keep track of shard follow stats inside shard follow stats' node task instead of persistent task status.
By maintaining the shard follow stats inside its node task the stats update is quicker as
no cluster state update is required. The stats are now transient; meaning if the task
is going to run a different node then the stats are gone too. Currently only the processed
global checkpoint is being tracked and this is being restored when a shard follow node task
starts via the indices stats api (the reason of the first change of this change). Other stats
that we may add in the future (like fetch_time, see: https://gist.github.com/s1monw/dba13daf8493bf48431b72365e110717)
it is ok if we start from zero in case a shard follow task moves to another node.
This limit is based on the number of estimate bytes in each translog
operation that fall between the minimum and maximum request sequence number.
If this limit is met then the shard follow task executor will make sure
that a subsequent shard changes request will be performed to fetch the
remaining translog operations.
This limit is needed in order to protect against returning too many
translog operations in a single shard changes response.
Relates to #2436
We check for the existence of both leader and follower index, then properly
report to the caller. However, we do not return after reporting failure. This
causes the caller receive exception twice: IllegalArgumentException then
NullPointerException. This commit makes sure to stop the action after reporting
failure.
Martijn van Groningen
Nhat Nguyen