Spring Kafka Consumer Configs - default values and at least once semantics

Question

I am writing kafka consumer using spring-kafka template. When I am instantiating consumers, Spring kafka takes in parameters like the following.

props.put(ConsumerConfig.ENABLE_AUTO_COMMIT_CONFIG, false);
props.put(ConsumerConfig.FETCH_MAX_BYTES_CONFIG, fetchMaxBytes);
props.put(ConsumerConfig.MAX_PARTITION_FETCH_BYTES_CONFIG, maxPartitionFetchBytes);

I read the documentation and it looks like there are lots of other parameters that can be passed as the consumer configs too. Interestingly, each of these parameter has a default value. My question is

1. On what basis were these arrived?
2. Will there be a real-need to change these values, if so what would be those
   (IMHO, this is on case by case basis. But still would like to hear it from experts)
3. The delivery semantic we have is atleast once. So, for this (atleast-once) delivery semantic, should these be left untouched and it would still process high volume of data.

Any pointers or answers would be of great help in clarifying my doubts.

Answer 1

The default values are an attempt to serve most of the use cases around Kafka. However, it would be an illusion to assume that those many different configurations can be set to serve all use cases.

A good starting point to understand the default values is the plain-Kafka ConsumerConfiguration and for Spring its documentation. In the Confluence docs you will also find for each configuration the "Importance". If this importance is set to high, it is recommended to really think about it. I have given some more background on the importance here.

at-least-once

For at least once semantics you want to control the commits of the consumed messages. For this, enable.autto.commit needs to be set to false which is the default value since spring version 2.3). In addition the AckMode is per default set to BATCH which is the basis for a at least once semantics.

So, depending on your Spring version it looks like you can leave the default configuration to achieve at-least-once semantics.