Source: I have implemented a OCI-compliant registry [1], though for the most part I've been following the behavior of the reference implementation [2] rather than the spec, on account of its convolutedness.
When the client finalizes a blob upload, they need to supply the digest of the full blob. This requirement evidently serves to enable the server side to validate the integrity of the supplied bytes. If the server only started checking the digest as part of the finalize HTTP request, it would have to read back all the blob contents that had already been written into storage in previous HTTP requests. For large layers, this can introduce an unreasonable delay. (Because of specific client requirements, I have verified my implementation to work with blobs as large as 150 GiB.)
Instead, my implementation runs the digest computation throughout the entire sequence of requests. As blob data is taken in chunk by chunk, it is simultaneously streamed into the digest computation and into blob storage. Between each request, the state of the digest computation is serialized in the upload URL that is passed back to the client in the Location header. This is roughly the part where it happens in my code: https://github.com/sapcc/keppel/blob/7e43d1f6e77ca72f0020645...
I believe that this is the same approach that the reference implementation uses. Because digest computation can only work sequentially, therefore the upload has to proceed sequentially.
When the client finalizes a blob upload, they need to supply the digest of the full blob. This requirement evidently serves to enable the server side to validate the integrity of the supplied bytes. If the server only started checking the digest as part of the finalize HTTP request, it would have to read back all the blob contents that had already been written into storage in previous HTTP requests. For large layers, this can introduce an unreasonable delay. (Because of specific client requirements, I have verified my implementation to work with blobs as large as 150 GiB.)
Instead, my implementation runs the digest computation throughout the entire sequence of requests. As blob data is taken in chunk by chunk, it is simultaneously streamed into the digest computation and into blob storage. Between each request, the state of the digest computation is serialized in the upload URL that is passed back to the client in the Location header. This is roughly the part where it happens in my code: https://github.com/sapcc/keppel/blob/7e43d1f6e77ca72f0020645...
I believe that this is the same approach that the reference implementation uses. Because digest computation can only work sequentially, therefore the upload has to proceed sequentially.
[1] https://github.com/sapcc/keppel [2] https://github.com/distribution/distribution