One of the most important use-case in any cloud is provisioning a VM . In this article we shall do a walk through about an instance(VM) being provisioned in a Openstack based cloud. This article deals with the request flow and the component interaction of various projects under Openstack. The end result will be booting up a VM.

Provisioning a new instance involves the interaction between multiple components inside OpenStack :

• CLI Command Line Interpreter for submitting commands to OpenStack Compute.
• Dashboard (“Horizon”) provides the interface for all the OpenStack services.
• Compute (“Nova”) retrieves virtual disks images(“Glance”) , attach flavor and associated metadata and transforms end user API requests into running instances.
• Network (“Quantum”) provides virtual networking for Compute which allows users to create their own networks and then link them to the instances.
• Block Storage (“Cinder”) provides persistent storage volumes for Compute instances.
• Image (“Glance”) can store the actual virtual disk files in the Image Store.
• Identity (“Keystone”) provides authentication and authorization for all OpenStack services.
• Message Queue(“RabbitMQ”) handles the internal communication within Openstack components such as Nova , Quantum and Cinder.

The request flow for provisioning an Instance goes like this:

1. Dashboard or CLI gets the user credential and does the REST call to Keystone for authentication.
2. Keystone authenticate the credentials and generate & send back auth-token which will be used for sending request to other Components through REST-call.
3. Dashboard or CLI convert the new instance request specified in  ‘launch instance’ or ‘nova-boot’ form to REST API request and send it to nova-api.
4. nova-api receive the request and sends the request for validation auth-token and access permission to keystone.
5. Keystone validates the token and sends updated auth headers with roles and permissions.
6. nova-api interacts with nova-database.
7. Creates initial db entry for new instance.
8. nova-api sends the rpc.call request to nova-scheduler excepting to get  updated instance entry with host ID specified.
9. nova-scheduler picks the request from the queue.
10. nova-scheduler interacts with nova-database to find an appropriate host via filtering and weighing.
11. Returns the updated instance entry with appropriate host ID after filtering and weighing.
12. nova-scheduler sends the rpc.cast request to nova-compute for ‘launching instance’ on appropriate host .
13. nova-compute picks the request from the queue.
14. nova-compute send the rpc.call request to nova-conductor to fetch the instance information such as host ID and flavor( Ram , CPU ,Disk).
15. nova-conductor picks the request from the queue.
16. nova-conductor interacts with nova-database.
17. Return the instance information.
18. nova-compute picks the instance information from the queue.
19. nova-compute does the REST call by passing auth-token to glance-api  to get the Image URI by Image ID from glance and upload image from image storage.
20. glance-api validates the auth-token with keystone.
21. nova-compute get the image metadata.
22. nova-compute does the REST-call by passing auth-token to Network API to allocate and configure the network such that instance gets the IP address.
23. quantum-server validates the auth-token with keystone.
24. nova-compute get the network info.
25. nova-compute does the REST call by passing auth-token to Volume API to attach volumes to instance.
26. cinder-api validates the auth-token with keystone.
27. nova-compute gets the block storage info.
28. nova-compute generates data for hypervisor driver and executes request on Hypervisor( via libvirt or api).

The table represents the Instance state at various steps during the provisioning :