Most of the functionality of MAAS is contained in a series of controllers. There are two basic types: a region controller and one or more rack controllers. The region controller deals with operator requests, while the rack controller(s) provides high-bandwidth services to the individual machines. In essence, the region controller interacts with the user, while the rack controllers manage the bare metal.
About region controllers
A region controller consists of the following components:
- REST API server (TCP port 5240)
- PostgreSQL database
- DNS
- caching HTTP proxy
- web UI
Region controllers are responsible for either a data centre or a single region. Multiple fabrics are used by MAAS to accommodate subdivisions within a single region, such as multiple floors in a data centre.
About rack controllers
A rack controller provides four services:
- DHCP
- TFTP
- HTTP (for images)
- power management
A rack controller is attached to each “fabric”. As the name implies, a typical setup is to have a rack controller in each data centre server rack. The rack controller will cache large items for performance, such as operating system install images, but maintains no independent state other than the credentials required to talk to the region controller.
Tell me about fabrics
A fabric is simply a way of linking VLANs (Virtual LANs) together. If you’re familiar with a VLAN, you know that it’s designed to limit network traffic to specific ports (e.g., on a switch) or by evaluating labels called “tags” (unrelated to MAAS tags). By definition, this would mean that two VLANs can’t communicate with each other – it would defeat the purpose of the VLAN – unless you implement some extraordinary measures.
For example, let’s say that your hospital has three key functions: Patient management, Accounting, and Facilities, each on their own VLAN. Let’s say that there are some situations in which you need to share data between all three of these functions. To accomplish this, you can create a fabric that joins these three VLANS. Since this fabric just makes it possible for these VLANs to communicate, you can manage the cross-VLAN access with additional software, or permissions, depending on your application software architecture.
You can learn more about fabrics in the Concepts and terms section of this documentation.
About controller communication
MAAS communication happens in a strict hierarchy, flowing from the UI/API through the region controller, to the rack controller, to the machines (and back). While high availability (HA) may add controllers, it does not change the flow of communication through the MAAS system. Understanding this message flow may help you with the machine topics which follow.
How machines communicate with the rack controller
All machine communication with MAAS is proxied through rack controllers, including HTTP metadata, DNS, syslog and APT (cache-and-forward proxies via Squid).
MAAS creates an internal DNS domain, not manageable by the user, and a unique DNS resource for each subnet that is managed by MAAS. Each subnet includes all rack controllers that have an IP on that subnet. Booting machines use the subnet DNS resource to resolve the rack controller available for communication. If multiple rack controllers belong to the same subnet, MAAS uses a round-robin algorithm to balance the load across numerous rack controllers. This arrangement ensures that machines always have a rack controller.
Machines use this internal domain for HTTP metadata queries, APT (proxying via Squid), and Syslog. DNS queries, PXE booting, and NTP polls use IP addresses.
The rack controller installs and configures bind as a forwarder. All machines communicate via the rack controller directly.
Zone management and maintenance still happen within the region controller.
How region and rack controllers communicate
The MAAS region and rack controllers interact in a number of different ways, depending upon the operation you’ve requested. Consider the process of commissioning a machine, that is, taking over the machine and gathering information on its available resources, including CPU, RAM, storage, and MIB information (obtainable via LLDP requests). Here’s a rough idea of what that sequence looks like – a sequence that is representative of the communication between rack and region controllers:
- An operator makes a request of MAAS, either via the Web UI or the API.
- MAAS translates this to an API request to the region controller.
- The region controller locates the rack controller that has BMC access to the machine in question, that is, the rack controller that can power on that machine.
- That same rack controller powers on the machine via IPMI request.
- The rack controller tasked with providing DHCP handles assigning an IP address to the machine via the DORA sequence (Discover, Offer, Request, Acknowledge). Note that this rack controller doesn’t have to be the same one that powers on the machine.
- The DHCP-managing rack controller inserts itself as the DHCP “next-server” and requests a network boot.
- (Still) the same rack controller RPCs the region controller to get what’s needed to boot an ephemeral Ubuntu kernel, namely the kernel, any kernel parameters, an initrd daemon, and a squashfs load.
- That same rack controller transforms the RPC response from the region controller into a valid PXE config and tells the machine to come get its files.
- The booting machine loads the kernel and initrd, boots with that initrd, and then loads the squashfs, eventually making its way up to an ephemeral Ubuntu instance.
- The booted machine pulls cloud-init metadata from the region controller, proxying through the rackd.
- cloud-init uses this metadata to gather resource information about the machine and pass it back to the region controller, again proxied by the rackd.
- The region controller (regiond or “region daemon”) stores this machine information in a postgres database that is accessible only to the regiond, making MAAS truly stateless with respect to machines.
Again, this list doesn’t represent every interaction between the controllers and machines, but it gives you a good idea of how MAAS works.
Tell me about the DHCP "next-server" statement
The next-server directive is used to specify the host address from which an initial boot file is to be loaded, usually a TFTP server. In the case of MAAS, the rack controller providing DHCP actually inserts itself, since it can proxy (broker) the delivery of boot bits to the machine in question.