Why is Kafka pull-based instead of push-based?

Question

Why is Kafka pull-based instead of push-based? I agree Kafka gives high throughput as I had experienced it, but I don\'t see how Kafka throughput would go down if it were to

Answer 1

Scalability was the major driving factor when we design such systems (pull vs push). Kafka is very scalable. One of the key benefits of Kafka is that it is very easy to add large number of consumers without affecting performance and without down time.

Kafka can handle events at 100k+ per second rate coming from producers. Because Kafka consumers pull data from the topic, different consumers can consume the messages at different pace. Kafka also supports different consumption models. You can have one consumer processing the messages at real-time and another consumer processing the messages in batch mode.

The other reason could be that Kafka was designed not only for single consumers like Hadoop. Different consumers can have diverse needs and capabilities.

Pull-based systems have some deficiencies like resources wasting due to polling regularly. Kafka supports a 'long polling' waiting mode until real data comes through to alleviate this drawback.

Answer 2

Refer to the Kafka documentation which details the particular design decision: Push vs pull

Major points that were in favor of pull are:

1. Pull is better in dealing with diversified consumers (without a broker determining the data transfer rate for all);
2. Consumers can more effectively control the rate of their individual consumption;
3. Easier and more optimal batch processing implementation.

The drawback of a pull-based systems (consumers polling for data while there's no data available for them) is alleviated somewhat by a 'long poll' waiting mode until data arrives.

Answer 3

Others have provided answers based on Kafka's documentation but sometimes product documentation should be taken with a grain of salt as an absolute technical reference. For example:

• Numerous push-based messaging systems support consumption at different rates, usually through their session management primitives. You establish/resume an active application layer session when you want to consume and suspend the session (e.g. by simply not responding for less than the keepalive window and greater than the in-flight windows...or with an explicit message) when you want to stop/pause. MQTT and AMQP, for example both provide this capability (in MQTT's case, since the late 90's). Given that no actions are required to pause consumption (by definition), and less traffic is required under steady stable state (no request), it is difficult to see how Kafka's pull-based model is more efficient.
• One critical advantage push messaging has vs. pull messaging is that there is no request traffic to scale as the number of potentially active topics increases. If you have a million potentially active topics, you have to issue queries for all those topics. This concern becomes especially relevant at scale.
• The critical advantage pull messaging has vs push messaging is replayability. This factors a great deal into whether downstream systems can offer guarantees around processing (e.g. they might fail before doing so and have to restart or e.g. fail to write messages recoverably).
• Another critical advantage for pull messaging vs push messaging is buffer allocation. A consuming process can explicitly request as much data as they can accommodate in a pre-allocated buffer, rather than having to allocate buffers over and over again. This gains back some of the goodput losses vs push messaging from query scaling (but not much). The impact here is measurable, however, if your message sizes vary wildly (e.g. a few KB->a few hundred MB).
• It is a fallacy to suggest that pull messaging has structural scalability advantages over push messaging. Partitioning is what is usually used to provide scale in messaging applications, regardless of the consumption model. There are push messaging systems operating well in excess of 300M msgs/sec on hard wired local clusters...125K msgs/sec doesn't even buy admission to the show. In fact, pull messaging has inferior goodput by definition and systems like Kafka usually end up with more hardware to reach the same performance level. The benefits noted above may often make it worth the cost. I am unaware of anyone using Kafka for messaging in high frequency trading, for example, where microseconds matter.

It may be interesting to note that various push-pull messaging systems were developed in the late 1990s as a way to optimize the goodput. The results were never staggering and the system complexity and other factors often outweigh this kind of optimization. I believe this is Jay's point overall about practical performance over real data center networks, not to mention things like the open Internet.