Status

Current state:  "Under Discussion"Accepted

Discussion thread: here

JIRA: hereKAFKA-5233

Released: 1.0..0

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

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Currently punctuate is triggered by the advance of the task's timestamp, which is the minimum of the timestamps of all input partitions. By default this means the event-time from the messages but a custom TimestampExtractor can be provided to use system-time instead of event-time. However, in that case the triggering of punctuate is still driven by the arrival of messages to all partitions and not by the advance of the system-time itself. The effect is that if any one of the input partitions has messages arriving irregularly, punctuate will be also be called at irregular intervals and in the extreme case not called at all if any one of the input partitions doesn't receive any messages.

Public Interfaces

org.apache.kafka.streams.processor.Processor

org.apache.kafka.streams.processor.ProcessorContext

org.apache.kafka.streams.kstream.Transformer

org.apache.kafka.streams.kstream.ValueTransformer

 See below

Terminology

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The proposal is to deprecate the current punctuate() method on Processor, Transformer and ValueTransformer interfaces:

@Deprecated
void punctuate(long timestamp); // current

...

@Deprecated
void schedule(long interval); //current, stream-time semantics

voidCancellable schedule(long interval, PunctuationType type, Punctuator callback); //new
// We could allow this to be called once for each value of PunctuationType to mix approaches.

...

enum PunctuationType {
  STREAM_TIME,
  WALL_CLOCK_TIME,
}

And Cancellable is 

interface Cancellable {
  void cancel();
}

Cancellable return type is provided to cater for more complicated use cases as such described in the Punctuate Use Cases sub page. For those cases requiring stream-time based punctuation with a system-time upper bound (aka "hybrid" punctuation semantics) the following pattern can be used:

ProcessorContext context;
long streamTimeInterval = ...;
long systemTimeUpperBound = ...; //e.g. systemTimeUpperBound = streamTimeInterval + some tolerance 
Cancellable streamTimeSchedule;
Cancellable systemTimeSchedule;
 
public void init(ProcessorContext context){
	this.context = context;
	streamTimeSchedule = context.schedule(PunctuationType.STREAM_TIME, streamTimeInterval,   this::streamTimePunctuate);
	systemTimeSchedule  SYSTEM_TIME,
}

Other alternatives under discussion:

= context.schedule(PunctuationType.WALL_CLOCK_TIME, systemTimeUpperBound, this::systemTimePunctuate);	
}
 
public void streamTimePunctuate(long streamTime){
	periodicBusiness(streamTime);

	systemTimeSchedule.cancel();
	systemTimeSchedule = context.schedule(PunctuationType.WALL_CLOCK_TIME, systemTimeUpperBound, this::systemTimePunctuate);
}
 
public void systemTimePunctuate(long systemTime){
	periodicBusiness(context.timestamp());

	streamTimeSchedule.cancel();
	streamTimeSchedule = context.schedule(PunctuationType.STREAM_TIME, streamTimeInterval, this::streamTimePunctuate);
}
 
public void periodicBusiness(long streamTime){ 
	// guard against streamTime == -1, easy enough.
	// if you need system time instead, just use System.currentTimeMillis()
 
	// do something businessy here
}

Compatibility, Deprecation, and Migration Plan

The following methods will be deprecated

• Processor.punctuate(long timestamp),
• Transformer.punctuate(long timestamp),
• ValueTransformer.punctuate(long timestamp),
• ProcessorContext.schedule(long interval);

The deprecated methods shall remain for some time along the newly added ones to allow for a smooth migration.

The semantics of the deprecated methods shall remain unchanged.

A call to the deprecated ProcessorContext.schedule(interval) from within a Processor will be equivalent to:

context.schedule(interval, PunctuationType.STREAM_TIME, this::punctuate);

A call to the deprecated ProcessorContext.schedule(interval) from within a Transformer will be equivalent to:

context.schedule(interval, PunctuationType.STREAM_TIME, timestamp -> {
	KeyValue<K,V> pair = punctuate(timestamp);
	if (record != null) {
		context.forward(pair.key, pair.value);
	}
});

Test Plan

Stream time, system time and a mix of both PunctuationTypes should be tested. Existing test for punctuation can be re-used to guide the test cases for stream time only. System time and mixed stream & system time tests will have to be developed.

Rejected Alternatives

(A) Change the semantics of punctuate() to be purely "system time driven", instead of "part time driven, and part data-driven". That is, the punctuate function triggering will no longer be dependent whether there are new data arriving, and hence not on the timestamps of the arriving data either. Instead it will be triggered only by system wall-clock time.

...

• The above approach changes the semantics of the punctuate method and therefore is not backward-compatible.
• It is not clear if doing data-driven periodic operations from the process() method without the intricate calculations of minimum timestamp per input partition is sufficient to cater for all use cases that may be attainable using present day stream-time based punctuate 

(B) An alternative could be to leave current semantics as the defaults for the punctuate method but allow a configuration switch between event and system time.

...

(E) Finally, the hybrid approach (this is convenient for the use cases in in Punctuate Use Cases but difficult to reason about):

/**
* Schedule punctuate at stream-time intervals with a system-time upper bound. 
* For pure system-time based punctuation streamTimeInterval can be set to -1 == infinite
* and systemTimeUpperBound to the desired interval
*/
schedule(Punctuator callback, long streamTimeInterval, long systemTimeUpperBound); 

/**
* Schedule punctuate at stream-time intervals without a system-time upper bound,
* i.e. pure stream-time based punctuation
*/
schedule(Punctuator callback, long streamTimeInterval);

...

• It's been argued this type of hybrid punctuation is more difficult to reason about than separate stream-time and system-time punctuations and the approach need further thought
• Some problems with this algorithm have been identified for edge case scenarios (see see discussion thread)
• The various trade-offs of this approach may better be left to the the users as per the mantra "make simple thing easy and complex things possible"

The current proposal opens the door to adding more PunctuationTypes in the future and so after further discussion and in a separate KIP, other approaches such as the hybrid one can be added later on.

Compatibility, Deprecation, and Migration Plan

We shall deprecate the following methods

Processor.punctuate(long timestamp), Transformer.punctuate(long timestamp), ProcessorContext.schedule(long interval);

The deprecated methods shall remain for some time along the newly added ones to allow a smooth migration.

The semantics of the deprecated methods shall remain unchanged.

A call to the deprecated ProcessorContext.schedule(interval) from within a Processor will be equivalent to:

context.schedule(PunctuationType.STREAM_TIME, this::punctuate);

A call to the deprecated ProcessorContext.schedule(interval) from within a Transformer will be equivalent to:

context.schedule(PunctuationType.STREAM_TIME, timestamp -> {
	KeyValue<K,V> pair = punctuate(timestamp);
	if (record != null) {
		context.forward(pair.key, pair.value);
	}
});

Test Plan

Stream time, system time and a mix of both PunctuationTypes should be tested. Existing test for punctuation can be re-used to guide the test cases for stream time only. System time and mixed stream & system time tests will have to be developed.

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 wayHowever, hybrid semantics can be implemented on top of the 2 PunctuationType callbacks, as show in the Proposed Changes section. This gives users more flexibility in addressing the various trade-offs inherent in this design as is most appropriate to their use case.