Dotted Version Vector Sets

This is an implementation of the Erlang's DVV on Python.

It is used in distributed systems, where UTC timestamp is unreliable value for object's version control.

Usage examples

• Creating a new version

from dvvset import DVVSet
dvvset = DVVSet()
dot = dvvset.create(dvvset.new("something"), "user_id_1")

• Incrementing version

context = dvvset.join(dot)
new_dot = dvvset.update(dvvset.new_with_history(context, "something else"), dot, "user_id_2")
dvvset.sync([dot, new_dot])

• Detecting conflicts

Conflict is situation when two branches of the history exist. It could happen when someone updates old version ( casual history ).

merged_history = dvvset.sync([OldVersion, NewVersion])
if len(dvvset.values(merged_history)) > 1:
    print("Conflict detected")
else:
    print("Ok")

Example

1. User 1 uploads file to the server, specifying version vector:

from dvvset import DVVSet
dvvset = DVVSet()
dot = dvvset.create(dvvset.new("something"), "user_id_1")

2. Server checks version on a subject of conflict. Then it stores file with version information and provides it to User 2.

merged_history = dvvset.sync([ExistingVersion, UploadedVersion])
if len(dvvset.values(merged_history)) > 1:
    return "409 Conflict"
else:
    return "200 OK"  # Casual history is linear

3. User 2 downloads file, edits it and increments its version, before uploading back to server.

context = dvvset.join(dot)  # ``dot`` is a downloaded version
new_dot = dvvset.update(dvvset.new_with_history(context, "something else"), dot, "user_id_2")
dvvset.sync([dot, new_dot])