Algorithm works in the following way. You have two repository: local and remote. They both contains a DAG of changelists.
The goal of the discovery protocol is to find one set of node common, the set of nodes shared by local and remote.
One of the issue with the original protocol was latency, it could potentially require lots of roundtrips to discover that the local repo was a subset of remote (which is a very common case, you usually have few changes compared to upstream, while upstream probably had lots of development).
The new protocol only requires one interface for the remote repo:
known(), which given a set of changelists tells you if they are present in the DAG.
The algorithm then works as follow:
- We will be using three sets,
unknown. Originally all nodes are in
- Take a sample from
- For each node that remote knows, move it and all its ancestors to
- For each node that remote doesn't know, move it and all its descendants to
- Iterate until
unknown is empty
There are a couple optimizations, first is instead of starting with a random sample of missing, start by sending all heads, in the case where the local repo is a subset, you computed the answer in one round trip.
Then you can do something similar to the bisecting strategy used when finding faulty changesets. Instead of random samples, you can try picking nodes that will maximize the number of nodes that will be classified with it (since all ancestors or descendants will be marked as well).
The end result worked pretty well.
The code is pretty readable if you're curious: http://selenic.com/hg/file/tip/mercurial/setdiscovery.py