Openstack Nova Internals of instance launching

• Api Server : Handle request from user and relays them to cloud controller
• Cloud Controller : Handles the communication between the compute nodes,the networking controller , api sever and scheduler.
• Scheduler : Select a host to run a command
• Compute Worker : managing compute instances,launch or terminate instance, attach or detach volumes.
• Network Controller : manages networking resources,allocated fix IP addresses, configure VLANs.

Flow of launching an Instance

• Api Server receives a run_instance command from the user.
• The Api server relays the message to the cloud controller.
• Authentication is performed to make sure this user has required permissions.
• If user is authenticated, cloud controller sends the message to the scheduler.
• Scheduler cast the message to random host and ask him to run a new instance.
• The compute worker on the host grabs the message.
• The compute worker needs a fixed ip to launch a new instance  so it sends the message to network controller i.e. (5,6,7,8) steps
  

Compute

• Compute worker receives the message and run_instance() method is called.
• run_instance()  does the following things:

Check if instance is already running

allocate a fixed ip address

setup VLAN and bridge if not already setup

spawn the instance using virtualization driver.