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

DVVSet dvvSet = new DVVSet();
Clock dot = dvvSet.create(dvvSet.newDvv("1611600920427"), "user_id_1");

• Incrementing version

List context = dvvSet.join(dot);
Clock newDot = dvvSet.update(dvvSet.newWithHistory(context, "1616682865530"), dot, "user_id_2");
List mergedHistory = dvvSet.sync(new Clock(dot.asList(), newDot.asList()));

• Detecting conflicts

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

List values = dvvSet.values(mergedHistory);
if (values.size() > 1) {
    System.out.println("CONFLICT");
} else {
    System.out.println("OK");
}

Example

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

DVVSet dvvSet = new DVVSet();
Clock dot = dvvSet.create(dvvSet.newDvv("1611600920427"), "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.

List mergedHistory = dvvSet.sync(new Clock(existingVersion, uploadedVersion));
List values = dvvSet.values(mergedHistory);
if (values.size() > 1) {
    // return 409 Conflict
} else {
    // return 200 OK
}

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

List context = dvvSet.join(dot);
Clock newDot = dvvSet.update(dvvSet.newWithHistory(context, "1616682865530"), dot, "user_id_2");
dvvSet.sync(new Clock(dot.asList(), newDot.asList()));