Objects in OSS have ETag values that are used to identify whether changes are made to data on the server. However, these ETag values are not necessarily equal to the MD5 hashes of objects. We recommend that you do not use ETag values to verify data consistency.
To check whether an uploaded object in OSS is consistent with the local file, you can include the Content-MD5 header value in the upload request. When OSS receives the object, OSS compares the MD5 hash with the Content-MD5 header value. The object can be uploaded only when the MD5 hash is consistent with the Content-MD5 header value. This way, data consistency is ensured.
For more information about how to modify object metadata, see Manage Object Meta.