Before you can decide on how to do this, you’re going to have to make a few choices:

Authentication and Access

Theres two main ways to expose a git repo, HTTPS or SSH, and they both have pros and cons here:

• HTTPS A standard sort of protocol to proxy, but you’ll need to make sure you set up authentication on the proxy properly so that only only thise who should have access can get it. The git client will need to store a username and password to talk to the server or you’ll have to enter them on every request. gitweb is a CGI that provides a basic, but useful, web interface.

• SSH Simpler to set up, and authentication is a solved problem. Proxying it isn’t hard, just forward the port to any of the backend servers, which avoids decrypting on the proxy. You will want to use the same hostkey on all the servers though, or SSH will refuse to connect. Doesn’t require any special setup.

Replication

Git is a distributed version control system, so you could replicate it at that level, alternatively you could use a replicated file system, or a simple file based replication. Each has it’s own trade-offs.

• Git replication Using git pull to replicate between repositories is probably going to be your most reliable option, as it’s the job git was built for, and doesn’t rely on messing with it’s underlying files directly. The one caveat is that, if you push to different servers in quick suscession you may cause a merge confict, which would break your replication. The cleanest way to deal with that is to have the load balancer send all requests to server1 if it’s up, and only switch to the next server if all the prior ones are down. That way writes will alk be going to the same place. Then set up replication in loop, with server2 pulling from server1, server3 pulling from server2, and so on up to server1 pulling from server5. With frequent pulls changes that are commited to server1 will quickly replicate to all the other servers. This would effectively be a shared nothing solution as none of the servers are sharing resources, which would make it easier to geigraphically separate them. The load balancer could be replaced by a CNAME record in DNS, with a daemon that updates it to point to the correct server.

• Replicated filesystem Git stores its data in a fairly simple file structure, so placing that on a replicated filesystem such as GlusterFS or Ceph would mean multiple servers could use the same data. From experience, this sort of thing is great when it’s working, but can be fragile and break in unexpected ways. You don’t want to be up at 2am trying to fix a file replication issue if you can avoid it.

• File replication. This is similar to the git replication option, in that you have to be very aware of the risk of conflicts. A similar strategy would probably work, but I’m not sure it brings you any advantages.

I think my prefered solution would be to have SSH access to the git servers and to set up pull based replication on a fairly fast schedule (where fast is relative to how frequently you push changes). You mention having a VPS as obe of the servers, so you might want to push changes to that rather than have be able to connect to your internal network.

A useful property of git is that, if the server is missing changesets you can just push them again. So if a server goes down before your last push gets replicated, you can just push again once the system has switched to the new server. Once the first server comes back online it’ll naturally get any changesets it’s missing and effectively ‘heal’.