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Concepts and Introduction
Introduction
Nova is the software that controls your Infrastructure as as Service (IaaS) cloud computing platform. It is similar in scope to Amazon EC2 and Rackspace CloudServers. Nova does not include any virtualization software, rather it defines drivers that interact with underlying virtualization mechanisms that run on your host operating system, and exposes functionality over a web API.
This document does not attempt to explain fundamental concepts of cloud computing, IaaS, virtualization, or other related technologies. Instead, it focues on describing how Nova's implementation of those concepts is achieved.
This page outlines concepts that you will need to understand as a user or administrator of an OpenStack installation. Each section links to more more detailed information in the Administration Guide, but you'll probably want to read this section straight-through before tackling the specifics presented in the administration guide.
Concept: Users and Projects
- access to images is limited by project
- access/secret are per user
- keypairs are per user
- quotas are per project
Concept: Virtualization
- KVM
- UML
- XEN
- HyperV
- qemu
Concept: Instances
An 'instance' is a word for a virtual machine that runs inside the cloud.
Concept: Volumes
A 'volume' is a detachable block storage device. You can think of it as a usb hard drive. It can only be attached to one instance at a time, and it behaves
Concept: Quotas
Nova supports per-project quotas. There are currently quotas for number of instances, total number of cores, number of volumes, total number of gigabytes, and number of floating ips.
Concept: RBAC
Nova provides roles based access control (RBAC) for access to api
commands. A user can have a number of different roles <auth_roles>.
Roles define which api_commands a user can perform.
It is important to know that there are user-specific (sometimes called global) roles and project-specific roles. A user's actual permissions in a particular project are the INTERSECTION of his user-specific roles and is project-specific roles.
For example: A user can access api commands allowed to the netadmin role (like allocate_address) only if he has the user-specific netadmin role AND the project-specific netadmin role.
More information about RBAC can be found in the auth.
Concept: API
- EC2
- OpenStack / Rackspace
Concept: Networking
Nova has a concept of Fixed Ips and Floating ips. Fixed ips are assigned to an instance on creation and stay the same until the instance is explicitly terminated. Floating ips are ip addresses that can be dynamically associated with an instance. This address can be disassociated and associated with another instance at any time.
There are multiple strategies available for implementing fixed ips:
Flat Mode
The simplest networking mode. Each instance receives a fixed ip from the pool. All instances are attached to the same bridge (br100) by default. The bridge must be configured manually. The networking configuration is injected into the instance before it is booted. Note that this currently only works on linux-style systems that keep networking configuration in /etc/network/interfaces.
Flat DHCP Mode
This is similar to the flat mode, in that all instances are attached to the same bridge. In this mode nova does a bit more configuration, it will attempt to bridge into an ethernet device (eth0 by default). It will also run dnsmasq as a dhcpserver listening on this bridge. Instances receive their fixed ips by doing a dhcpdiscover.
VLAN DHCP Mode
This is the default networking mode and supports the most features. For multiple machine installation, it requires a switch that supports host-managed vlan tagging. In this mode, nova will create a vlan and bridge for each project. The project gets a range of private ips that are only accessible from inside the vlan. In order for a user to access the instances in their project, a special vpn instance (code name cloudpipe) needs to be created. Nova generates a certificate and key for the userto access the vpn and starts the vpn automatically.
The following diagram illustrates how the communication that occurs between the vlan (the dashed box) and the public internet (represented by the two clouds)
Concept: Services
- nova-api
- nova-scheduler
- nova-compute
- nova-volume
- nova-network
- nova-instancemonitor
Concept: nova-manage
nova-manage is a command line utility for performing administrative tasks and checking on the health of the system.
Concept: Flags
python-gflags
Concept: Plugins
- Managers/Drivers: utils.import_object from string flag
- virt/connections: conditional loading from string flag
- db: LazyPluggable via string flag
- auth_manager: utils.import_class based on string flag
- Volumes: moving to pluggable driver instead of manager
- Network: pluggable managers
- Compute: same driver used, but pluggable at connection
Concept: IPC/RPC
Rabbit!
Concept: Fakes
- auth
- ldap
Concept: Scheduler
- simple
- random
Concept: Security Groups
Security groups
Concept: Certificate Authority
Per-project CA * Images * VPNs
Concept: Images
- launching
- bundling