Status

Current state: Under Discussion

Discussion thread: TBD 

JIRA: KAFKA-8770 

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

Motivation

Currently, in Kafka Streams, we support two modes of processing:

• emit on update
• emit on window close

The first one has been present for the majority of Kafka Stream's inception, when results are forwarded regardless . The second option has been added by implementing the so-called "suppression" operator which collects results throughout the window, and extending into the grace period. However, one mode of processing should've been implemented long ago, and that is emit on change. As mentioned in the JIRA, in many cases, the user would process a prior result to yield a new one, yet, this new result is forwarded to a downstream processor even though the operation performed had been effectively a no-op. This is problematic in certain circumstances where there is a high number of such operations, leading to an enormous number of useless results being forwarded down the topology.

Proposed Behavior Changes

For behavior changes, the user should expect the following:

1. No-op operations will only be dropped from KTables, not from other classes.
2. Streams will drop no-ops for stateful operations. 
   1. If the user is very concerned about a particular stateless operation producing a lot of no-ops, than the user can simply materialize the operation, and then Streams will automatically drop all no-ops.
   2. In certain situations where there is a high volume of no-ops throughout the Streams DAG, it will be recommended practice to materialize all operations to reduce traffic overall across the entire network of nodes.

Above are the main behavior changes that we should expect with this KIP. To reiterate, the primary benefit of this KIP is a performance enhancement. In some topologies, records being sent could have a large proportion of no-ops which leads to increased traffic while at the same time, the extra records sent are of no value. This KIP intends to eliminate this flaw of Kafka Streams.

Design Reasoning

With the current default model of emission, we are forwarding the processed results downstream regardless if  it has changed or not. After some discussion, there are a couple of points that I would like to emphasize:

1. There has been some thoughts as to only coupling emit on change with the suppress operator. However, that approach has been abandoned in favor of a more extensive change which would impact almost all KTable operations supported by Kafka Streams (it might be that we will restrict emit-on-change to materialized KTables). Our justification is that no-op changes should be suppressed even before they are forwarded downstream, as the duplication of these useless results has the potential to explode across multiple downstream nodes once it has been forwarded. The expected behavior is the following:
   1. Any operations that have resulted in a no-op would be discarded. Exceptions would potentially be made for non-materialized KTables. Some might have concerns about a KTable constructed through aggregations. In this case, if a no-op result is produced, how do we determine which timestamp to use? In this case, we would need some configuration i.e. timestamp.aggregation.selection.policy  which will determine if the minimum or maximum timestamp of the aggregation is selected (this configuration is still up for debate).
   2. Any subsequent results which yield a no-op will not be forwarded. That also means that timestamps will not be updated for the corresponding keys. This applies for all non-aggregation operations.
2. About configurations. As noted, we intend that Kafka Streams be as simple as possible when it comes to configurations. In that case, this KIP proposes that instead, emit-on-change becomes the new behavior of Kafka Streams. Since emit-on-change is more correct, (no-ops shouldn't really be sent in the first place), there is a strong case for no additional config that allows the user to "opt-out" of this change (and remain with emit-on-update).

The other crucial aspect of our proposal is why we restrict ourselves to dropping no-ops for stateful operations. The reasoning is two-fold:

1. If we are loading the prior result in a stateless operation, there will be a significant performance impact. NOTE: However, this can be subject to change: if we can generate some integer id (such id generation would likely fall on the user for implementation) which can reflect changes in the processed result, then we might be able to extend dropping no-ops to stateless operations as well. This is still an alternative worth considering.
2. The other reason is that if we are loading a prior result in its entirety for a stateless operation, we are essentially replicating some functions of a stateful operator into a stateless one. After all, a stateless operator was never intended to load a prior result (only a stateful operator should do such a thing). That means there would be some redundancy between stateful and stateless operators. However, this discrepancy (stateless operations don't drop no-ops while stateful operations do) can result in much confusion from user.

Alternative Approaches

There is a possibility where we can support emit-on-change for all operations.

There is more than one way to yield the prior result. After all, we can obtain it from an upstream processor. For most operations, we can forward downstream both the old and new results of the upstream processor. In this case, the same operation will be performed twice. However, each operation can be very expensive. Performing it twice will in other words has the potential to incur horrendous performance hits. It might be that this is not a serious issue, but it is of significant concern. 

