Status

Current state: Under Discussion

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

Motivation

Kafka Streams do not provide a way to easily trigger periodic callbacks at specific times. Wall-clock time punctuation allows to schedule periodic callbacks based on wall-clock time progress, but the punctuation time starts when the punctuation is scheduled. As a result, the callback is triggered at a non-deterministic time. It would be nice to allow a punctuation to be triggered at a fixed/anchored time, independent of when the punctuation was registered. For instance, this will allow for triggering a punctuation at the start of every hour, i.e. HH:00:00.

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Generally, the work-around can be characterised as flag-polling, where the poll frequency is determined by a wall-clock punctuation. Hence, the punctuation trigger time is only as precise as the wall-clock punctuation trigger interval. A more frequent wall-clock punctuation will give a more precise trigger time, at the cost of firing an increased number of flag checks (i.e. punctuations). This is a trade-off that can be removed with anchored punctuations. 

Public API Interfaces

The goal is to introduce an anchored wall-clock punctuation that will have functionally similarities with that of running a cron job. In short, the user should be allowed to specify the start time for the schedule, instead of relying on the non-deterministic time for when the schedule was registered.

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package org.apache.kafka.streams.processor.api;

public interface ProcessingContext {

	// New method allowing for anchored punctuation
	Cancellable schedule(final Duration interval, final Instant startTime, final PunctuationType type, final Punctuator callback);

	// Existing method
 	Cancellable schedule(final Duration interval, final PunctuationType type, final Punctuator callback) {
		schedule(interval, null, type, callback);
	}
}

Proposed changes

PunctuationType support

It is planned to support both system time and stream time in this first iteration, as it is deemed more of a hassle to gracefully handle non-supported punctuation types rather than implementing support for both punctuation types.

Punctuation semantics

The startTime, together with the interval and the current time, will determine the next trigger time for the callback. The method for calculating the next trigger time could look something like:

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Edge cases, and possibly unintuitive cases, will be described in the following paragraphs. 

Start time is defined in the past

If the startTime is defined as a point in time that is before the current time, the implementation will just skip forward and calculate the next trigger time.

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As a result, there will exist several schedule configurations yielding the same trigger times.

Starts and restarts

If the running Streams app start, or restarts, close to the scheduled trigger time, there is a possibility that callbacks will be missed. 

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However, if the app happened to shut down at t=109, and did not come back up again before t=111, the app will miss the punctuation at t=110 and will have to wait until t=120 for the next punctuation.

Compatibility, Deprecation, and Migration Plan

The goal is to not enforce any changes upon the existing users of the schedule method, but instead expand the current schedule options with a new anchored schedule option. This can be achieved by using method overloading.

Test Plan

The plan is to test the new schedule option in the same way that the current schedule options are tested. The anchored wall-clock punctuation is a new feature, and the feature should therefore not affect any current features or users. 

Rejected Alternatives

Cron job

Creating a new schedule method that takes in a cron job expression as a parameter:

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