To me, MAAS is a way of more quickly installing something like Ubuntu Server onto many computers without having to 1) put the CD in, 2) follow the same script of responses to questions and then iteratively 3) run a lot of command line commands like "sudo apt-get install..." for each computer.
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| OpenStack (a.k.a. Cloud Management Layer) |
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| Juju |
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| Metal-as-a-Service (a.k.a. Deployment/Management Automation Layer) |
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---------- ------------- ----------- ------------ -----------
| Node | | Node | | Node | | Node | | Node |
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At the point where you've enlisted and commissioned nodes into MAAS you may then acquire/start them so that you could remote control into one and have a fully-functional Ubuntu server. You wouldn't necessarily need Juju or OpenStack on top of this.
Juju makes it easy to go further and then to automate deployments of services across the collection of available nodes. OpenStack is a collection of services which allow you to, in theory, spin up virtual computers from this collective (like Amazon EC2). Additionally, you have the ability to serve up virtual drives (like Amazon S3). And you can combine these two together so that you can spin up a virtual computer that attaches to a persistent drive with your data on it.
OpenStack is getting more rubust in that it has "high availability" features built in--it's forgiving to system failures on a single node, for example.
Once you've seen the PXE boot in action and watched it successfully enlist and commission a computer you'll be impressed. Unfortunately there are numerous bugs in all versions that I've worked with and you will end up spending plenty of time troubleshooting everything you've attempted to put into place.