Keeping Multiple Subversion Repositories in Sync

With Subversion 1.4svnsync was introduced for this purpose. The key problem with using svnsync for multiple Subversion repositories distributed over the WAN is its reliance on a master-slave architecture. While svnsync does provide the advantage of having local read-only repositories at each of the remote development sites, only the master repository is writeable. The master repository is then replicated to the read only slaves. However, the replication process can place a significant load on the network and servers. Because of this, replication tends to happen on an infrequent basis, leaving the read-only slave repositories that remote sites do their checkouts from out of sync with the master much of the time. As a result, commit failures due to update conflicts on the master repository can become a problem. In order to avoid commit failures, developers at the slave repository sites have to do updates over the WAN against the master Subversion repository before doing their commits. This can negate most of the expected network performance and developer productivity benefits of using svnsync in a distributed development environment.

Other solutions such as svk do allow multiple repositories to be readable as well as writeable, but there are no guarantees of consistency across the repositories. A commit can succeed on a developer’s local repository where there are no conflicts, and fail when it’s copied to other sites’ repositories due to update conflicts. This can make administration extremely difficult.

WANdisco solves these problems by turning distributed Subversion repositories into peers. All of the repositories are writeable, and consistency across the repositories is guaranteed. WANdisco’s active-active replication capabilities allow developers to work at LAN speed over the WAN for both read and write operations, while keeping all of the repositories in sync, in effect in real-time. WANdisco also provides self-healing capabilities that automate disaster recovery after a network outage or server failure.

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