Thanks for the reply, I have a question about rolling upgrades, the part marked in the following text, is it understood that it is not allowed to accept multiple ES versions for a long time during the upgrade? However, the actual rolling upgrade requires a gradual upgrade of nodes one by one, and needs to wait for the node status to recover and the cluster status to recover to green, which may take a long time. During this period, two versions of ES must exist in the cluster at the same time.
Rolling upgrades
A rolling upgrade allows an Elasticsearch cluster to be upgraded one node at a time so upgrading does not interrupt service. Running multiple versions of Elasticsearch in the same cluster beyond the duration of an upgrade is not supported, as shards cannot be replicated from upgraded nodes to nodes running the older version.