With this change, we won't wait for the local checkpoint to advance to
the max_seq_no before starting phase2 of peer-recovery. We also remove
the sequence number range check in peer-recovery. We can safely do these
thanks to Yannick's finding.
The replication group to be used is currently sampled after indexing
into the primary (see `ReplicationOperation` class). This means that
when initiating tracking of a new replica, we have to consider the
following two cases:
- There are operations for which the replication group has not been
sampled yet. As we initiated the new replica as tracking, we know that
those operations will be replicated to the new replica and follow the
typical replication group semantics (e.g. marked as stale when
unavailable).
- There are operations for which the replication group has already been
sampled. These operations will not be sent to the new replica. However,
we know that those operations are already indexed into Lucene and the
translog on the primary, as the sampling is happening after that. This
means that by taking a snapshot of Lucene or the translog, we will be
getting those ops as well. What we cannot guarantee anymore is that all
ops up to `endingSeqNo` are available in the snapshot (i.e. also see
comment in `RecoverySourceHandler` saying `We need to wait for all
operations up to the current max to complete, otherwise we can not
guarantee that all operations in the required range will be available
for replaying from the translog of the source.`). This is not needed,
though, as we can no longer guarantee that max seq no == local
checkpoint.
Relates #39000Closes#38949
Co-authored-by: Yannick Welsch <yannick@welsch.lu>
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