OpenStack has a very modular design, and because of this design, there are lots of moving parts. It is overwhelming to start walking through installing and using OpenStack without understanding the internal architecture of the components that make up OpenStack. In this chapter, we'll look at these components. Each component in OpenStack manages a different resource that can be virtualized for the end user. Separating the management of each of the types of resources that can be virtualized into separate components makes the OpenStack architecture very modular. If a particular service or resource provided by a component is not required, then the component is optional to an OpenStack deployment. Once the components that make up OpenStack have been covered, we will discuss the configuration of a community-supported distribution of OpenStack called RDO.
Let's start by outlining some simple categories to group these services into. Logically, the components of OpenStack are divided into three groups:
- Control
- Network
- Compute
The control tier runs the Application Programming Interface (API) services, web interface, database, and message bus. The network tier runs network service agents for networking, and the compute tier is the virtualization hypervisor. It has services and agents to handle virtual machines. All of the components use a database and/or a message bus. The database can be MySQL, MariaDB, or PostgreSQL. The most popular message buses are RabbitMQ, Qpid, and ActiveMQ. For smaller deployments, the database and messaging services usually run on the control node, but they could have their own nodes if required.
In a simple multi-node deployment, the control and networking services are installed on one server and the compute services are installed onto another server. OpenStack could be installed on one node or more than two nodes, but a good baseline for being able to scale out later is to put control and network together and compute by itself. An OpenStack cluster can scale far beyond a few nodes, and we will look at scaling beyond this basic deployment in Chapter 11, Scaling Horizontally.
Now that a base logical architecture of OpenStack has been defined, let's look at what components make up this basic architecture. To do that, we'll first touch on the web interface and then work toward collecting the resources necessary to launch an instance. Finally, we will look at what components are available to add resources to a launched instance.