Logically, the components of OpenStack can be divided into three groups:

The control tier runs the Application Programming Interfaces (API) services, web interface, database, and message bus. The network tier runs network service agents for networking, and the compute node 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, each of these groups is installed onto a separate server. OpenStack could be installed on one node or two nodes, but a good baseline for being able to scale out later is to put each of these groups on their own node. An OpenStack cluster can also scale far beyond three nodes, and we'll look at scaling beyond this basic deployment in Chapter 11, Scaling Horizontally.

Now that a base logical architecture of OpenStack is 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 towards collecting the resources necessary to launch an instance. Finally, we will look at what components are available to add resources to a launched instance.