How to monitor AWS S3 cross-region replication process?
Still no answers, and still no CRR monitoring in place on AWS, here's minimalistic solution, any feedback and suggestions are welcome. This monitor will only show which S3 objects are missing from destination, it will not provide visibility on versions of replicated objects.

Prerequisites




Enable inventory reports on src (Source) and dst
(Destination) buckets
Create Athena tables (src_inventory, dst_inventory) based on inventory reports for each bucket


Monitor



Run Athena query after both inventory reports generated e.g. on daily basis, can be executed from Lambda, Step Function, build server or any type of cron job.

Example of Athena query which should show number of missing objects on destination bucket at the time of generation of inventory reports:

FROM src_inventory src 
LEFT JOIN dst_inventory dst 
ON src.key = dst.key 
WHERE dst.key is NULL
AND src.dt = '2018-11-11-08-00';


The result can be reported as CloudWatch metric and monitored with CloudWatch alarm.