The Heat workflow

As already mentioned in the previous sections of this chapter, Heat provides two types of interfaces, including a web-based interface integrated into the OpenStack dashboard and also a command-line interface (CLI), which can be used from inside a Linux shell.

The interfaces use the heat-api to send commands to the Heat engine via the messaging service (for example RabbitMQ). A metering service such as Ceilometer or CloudWatch API is used to monitor the performance of resources in the stack. These monitoring/metering services are used to trigger actions upon reaching a certain threshold. An example of this could be automatically launching a redundant web server behind a load balancer when the CPU load on the primary web server reaches above 90 percent.