The main bottleneck for emit-on-change for stateless operations is really how to load the prior result. That does not necessarily have to be done. Earlier in our discussion, we have talked about using a hash code as a way of uniquely identifying the results, and then comparing those hash codes. But as noted, hash codes can vary across JVMs, and it is not a requirement that the programs on different runs return the same hash code for the exact same object. Instead, we can consider using some method distinct from Object#hashCode(). There is the possibility here that we can add a configuration for allowing emit-on-change for stateless operations. If emit-on-change is enabled, then we can use some method defined by the user i.e. generateUniqueObjectId(V result)  returning a 32-bit or 64-bit integer as an id – this method which will have stricter constraints than a normal hash code function.  This method would be used as is the hash codes described in the Implementation section below. We store these ids instead, and compare these for equality.

This potentially can work, but the user must implement the provided method correctly. This must be stressed in further documentation.

Implementation [DISCARDED]

NOTE: THIS SECTION HAS BEEN DISCARDED AS A PORTION OF THE KIP. IT HAS BEEN LEFT ONLY IN THE KIP FOR RECORD-KEEPING PURPOSES. FOR THOSE WHO ARE INTERESTED, THE CORE APPROACH (STORING HASH CODES INSTEAD OF ACTUAL VALUES) HAS BEEN REJECTED. THIS IS SINCE THE BEHAVIOR OF HASH CODES IS SUBJECT TO WIDE VARIATION ACROSS MULTIPLE SYSTEMS, AND COULD NOT BE USED RELIABLY TO COMPARE THE ACTUAL VALUES OF PROCESSED RESULTS.

This section is to discuss some points on how we should approach the implementation of this KIP. Some extra configurations will probably result from the implementation, but that is still debatable. See below for possible additions.   

Details on Core Improvement

Emit on change, in many aspects, is basically an improved version of emit on update, except the crucial improvement is that we check if the new result had been changed by a given operation. And for doing such a check, if we want to be thorough, load the previous result and compare it with the most recent one. However, this would not be doable in some cases without incurring extra performance hits. Therefore, rather than load the entire result, we should compare the previous result's hash code with the current result's hash code. Admittedly, there might be a small chance of collisions where the new result with changed values would still yield the same hash code. Yet, it could be safe to say that a good hash function would never let such a thing happen.

However, the main constraint for this approach, if we were to follow through, is that the hash code must reflect any changes in the individual component fields of the result. The key building block for this entire optimization is that the user provides a good hash code override of the given value type. Of course, this isn't always the case.  After all, it isn't safe to assume that the user has provided a safe hash code function for us to take advantage of. In conclusion, we can probably come up with the following choices for the user:

1. Completely disregard the hash code and load the prior result and compare it to the new one.
2. Compare only the hash codes of the given objects instead.

The second option quite obviously is the recommended one. While it is uncertain if we will need to give the user the ability to choose between these two options, it would probably be necessary. In the latter option, it should be emphasized that the hash function must be chosen carefully. In many cases, Kafka already loads the prior result anyway. So for instances where we already load the previous result, we will just use that instead of the hash code (if we are to implement option 2). 

Discourse on other areas

The timestamps of the results processed would also present a serious design challenge for this KIP. The requirements of emit on change would entail the following behavior:

1. Let's say we receive record 1 with timestamp 0 which leads to a change in the result processed. 
2. Afterwards, we receive another record with timestamp 100 which would lead to a no-op. 

It is required that the timestamp be 0. 

To resolve this situation, the current best bet is probably to load the timestamp along with the hash code / full prior result. 

Compatibility, Deprecation, and Migration Plan

If the user has logic that involves actively filtering out no-op operations, then they will be able to remove this code in a future release once this KIP has been integrated into Kafka Streams. For now, this KIP will aim to add a minimum number of configurations, and avoid adding APIs. If the user has any logic regarding timestamps, there might be a chance that their program will be affected. This is specific however to the individual user, and behavior will have to be changed accordingly.

Rejected Alternatives

For configurations, we have decided that an opt-out config is not necessary. Emit-on-change as a whole is more correct when it comes to Kafka Streams. The user should have little reason not to want this change, particularly since this KIP should not have a severe impact on performance (it is possible in implementation that we will only check if two values are equal in KTables where prior results has already been loaded).