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.
