Eventual Consistency Latency in IBM Cloud Object Storage Cross-Region Buckets

Direct Answers

Documented maximum propagation window: IBM does not publish a service-guaranteed maximum propagation window for metadata updates in Cross-Region buckets. The official documentation describes eventual consistency for overwrites but does not specify an upper bound.

Distance-based predictability: Observed latency generally correlates with physical distance between the writing and reading regional endpoints because replication traverses IBM's global backbone. However, predictability is limited by dynamic routing, regional congestion, and replication topology changes. You cannot reliably calculate a latency budget from distance alone.

Confirmed Behavior vs. Observed Patterns

• New object creation — strong read-after-write consistency in all regions.
• Overwrites/updates — eventual consistency with Last-Writer-Wins resolution across all regions, including Cross-Region buckets.
• Typical observed latency — seconds to a few minutes under normal load; can extend further under heavy concurrent writes to the same key or during regional congestion.
• Staleness window — unbounded by SLA; a GET may return a stale version until the next replication cycle completes.

Practical Verification Steps

1. From Region A, issue a PUT to overwrite an existing object in a Cross-Region bucket.
2. Immediately issue HEAD or GET requests against the same key from Region B and Region C endpoints.
3. Record the timestamp of the PUT response and the first response from each reading region that returns the new ETag/version ID.
4. Repeat under your expected concurrent load to capture tail latency.

# Example using AWS CLI (compatible with COS S3 API)
aws s3api put-object --bucket my-cross-region-bucket --key shared-key --body /path/to/file --endpoint-url https://s3.us-south.cloud-object-storage.appdomain.cloud

# Poll from another region
aws s3api head-object --bucket my-cross-region-bucket --key shared-key --endpoint-url https://s3.eu-gb.cloud-object-storage.appdomain.cloud

Assumptions and Uncertainty

• Behavior described applies to IBM Cloud Object Storage Standard and Vault tiers as of 2024–2025; Smart Tier may have different replication characteristics.
• Cross-Region bucket topology (e.g., us-south + eu-gb + ap-tokyo) affects replication paths; not all region pairs share the same backbone links.
• No public SLA metric covers propagation latency; IBM Cloud Service Level Documentation for COS focuses on availability and durability, not consistency windows.

One Diagnostic Detail Needed

What is the typical concurrent write rate (PUTs/second) targeting the same object key in your workload? This determines whether you should design for seconds-scale or minutes-scale staleness and whether application-level version checks or conditional writes are warranted.