the application causes a ShareFetch RPC to be sent (poll() call) on which RENEW acknowledgements are piggybacked, it could happen that the renewed acquisition lock again times out before the poll() completes. To get around this, we will not return any data from the broker side on ShareFetch requests containing RENEW acknowledgements. That way we are guaranteed that the poll()  completes timely.

We assume the application uses appropriate concurrency constructs to process records in separate threads.

Compatibility, Deprecation, and Migration Plan

  • The existing functionality is not modified. Applications using share consumers in explicit mode will get a new capability of renewing records.
  • We are increasing the version number of ShareFetch and ShareAcknowledge requests. If a broker running the old version of the code receives the RENEW  ack type in the request, it should consider this an error and return the appropriate error code to the consumer (Errors.INVALID_REQUEST at the time of writing).
  • Though the broker with old and new code can handle RENEW (return error or renew lock), the share consumer should be smart enough to figure out based on the broker API versions whether to send the RENEW ack or not. There could be 2 ways in which this can be done:
    • If the application makes a RENEW acknowledgement call, and the broker does not support it - we mutate the ack type to RELEASE in the share consumer impl.
    • We can modify the ShareConsumer.acknowledge(ConsumerRecord, AcknowledgementType)method to throw an exception if broker does not support RENEW (preferred since more explicit).

Test Plan

  • We will be adding new unit tests mainly in core/src/test/java/kafka/server/share/SharePartitionTest.java to verify batch and offset level renewal as well as mix acknowledgement type handling.
  • New integration tests will be added in clients/clients-integration-tests/src/test/java/org/apache/kafka/clients/consumer/ShareConsumerTest.java  to verify mix and renew acknowledgement type handling.

Rejected Alternatives

None

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