How do you check replication is happening between 2 data centers in
different geographical location?
Replication happens in real time. If a keyspace is replicated to both DC1 and DC2, the writes will be sent at the same time to all replicas in all DCs.
If a replica happens to be down (for whatever reason), the coordinator of the write request will store a hint (mutation) provided the consistency level for the write has been satisfied. When the replica comes back online, the coordinator will handoff the hint (replay the missed mutation) to the replica.
You will know when replication isn't being satisfied when nodes go down, or when you see dropped mutations either in the logs or the output of
nodetool tpstats. You would have learned all this when you completed the DS201 Cassandra Foundations and DS210 Cassandra Operations courses at the DataStax Academy when you got certified so I don't think I'm telling you anything you don't already know.
In any case, you can read more about the topic in Data replication in Cassandra. Cheers!
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