Delta transfer

How it works

Both sides need the previous version and the same block rules. A hash miss falls back to a full send. Object stores that only replace whole objects need a client that speaks deltas. The store itself may not.

A 9 GB disk image changes 2 percent. A delta job sends about 180 MB. A PUT of the whole image sends 9 GB. The recipient who deleted yesterday's file forces a full send.

How it differs

Delta transfer is not compression. Compression shrinks one version. Delta skips bytes the other side already has.

A delta against the wrong base rebuilds a corrupt file that still has the right size.

Hash the result, not only the delta.

On the ticket

• The practical close is a log line: time, actor, byte count, result.
• Without that line the transfer is a story.
• With it, the next person can see whether this door did what the ticket claimed.
• If the path is shared, say so in the partner profile so a later change does not silently pick a different limit, key, or region.
• On a real ticket, write down the door, the byte count, and the clock.
• For delta transfer, that means naming the host or bucket, the expected size, and the time the other side must have a complete file.
• A progress bar is not that record.
• A 200 response that arrives before the complete call is not that record.
• If a retry is allowed, say how many and whether it resumes.
• If a person must approve the send, name the person.
• Partners who receive delta transfer files should match on hash or size before they import.
• A same-length corrupt file passes a size check and fails a hash.
• Keep the published hash off the only channel an attacker can edit, or treat it as a corruption check rather than a substitution check.
• When the path changes, new key, new region, new cap, update the profile the same day so the next run does not use a stale limit.