Luke Chen created KAFKA-13008:
---------------------------------
Summary: Stream will stop processing data for a long time while
waiting for the partition lag
Key: KAFKA-13008
URL: https://issues.apache.org/jira/browse/KAFKA-13008
Project: Kafka
Issue Type: Bug
Affects Versions: 3.0.0
Reporter: Luke Chen
In KIP-695, we improved the task idling mechanism by checking partition lag.
It's a good improvement for timestamp sync. But I found it will cause the
stream stop processing the data for a long time while waiting for the partition
metadata.
I've been investigating this case for a while, and figuring out the issue will
happen in below situation (or similar situation):
# start 2 streams (each with 1 thread) to consume from a topicA (with 3
partitions: A-0, A-1, A-2)
# After 2 streams started, the partitions assignment are: (I skipped some
other processing related partitions for simplicity)
stream1-thread1: A-0, A-1
stream2-thread1: A-2
# start processing some data, assume now, the position and high watermark is:
A-0: offset: 2, highWM: 2
A-1: offset: 2, highWM: 2
A-2: offset: 2, highWM: 2
# Now, stream3 joined, so trigger rebalance with this assignment:
stream1-thread1: A-0
stream2-thread1: A-2
stream3-thread1: A-1
# Suddenly, stream3 left, so now, rebalance again, with the step 2 assignment:
stream1-thread1: A-0, *A-1*
stream2-thread1: A-2
# Now, note that, the partition A-1 used to get assigned to stream1-thread1,
and now, it's back. And also, assume the partition A-1 has slow input (ex: 1
record per 30 mins), and partition A-0 has fast input (ex: 10K records / sec).
So, now, the stream1-thread1 won't process any data until we got input from
partition A-1 (even if partition A-0 is buffered a lot, and we have
`{{max.task.idle.ms}}` set to 0).
The reason why the stream1-thread1 won't process any data is because we can't
get the metadata of partition A-1. And why we can't get the metadata? It's
because
# In KIP-695, we use consumer's cache to get the partition lag, to avoid
remote call
# The lag for a partition will be cleared if the assignment in this round
doesn't have this partition. check here. So, in the above example, the metadata
cache for partition A-1 will be cleared in step 4, and re-initialized (to null)
in step 5
# In KIP-227, we introduced a fetch session to have incremental fetch
request/response. That is, if the session existed, the client(consumer) will
get the update only when the fetched partition have update (ex: new data). So,
in the above case, the partition A-1 has slow input (ex: 1 record per 30 mins),
it won't have update until next 30 mins, or wait for the fetch session become
inactive for (default) 2 mins to be evicted. Either case, the metadata won't be
updated for a while.
In KIP-695, if we don't get the partition lag, we can't determine the partition
data status to do timestamp sync, so we'll keep waiting and not processing any
data. That's why this issue will happen.
*Proposed solution:*
# If we don't get the current lag for a partition, or the current lag > 0, we
start to wait for max.task.idle.ms, and reset the deadline when we get the
partition lag, like what we did in previous KIP-353
# Introduce a waiting time config when no partition lag, or partition lag
keeps > 0 (need KIP)
[~vvcephei] [~guozhang] , any suggestions?
cc [~ableegoldman] [~mjsax] , this is the root cause that in
[https://github.com/apache/kafka/pull/10736,] we discussed and thought there's
a data lose situation. FYI.
--
This message was sent by Atlassian Jira
(v8.3.4#803005)
