Blocker Description
The new built-in "sticky" partitioner switches partitions based on the amount of bytes produced to a partition. It doesn't use batch creation as a switch trigger. The previous "sticky" DefaultPartitioner switched partition when a new batch was created and with small linger.ms (default is 0) could result in sending larger batches to slower brokers potentially overloading them. See https://cwiki.apache.org/confluence/display/KAFKA/KIP-794%3A+Strictly+Uniform+Sticky+Partitioner for more detail.
However, the with large linger.ms, the new built-in partitioner may create suboptimal batches. Let's consider an example, suppose linger.ms=500, batch.size=16KB (default) and we produce 24KB / sec, i.e. every 500ms we produce 12KB worth of data. The new built-in partitioner would switch partition on every 16KB, so we could get into the following batching pattern:
- produce 12KB to one partition in 500ms, hit linger, send 12KB batch
- produce 4KB more to the same partition, now we've produced 16KB of data, switch partition
- produce 12KB to the second partition in 500ms, hit linger, send 12KB batch
- in the mean time the 4KB produced to the first partition would hit linger as well, sending 4KB batch
- produce 4KB more to the second partition, now we've produced 16KB of data to the second partition, switch to 3rd partition
so in this scenario the new built-in partitioner produces a mix of 12KB and 4KB batches, while the previous DefaultPartitioner would produce only 12KB batches – it switches on new batch creation, so there is no "mid-linger" leftover batches.
To avoid creation of batch fragmentation on partition switch, we can wait until the batch is ready before switching the partition, i.e. the condition to switch to a new partition would be "produced batch.size bytes" AND "batch is not lingering". This may potentially introduce some non-uniformity into data distribution, but unlike the previous DefaultPartitioner, the non-uniformity would not be based on broker performance and won't re-introduce the bad pattern of sending more data to slower brokers.
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