S3 to Lambda event ordering across object keys: is there a guarantee?

When multiple objects are uploaded to a bucket in rapid succession, the application expects the corresponding Lambda functions to process events in the same order as the uploads. The goal is to determine whether S3 event notifications preserve a global order across all object keys, or if ordering is only guaranteed per key, and how Lambda concurrency limits might introduce reordering.

Constraints include the asynchronous nature of S3 notifications, the lack of documented global ordering guarantees, and the potential for Lambda pool saturation to delay or reorder events. Understanding these behaviors is essential before applying any optimization or ordering logic in downstream services.

Specific questions:

• Does S3 guarantee a global order of event notifications across all object keys in a bucket?
• If not, what mechanisms can be employed to detect or mitigate reordering when processing events in Lambda?
• Are there regional or account-level settings that influence the degree of reordering observed?