Heat is the orchestration component. Orchestration is the process of launching multiple instances that are intended to work together. In orchestration, there is a file, known as a template, used to define what will be launched. In this template, there can also be ordering or dependencies set up between the instances. Data that needs to be passed between the instances for configuration can also be defined in these templates. Heat is also compatible with AWS CloudFormation templates and implements additional features in addition to the AWS CloudFormation template language.
To use Heat, one of these templates is written to define a set of instances that needs to be launched. When a template launches, it creates a collection of virtual resources (instances, networks, storage devices, and so on); this collection of resources is called a stack. When a stack is spawned, the ordering and dependencies, shared configuration data, and post-boot configuration are coordinated via Heat.
Heat is not configuration management. It is orchestration. It is intended to coordinate launching the instances, passing configuration data, and executing simple post-boot configuration. A very common post-boot configuration task is invoking an actual configuration management engine to execute more complex post-boot configuration. In Chapter 9, Orchestration, we'll explore creating a Heat template and launching a stack using Heat.
