Suppose a broker is down for long time, now what happens to the followers and leaders that this broker was containing?
if broker was containing a leader and one of in-sync replica is chosen as leader, does it create another in-sync replica(in case we have specific replication factor)?
if broker was containing a follower, does it create another follower elsewhere in cluster?
now suppose broker awakens after long time, now do leaders and followers are restored as it left when it went down?
What happens when a broker is down depends on your configuration. It mostly depends on these configuration settings:
min.insync.replicasdefault.replication.factorunclean.leader.election.enableKafka does not create a new replica when a broker goes down.
If the offline broker was a leader, a new leader is elected from the replicas that are in-sync. If no replicas are in-sync it will only elect an out of sync replica if unclean.leader.election.enable is true, otherwise the partition will be offline.
If the offline broker was a follower, it will be marked a out of sync by the leader.
When restarting the broker, it will try to get back in sync. Once done, whether it stays a follower or becomes the leader depends if it is the prefered replica.
Finally if you know a broker will be offline for a long time and still require a replica, you can use the reassignment tool kafka-reassign-partitions.sh to move partitions to online brokers.