Amazon S3 can now perform conditional writes that evaluate if an object is unmodified before updating it. This helps you coordinate simultaneous writes to the same object and prevents multiple concurrent writers from unintentionally overwriting the object without knowing the state of its content. You can use this capability by providing the ETag of an object using S3 PutObject or CompleteMultipartUpload API requests in both S3 general purpose and directory buckets.

Conditional writes simplify how distributed applications with multiple clients concurrently update data across shared datasets. Similar to using the HTTP if-none-match conditional header to check for the existence of an object before creating it, clients can now perform conditional-write checks on an object’s Etag, which reflects changes to the object, by specifying it via the HTTP if-match header in the API request. S3 then evaluates if the object’s ETag matches the value provided in the API request before committing the write and prevents your clients from overwriting the object until the condition is satisfied. This new conditional header can help improve the efficiency of your large-scale analytics, distributed machine learning, and other highly parallelized workloads by reliably offloading compare and swap operations to S3.

This new conditional-write functionality is available at no additional charge in all AWS Regions. You can use the AWS SDK, API, or CLI to perform conditional writes. To learn more about conditional writes, visit the S3 User Guide.