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This is the process for in-place upgrades of Elasticsearch (ES), using an existing Search feed and index data.

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1. It fixes /etc/sysconfig/elasticsearch to the correct ES6 version (the same as ES7).

2. It increases the systemd timeout in /etc/systemd/system/elasticsearch.service.d/override.conf (see github.com/elastic/elasticsearch/issues/60140)

3. A prompt to continue with the yum upgrade to 7.5.2 appears after refreshing the config files for Elasticsearch 6.

4. It disables shard allocation and does a POST synced-flush for safer rolling upgrades. 

   Info
   title Important
   Disabling shard allocation or sync-flush can fail to contact the node, but do not proceed to upgrade the next node until the cluster health is green again.

5. It uninstalls the Prometheus Exporter plugin if it exists.

6. It shells out to yum to install the Elasticsearch 7 RPM in the current directory or from artifacts.elastic.co, if unavailable.

7. It updates elasticsearch.yml for version 7 compatibility, including discovery.initial_master_nodes instead of discovery.zen.unicast.hosts, and jvm.options.

8. It starts the upgraded Elasticsearch 7 and waits for it to be ready.

9. The cluster re-enables shard allocation and prompts to repeat these two steps on the next node if the cluster health is green or yellow.