Table of Contents |
---|
Status
Current state: Accepted
Discussion thread: here
...
Please keep the discussion on the mailing list rather than commenting on the wiki (wiki discussions get unwieldy fast).
Motivation
Kafka can be used in a stream processing pipeline to pass intermediate data between processing jobs. The amount of intermediate data generated from stream processing jobs can taken a large amount of disk space in the Kafka. It is important that we can delete this data soon after it is consumed by downstream application, otherwise we have to pay significant cost to purchase disks for Kafka clusters to keep those data.
...
Note that this KIP is related to and supersedes KIP-47.
Public Interfaces
1) Java API
- Add the following API in Admin Client. This API returns a future object whose result will be available within RequestTimeoutMs, which is configured when user constructs the AdminClient.
Future<Map<TopicPartition, DeleteDataResult>> deleteDataBeforedeleteRecordsBefore(Map<TopicPartition, Long> offsetForPartition)
...
DeleteDataResult(long: low_watermark, error: Exception)
2) Protocol
Create DeleteRecordsRequest
...
Code Block | ||
---|---|---|
| ||
FetchResponsePartitionHeader => partition error_code high_watermark low_watermark partition => int32 error_code => int16 high_watermark => int64 log_start_offset => int64 <-- NEW. This is the low_watermark of this partition on the leader. |
3) Checkpoint file
We create one more checkpoint file, named "log-begin-offset-checkpoint", in every log directory. The checkpoint file will have the same format as existing checkpoint files (e.g. replication-offset-checkpoint) which map TopicPartition to Long.
4) Script
Add kafka-delete-data.sh that allows user to delete data in the command line. The script requires for the following arguments:
...
Code Block |
---|
{ "version" : int, "partitions" : [ { "topic": str, "partition": int, "offset": long }, ... ] } |
Proposed Changes
The idea is to add new APIs in Admin Client (see KIP-4) that can be called by user to delete data that is no longer needed. New request and response needs to be added to communicate this request between client and broker. Given the impact of this API on the data, the API should be protected by Kafka’s authorization mechanism described in KIP-11 to prevent malicious or unintended data deletion. Furthermore, we adopt the soft delete approach because it is expensive to delete data in the middle of a segment. Those segments whose maximum offset < offset-to-delete can be deleted safely. Brokers can increment log_start_offset of a partition to offset-to-delete so that data with offset < offset-to-delete will not be exposed to consumer even if it is still on the disk. And the log_start_offset will be checkpointed periodically similar to high_watermark to be persistent.
...
Please refer to public interface section for our design of the API, request and response. In this section we will describe how broker maintains low watermark per partition, how client communicates with broker to delete old data, and how this API can be protected by authorization.
1) Interaction between user application and brokers
1) User application determines the maximum offset of data that can be deleted per partition. This information is provided to deleteDataBeforedeleteRecordsBefore()
as Map<TopicPartition, Long>. If users application only knows timestamp of data that can be deleted per partition, they can use offsetsForTimes()
API to convert the cutoff timestamp into offsetToDelete per partition before providing the map to deleteData
Before
to deleteRecordsBefore() API.
2) Admin Client builds DeleteRecordsRequest using the offsetToDelete from deleteDataBefore
deleteRecordsBefore
() parameter and the requestTimeoutMs
is taken from the AdminClient
constructor. One DeleteRecordsRequest
is sent to each broker that acts as leader of any partition in the request. The request should only include partitions which the broker leads.
...
9) If admin client does not receive DeleteRecordsResponse
from a broker within RequestTimeoutMs
, the DeleteDataResult
of the partitions on that broker will beDeleteDataResult
(low_watermark = -1, error = TimeoutException). Otherwise, the DeleteDataResult
of each partition will be constructed using the low_watermark
and the error
of the corresponding partition which is read from the DeleteDataResponse
received from its leader broker. delete
DataBeforedeleteRecordsBefore(...).get()
will unblock and return Map<TopicPartition, DeleteDataResult>
when DeleteDataResult
of all partitions specified in the offsetForPartition
param are available.
2) Routine operation in the broker
- Broker will delete those segments whose largest offset < log_start_offset
.
...
- Broker will checkpoint log_start_offset
for all replicas periodically in the file "log-begin-offset-checkpoint", in the same way it checkpoints high_watermark
of replicas. The checkpoint file will have the same format as existing checkpoint files which map TopicPartition to Long.
3) API Authorization
Given the potential damage that can be caused if this API is used by mistake, it is important that we limit its usage to only authorized users. For this matter, we can take advantage of the existing authorization framework implemented in KIP-11. deleteDataBeforedeleteRecordsBefore()
will have the same authorization setting as deleteTopic()
. Its operation type is be DELETE and its resource type is TOPIC.
4) ListOffsetRequest
log_start_offset
of a partition will be used to decide the smallest offset of the partition that will be exposed to consumer. It will be returned when smallest_offset option is used in the ListOffsetRequest.
...
The KIP changes the inter-broker protocol. Therefore the migration requires two rolling bounce. In the first rolling bounce we will deploy the new code but broker will still communicate using the existing protocol. In the second rolling bounce we will change the config so that broker will start to communicate with each other using the new protocol.
Test Plan
- Unit tests to validate that all the individual components work as expected.
- Integration tests to ensure that the feature works correctly end-to-end.
Rejected Alternatives
- Using committed offset instead of an extra API to trigger data delete operation. Delete data if its offset is smaller than committed offset of all consumer groups that need to consume from this partition.
deleteDataBefore
deleteRecordsBefore
() can be called, which can be hard to do if there are multiple consumer groups interested in consuming this topic. The disadvantage of this approach is that it is less flexible than deleteDataBefore
deleteRecordsBefore
() API because it re-uses committed offset to trigger data delete operation. Also, it adds complexity to broker implementation and would be more complex to implement than the deleteDataBefore
deleteRecordsBefore
() API. An alternative approach is to implement this logic by running an external service which calls deleteDataBefore
deleteRecordsBefore
() API based on committed offset of consumer groups.low_watermark
of all followers to increase above the cutoff offset