Heat is the main project in the Open Stack Orchestration program. It implements an orchestration engine to launch multiple composite cloud applications based on templates in the form of text files that can be treated like code.
Heat provides both an Open Stack-native ReST API and a Cloud Formation-compatible Query API. A stack is a collection of resources or objects that will be created by Heat. A Heat template has three main parts or sections, one of which defines the specific template, the second part being made up of resources and the third section comprising output. Resources are the specific objects that Heat will either create and/or modify during the course of its operation. Output, as we already know, is the information passed to the user either via the means of Open Stack Dashboard or through the heat stack-list and heat stack-show commands.
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The main objectives of Open Stack when they created Heat is to provide a templates based orchestration for describing a cloud application by executing appropriate Open Stack API calls to generate running cloud applications. A heat template describes the infrastructure for a cloud application in text files which are readable and writable by humans and can be managed by version control tools.
Heat Orchestration Template (HOT) is one of the two template formats used by HEAT. This template is not backwards compatible with AWS Cloud Formation templates and can only be used with Open Stack. Heat, like many of the other Open Stack products and services uses a back end database for maintaining state information. Heat consists of a god number of Python applications, some of which will be outlined below:
Heat API: This component provides and Open Stack-native ReST API that processes API requests by sending them to the heat-engine over RPC.
Heat: This tool is a CLI which communicates with the heat API to execute AWS Cloud Formation APIs. However, this is not compulsory as developers could also use the heat API directly.
Heat API CFN: This component provides and AWS style Query API that is compatible with AWS Cloud Formation and processes API requests by sending them to the heat-engine over RPC.
Heat Engine: This component of Open Stack is the component that does the main work of orchestrating the launch of templates and providing events back to the API consumer.
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Templates specify the relationships between resources (e.g., this volume is connected to this server). This enables Heat to call out to the Open Stack APIs to create your entire infrastructure in the correct order to completely launch your application. The software integrates other components of Open Stack. The templates allow creation of most Open Stack resource types (such as instances, floating ips, volumes, security groups, users etc.), as well as some more advanced functionality such as instance high availability, instance auto scaling and nested stacks. Heat primarily manages infrastructure, but the templates integrate well with software configuration management tools such as Puppet and Ansible. All these tools and components are to be installed on an Open Stack controller node that also houses the API servers for Nova, Glance, Neutron etc. Operators can customise the capabilities of Heat by installing plugins.
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