cf plugin for hands-off, zero downtime application deploys

*On nix

$ go get github.com/concourse/autopilot
$ cf install-plugin $GOPATH/bin/autopilot

On Windows

$ go get github.com/concourse/autopilot
$ cf install-plugin $env:GOPATH/bin/autopilot.exe

$ cf zero-downtime-push application-to-replace \
    -f path/to/new_manifest.yml \
    -p path/to/new/path

Your application manifest must be up to date or the new application that is created will not resemble the application that it is replacing.

Autopilot takes a different approach to other zero-downtime plugins. It doesn't perform any complex route re-mappings instead it leans on the manifest feature of the Cloud Foundry CLI. The method also has the advantage of treating a manifest as the source of truth and will converge the state of the system towards that. This makes the plugin ideal for continuous delivery environments.

1. The old application is renamed to <APP-NAME>-venerable. It keeps its old route mappings and this change is invisible to users.

2. The new application is pushed to <APP-NAME> (assuming that the name has not been changed in the manifest). It binds to the same routes as the old application (due to them being defined in the manifest) and traffic begins to be load-balanced between the two applications.

3. The old application is deleted along with its route mappings. All traffic now goes to the new application.