“Amazon S3 can now perform conditional writes that evaluate if an object is unmodified before updating it.”
S3 now checks an object’s ETag via an if-match header before PutObject or CompleteMultipartUpload overwrites it. That is compare-and-swap, at no extra charge, in every Region. It took AWS until 2024 to ship the primitive every storage system needs for safe concurrent writes. Customers spent the years before that building it themselves.