Request Flow for Provisioning Instance in Openstack

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.

request flow

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 :

Status Task Power state Steps
Build scheduling None 3-12
Build networking None 22-24
Build block_device_mapping None 25-27
Build spawing None 28
Active none Running
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