crspybits
commented
2 years ago Neebla: The client app itself doesn't directly use checksum's.
iOSBasics: (The SyncServer API library)
1) On v0 file uploads, the server endpoint UploadFileRequest takes a checksum as input. The client computes the checksum from the file contents about to be uploaded.
2) On file downloads, the server endpoint DownloadFileResponse provides a checksum so the client can recompute the checksum locally and compare it to the checksum coming back from the server. It also receives a contentsChanged value computed on the server, based on checksums (see below).
Server: 1) On file uploads:
a) The checksum is used as part of a heuristic check, prior to uploading to cloud storage, for a duplicate v0 upload. i.e., the checksum coming up from the client request is compared to the lastUploadedCheckSum for a v0 file.
b) When handling a v0 upload, after uploading the file to the specific cloud storage, the checksum obtained from the specific cloud storage system upload is compared to that obtained from the client.
c) This checksum is saved in a server database table (lastUploadedCheckSum).
2) Applying mutable file changes
a) After applying changes, and uploading them, the resulting checksum from cloud storage is stored back again in lastUploadedCheckSum.
3) On file downloads:
a) A checksum is obtained from the cloud storage system as part of the download.
b) The lastUploadedCheckSum is compared with the checksum obtained from the cloud storage system download to test if the file was changed at rest (or possibly, during transmission). E.g., this can happen if a user goes in directly and modifies their file. This "contentsChanged" is passed back to the client along with the download data.
kjetilk
AJamesPhillips
In getting ready to add support for Solid to Neebla and SyncServerII the question of the kind of use of checksums has come up. I'm opening this issue to have a place to record the specifics of how checksums are currently used.