This is the third and final part of a little blog series about a new chunking algorithm that we discussed in ownCloud. You might be interested to read the first two parts ownCloud Chunking NG and Announcing an Upload as well.
This part makes a couple of ideas how the new chunking could be useful with a future feature of incremental sync (also called delta sync) in ownCloud.
In preparartion of delta sync the server could provide another new WebDAV route: remote.php/dav/blocks.
For each file, remote.php/dav/blocks/file-id exists as long as the server has valid checksums for blocks of the file which is identified by its unique file id.
A successful reply to remote.php/dav/blocks/file-id returns an JSON formatted data block with byte ranges and the respective checksums (and the checksum type) over the data blocks for the file. The client can use that information to calculate the blocks of data that has changed and thus needs to be uploaded.
If a file was changed on the server and as a result the checksums are not longer valid, access to remote.php/blocks/file-id is returning the 404 "not found" return code. The client needs to be able to handle missing checksum information at any time.
The server gets the checksums of file blocks along the upload of the chunks from the client. There is no obligation of the server to calculate the checksums of data blocks that came in other than through the clients, yet it can if there is capacity.
To implement incremental sync, the following high level processing could be implemented:
- The client downloads the blocklist of the file: GET remote.php/dav/blocks/file-id
- If GET succeeded: Client computes the local blocklist and computes changes
- If GET failed: All blocks of the file have to be uploaded.
- Client sends request MKCOL /uploads/transfer-id as described in an earlier part of the blog.
- For blocks that have changed: PUT data to /uploads/transfer-id/part-no
- For blocks that have NOT changed: COPY /blocks/file-id/block-no /uploads/transfer-id/part-no
- If all blocks are handled by either being uploaded or copied: Client sends MOVE /uploads/transfer-id /path/to/target-file to finalize the upload.
This would be an extension to the previously described upload of complete files. The PROPFIND semantic on /uploads/transfer-id remains valid.
Depending on the amount of not changed blocks, this could be a dramatic cut for the data that have to be uploaded. More information has to be collected to find out how much that is.
Note that this is still in the idea- and to-be-discussed state, and not yet an agreed specification for a new chunking algorithm.
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