Working Copy Sync Plan

1. The Overall Problem

There should be a lightweight way to transparently sync working copies across repositories in the background. This document covers one part of a proposed implementation.

The initial implementation's goals are:

Outside this document's scope are:

1.1. Why would you even want to sync working copies?

Imagine that you're working in a local repository and have local changes. At many organizations, to test your changes you would need to sync the contents over to a remote server to e.g. test by sending a small amount of production traffic.

The usual way to do that is to make an explicit commit and push, then have a hook on the server that updates the test repo to that commit.

hg commit && hg push test-server -r .

This sucks. Making a commit is a heavyweight operation that requires a mental context switch. It adds unnecessary friction to what should be a seamless process -- for a web application, if the server were running locally you would just need to save the file you're editing, run any build steps, then hit refresh in your browser.

The overall plan allows developers to get the same workflow as with a local server.

1.2. Why not use rsync/Unison/Dropbox/<insert favorite file sync tool here>?

For small repositories, this works great! Not really for larger ones, though. Consider the case where a large update happens from one public commit to another, which changes > 10,000 files. Any of the above tools will try to sync all changed files, even though the only information the remote needs to know is that the public commit you're on has changed -- at that point the server can update to that commit (including possibly fetching those changes over the network, with RemotefilelogExtension).

1.3. Why not just make real commits on disk and strip them when we're done?

That is not atomic -- if hg log is run while a commit is being made and pushed, it's going to result in weird temporary commits being visible. Even if that is taken care of with e.g. a pre-secret phase, the fact that the lock is taken by a background process is still going to be visible to other Mercurial processes.

1.4. What else are in-memory commits good for?

2. The Plan

2.1. On the client

2.2. On the server

2.3. Performance tradeoffs and optimizations

WorkingCopySyncPlan (last edited 2015-08-26 17:34:10 by SiddharthAgarwal)