Current state: WIP
Discussion thread: here
Please keep the discussion on the mailing list rather than commenting on the wiki (wiki discussions get unwieldy fast).
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.
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.
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.
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.
The thread that is executing the user's code that interacts with the
|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
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.|
Providing Data to the Background Thread
Getting Data from the Background Thread
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?
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?
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.