This setting, which prior to Elasticsearch 5 was enabled by default and caused all kinds of
confusion, has since been disabled by default and is not recommended for production use. The
preferred way going forward is for users to explicitly specify separate data folders for each started
node to ensure that each node is consistently assigned to the same data path.
Relates to #42426
Downgrading an Elasticsearch node to an earlier version is unsupported, because
we do not make any attempt to guarantee that a node can read any of the on-disk
data written by a future version. Yet today we do not actively prevent
downgrades, and sometimes users will attempt to roll back a failed upgrade with
an in-place downgrade and get into an unrecoverable state.
This change adds the current version of the node to the node metadata file, and
checks the version found in this file against the current version at startup.
If the node cannot be sure of its ability to read the on-disk data then it
refuses to start, preserving any on-disk data in its upgraded state.
This change also adds a command-line tool to overwrite the node metadata file
without performing any version checks, to unsafely bypass these checks and
recover the historical and lenient behaviour.
Added documentation for node repurpose tool and included documentation on how to repurpose nodes safely. Adjusted order of tools in `elasticsearch-node` tool since the repurpose tool is most likely to be used.
Co-Authored-By: David Turner <david.turner@elastic.co>
