Cross-Region Replication (CRR): Geographic Data Copying
Cross-Region Replication automatically copies data to another geographic region, like a live backup. Use it for disaster recovery, lower latency for global users, or compliance. The footgun: it only copies *new* objects, not what's already in the bucket.
Why it exists
Storing critical data in a single geographic location creates risk. A regional outage could take your service offline, latency could be high for distant users, and you might not meet data residency laws. Cross-Region Replication was created to solve this by maintaining a synchronized data copy in a separate physical region.
The mental model
Think of CRR as a live sync to a remote office. As new documents arrive at your main office (the source bucket), a copy is automatically sent to the branch office (the destination bucket) in another city. This ensures the branch has the latest files for local access and business continuity if the main office becomes unavailable.
How it works
CRR is an asynchronous, live replication feature for S3. When you upload a new object or update an existing one in the source bucket, S3 automatically copies it to a pre-configured destination bucket in a different AWS Region. The process copies the object data and its metadata, like creation time and version ID. While not instantaneous, S3 Replication Time Control (S3 RTC) offers an SLA to replicate 99.99% of new objects within 15 minutes. Crucially, CRR only applies to objects created after you enable it.
When to use it
Use CRR for three main scenarios. First, for disaster recovery, allowing you to fail over to a secondary region during an outage. Second, to minimize latency for a global user base by serving content from the region closest to them. Third, to meet compliance requirements that mandate data be stored in specific, geographically separate locations.
When not to use it
Do not rely on CRR as a standalone solution for backing up an existing bucket. It does not replicate objects that were present before replication was configured; for that, you must use S3 Batch Replication. If your only goal is a logical copy within the same region (e.g., against accidental deletion), Same-Region Replication (SRR) is the correct tool.
One canonical example
A media company stores its video masters in an S3 bucket in us-east-1. To ensure business continuity and serve European customers faster, they configure CRR to replicate all new video uploads to a bucket in eu-west-1. If the US region has an issue, they can fail over their applications to serve content from the European bucket.
Interview question
A developer configures Cross-Region Replication (CRR) for an existing S3 bucket. What is a critical behavior they should be aware of regarding the replication process?
- a.The replication process is synchronous, ensuring immediate consistency across regions.
- b.Objects already present in the source bucket before CRR activation will not be replicated.Correct
- c.CRR will only replicate objects that have public read access enabled.
- d.Replication only occurs for objects updated, not newly uploaded, after CRR is enabled.
Why? this is the answer
The card explicitly states that CRR "does not replicate objects that were present before replication was configured." It only applies to new objects or updates made after CRR is enabled. Option A is incorrect because the card describes CRR as an "asynchronous" feature.
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