Hi Gil, thank you very much for these information, especially the links. Those are mighty useful indeed!
I understand and fully subscribe to these changes and see the benefits of having the Common Schema: it actually helps our single observability stack project moving forward.
From our side, we created ingest rules for user_agent and geoip, and as an example, mapped context.user.ip and context.system.ip to normalise on our custom user.geoip.* fields with a definition for geopoint and other related data.
With the coming change, it means either our current ingest and fields definitions have to change (easier but may not conform to ECS intentions), or consumers of our custom user.geoip.* fields have to be modified (visualisations, Query DSLs consumed by external systems, Timelion, and aggregations). No big deal really; we'll just have to make sure we align with the core principles of the ECS.
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One other thing, we were looking to use some 3rd party Kibana plugins but are no longer sure if it would be a good idea to proceed with that due to the impending 7.x release.