The first part of this little blog series explained the basic operations of chunk file upload as we set it up for discussion. This part goes a bit beyond and talks about an addition to that, called announcing the upload.
With the processing described in the first part of the blog, the upload is done savely and with a clean approach, but it also has some drawbacks.
Most notably the server does not know the target filename of the uploaded file upfront. Also it does not know the final size or mimetype of the target file. That is not a problem in general, but imagine the following situation: A big file should be uploaded, which would exceed the users quota. That would only become an error for the user once all uploads happened, and the final upload directory is going to be moved on the final file name.
To avoid useless file transfers like that or to implement features like a file firewall, it would be good if the server would know these data at start of the upload and stop the upload in case it can not be accepted.
To achieve that, the client creates a file called _meta in /uploads/ before the upload of the chunks starts. The file contains information such as overall size, target file name and other meta information.
The server’s reply to the PUT of the _meta file can be a fail result code and error description to indicate that the upload will not be accepted due to certain server conditions. The client should check the result codes in order to avoid not necessary upload of data volume of which the final MOVE would fail anyway.
This is just a collection of ideas for an improved big file chunking protocol, nothing is decided yet. But now is the time to discuss. We’re looking forward to hearing your input.
The third and last part will describe how this plays into delta sync, which is especially interesting for big files, which are usually chunked.