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Status
Current state: [One of "Under Discussion", "Accepted", "Rejected"]WIP
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).
Note this is a joint proposal by Philip Nee, Kirk True, and Lianet Magrans.
Motivation
...
This KIP documents the updated threading model of the Consumer
implementation of the client.
The complexity of the consumer has increased and with it the code to support and fix bugs. Patches and hotfixes in the past years have heavily impacted the readability of the code. The complex code path and intertwined logic make the code difficult to modify and comprehend. Additionally, logic is at times executed on application threads and at other times on the dedicated, internal heartbeat thread. The asynchronous nature of the current implementation has lead to many bugs (which are labeled with the new-consumer-threading-should-fix label. The motivation is to simplify the structure of the code by clearly defining—and removing, where possible—the asynchronous code.
The simplification will also allow us to implement the necessary primitives for KIP-848.
Public Interfaces
This KIP has the explicit goal of making no changes to the public interfaces. The protocol, configuration, APIs, etc. will remain as they currently are. The internal behavior of the consumer is substantially changing and we want to ensure it is reviewed and vetted by the community.
Proposed Changes
Terminology
To help understand the design, we need to introduce some terminology. Terms designated with 1 apply to the current KafkaConsumer
implementation and terms with 2 apply to the new implementation; a term may apply to both.
Term | Definition |
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Application event | A data structure specific to each Application events can optionally include a |
Application event processor | Logic which processes application events on the background thread, interacting with the request managers. |
Application event queue | A shared queue which stores application events enqueued by the application thread. These events are later dequeued by the background thread and given to the application event processor for execution. |
Application thread | The thread that is executing the user's code that interacts with the |
Background event | |
Background event queue | A shared queue which stores background events enqueued by the background thread. The events are later dequeued by the application thread inside the |
Background thread | An internal thread created for each
|
Event handler | Logic that pulls events from the event queue for processing on the background thread. |
Heartbeat | Logic related to communicating liveness, group membership, etc. as introduced in KIP-62. |
Network client delegate | |
Request manager | An internal interface that is used by the background thread to handle the management of requested, inflight, and responded network I/O. |
Threading Model
<TBD>
Background thread
<TBD>
Providing Data to the Background Thread
<TBD>
Getting Data from the Background Thread
<TBD>
Network I/O
<TBD>
Describe the problems you are trying to solve.
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.
A public interface is any change to the following:
Binary log format
The network protocol and api behavior
Any class in the public packages under clientsConfiguration, especially client configuration
org/apache/kafka/common/serialization
org/apache/kafka/common
org/apache/kafka/common/errors
org/apache/kafka/clients/producer
org/apache/kafka/clients/consumer (eventually, once stable)
Monitoring
Command line tools and arguments
- Anything else that will likely break existing users in some way when they upgrade
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.