A framework for building your own configuration management tool
There are Puppet, Chef, SaltStack and Ansible. These are good products, so why create another configuration management tool? Sure, all of them have minor downsides, but that's not enough for kind of reinventing the wheel. But there is one property that they all share: They are tools that you feed with configuration files and scripts. They are big machines with thousands of knobs and switches. Imagine a factory intended for building every model of cars that is available nowadays. How huge and complex would it be? Could you really build ALL models? How many robotic arms would be hanging useless, because they are not required for the current model? Wouldn't it be better to just pick the robotic arms that you need and combine them into a relatively small and effective factory? That's the idea behind Optimus. It is not a tool, it is a framework for building your own configuration management tool.
Proof of concept.
go get github.com/neelance/optimus
Install Docker and set $DOCKER_HOST accordingly, see https://docs.docker.com/installation/#installation.
Create three docker containers that run sshd, available at ports 50001, 50002 and 50003 of the docker host with root as username and password:
$GOPATH/src/github.com/neelance/optimus/example/create-docker-containers.sh
Those encapsulated containers will be our test hosts. Run this script again at any time to reset them to their initial state.
Now you can build and run the example tool:
go run $GOPATH/src/github.com/neelance/optimus/example/example.go
This will show the available commands. The run command will execute a shell command on each host in parallel, for example:
go run $GOPATH/src/github.com/neelance/optimus/example/example.go run hostname
More powerful is the up command, which will bring the hosts to the configuration state described inside of example.go:
go run $GOPATH/src/github.com/neelance/optimus/example/example.go up
On first run, it will create some users. Subsequent runs will do nothing, since the users already exist. You can modify example.go to describe other configurations and then apply them with the up command.
The up command consists of two phases, the analyze and the modify phase.
In the analyze phase, the optimus.Configurator function gets executed multiple times in parallel for each host. It can fetch information about the current state of the host and apply actions (changes) to the optimus.HostState. However, these actions are not yet applied to the actual server, but queued for the modify phase. During the analyze phase, all communication with the host is required to be read-only. This way, any information about the host's state need to be fetched only once per analyze phase. The phase ends by listing all queued actions and asking for confirmation to proceed.
The modify phase runs the queued actions on the actual hosts in parallel. If an error occurs while executing an action, the affected host will be marked as failed and it will be ignored for the rest of the up command. The modify phase continues for the remaining hosts.
Another analyze phase begins after all actions have been executed. It examines the hosts (except the failed ones) and ideally it should detect that no further actions are required. If this is the case, the up command terminates. In some scenarios, however, multiple cycles are required to reach a stable state.