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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: here [Change the link from KAFKA-1 to your own ticket]

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

Motivation

Describe the problems you are trying to solve.

Problem

I am an operator of a Tiered Storage Kafka cluster and I would like to know whether interactions with the remote tier, uploads and deletions, are progressing at a steady pace.

For example, if uploads are not progressing at a constant rate I will have data building up in local storage and I might need to take corrective actions (like adding more storage temporarily). Likewise, if deletes are not progressing at a constant rate this might indicate a problem with the retention settings of my topics which I would like to remedy.

Solution

To get this observability we would like to expose a tier-upload and tier-delete metrics from the point of view of Kafka. Since upload and deletion are carried out by plugins such metrics emitted by Kafka are going to be best estimates on what it believes the state of the world is. We propose the new metrics to be emitted on per topic and per partition granularity.

Other Tiered Storage metrics are emitted only at a topic level, but we would like to also expose these at a partition level because this will allow us to easily track whether a single broker is experiencing a slowdown or the issue is spread across the cluster.

We leave more detailed progress metrics to be emitted by the developers of Tiered Storage plugins.

Public Interfaces

Briefly list any new interfaces that will be introduced as part of this proposal or any existing interfaces that will be removed or changed. The purpose of this section is to concisely call out the public contract that will come along with this feature.

New Metrics

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.

Constraints

The metric calculation will not acquire any locks. If it had to acquire locks then it would be contending with the archival/deletion paths of Tiered Storage.

Reference implementation

Reference implementation for UploadTierLag on topic level: https://github.com/satishd/kafka/commit/c96d3af4d02bf515a4355b14f33793211c5b3745

Details

The metrics for upload and delete will be updated on every respective archival and deletion cycle. If no archival or deletion happened in a particular cycle then the metric will not change.

On the archive path of Tiered Storage we go through segments which are eligible for upload in a sequential manner. This allows us to emit the UploadTierLag metric by taking the difference between the segment's base offset and the base offset of the first segment not eligible for tiering.

In a similar manner, when we expire segments from Tiered Storage we go through them in a sequential manner. We can emit the DeleteTierLag metric by taking the difference between the segment's base offset and the end offset + 1 of the last eligible for deletion segment.

Compatibility, Deprecation, and Migration Plan

These are new metrics and as such shouldn't have compatibility concerns.

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?

Unit and integration tests.

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.

N/A