A method writes a data block from a client application onto object storage. Object storage receives a write request with a data block over an API from a coupling server, the data block being an update to a target file stored within object storage. Next, object storage receives a completion request from the coupling server for the target file. Then, object storage receives a final manifest from the coupling server, the final manifest includes for each data block of the target file an identifier for said data block, a start location, an end location or a size, and a unique identifier for the data block; the manifest has the data blocks ordered by positions in the target file. Finally, object storage assembles of new version of the target file using the final manifest, the received data block and other data blocks of the target file, and writes a new version into object storage.