Status

Current state: WIP

Discussion thread: here

JIRA: here

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.

• Application event processor²: Processes the events on the background thread, interacting with the request managers.
• Application thread^{1, 2}: The thread that is executing the user's code that interacts with the Consumer API. Per the current implementation in KafkaConsumer, only one thread may call APIs at a time.
• Background thread²: The thread created for each Consumer that is dedicated to executing events on the event handler.  
• Event²: A data structure specific to each Consumer API call that encapsulates the relevant data. For example, a seek event would include topic information and an offset. 
• Event handler²: Logic that pulls events from the event queue for processing on the background thread.
• Event queue²: Queue of events that is shared between the application thread and the background thread, serving as a conduit between the two.
• Heartbeat thread¹: Thread in the existing KafkaConsumer implementation that communicates liveness to the brokers, processes fetch results, etc. Introduced in KIP-62.
• Network client delegate²: TBD
• Request manager²: 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>

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.