A bucket lives in one region. If the region has a bad day, so does the data. Replication keeps a copy of every new object in a second bucket, usually in another region (cross-region replication); the same region also works.

Three things must be in place:

1. Versioning on both buckets.
2. A role that trusts s3.amazonaws.com, with permission to read the source and write to the destination. S3 does the copying, as itself, using that role.
3. A replication configuration on the source.

The configuration

{
  "Role": "arn:aws:iam::123456789012:role/s3-replication",
  "Rules": [{
    "ID": "to-ireland",
    "Status": "Enabled",
    "Priority": 1,
    "Filter": {},
    "DeleteMarkerReplication": { "Status": "Disabled" },
    "Destination": { "Bucket": "arn:aws:s3:::market-docs-dr-1bocwlg" }
  }]
}

aws s3api put-bucket-replication --bucket market-docs-1bocwlg --replication-configuration file://replication.json

From then on, every new object is copied within seconds to minutes. Objects that were already there are not copied.

Checking a copy

aws s3api head-object --bucket market-docs-1bocwlg --key prices.csv --query ReplicationStatus

On the source, PENDING and then COMPLETED; on the copy in the other bucket, REPLICA. FAILED usually means the role cannot write to the destination.