In the previous chapter, we looked at Ceilometer and used telemetry in OpenStack. In this chapter, we will take a look at orchestration using OpenStack's Heat component. We will take a look at what orchestration is and how to write a template for Heat. Then, we will use the template to launch a Heat stack.
In Chapter 5, Instance Management, we used Nova to launch instances in OpenStack. This example walked through launching a single instance or, if the instance count was increased, multiple instances with the same configuration. What if a collection of instances needed to be launched that required each of them to have a different configuration or they all needed to know about each other as part of their post-boot configuration? For example, maybe a different Glance image is needed for each instance or a different flavor is needed for the different roles within this collection of instances. It is even a possible requirement to control the order in which the instances are spawned to make sure that they are available in a specific order for post-boot configuration purposes. Enter orchestration. With OpenStack's orchestration component, Heat, all of these requirements and more can be met with Heat's capabilities.