nova/doc/source/nova.concepts.rst

Nova 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 focuses 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 adminguide/index, 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: Storage

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, so it does not work like a SAN. If you wish to expose the same volume to multiple instances, you will have to use an NFS or SAMBA share from an existing instance.

Local Storage

Every instance larger than m1.tiny starts with some local storage (up to 160GB for m1.xlarge). This storage is currently the second partition on the root drive.

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 named cloudpipe <cloudpipe>) needs to be created. Nova generates a certificate and key for the user to access the vpn and starts the vpn automatically. More information on cloudpipe can be found here <cloudpipe>.

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: Binaries

Nova is implemented by a number of related binaries. These binaries can run on the same machine or many machines. A detailed description of each binary is given in the binaries section <binaries> of the developer guide.

Concept: nova-manage

The nova-manage command is used to perform many essential functions for administration and ongoing maintenance of nova, such as user creation, vpn management, and much more.

See doc:nova.manage in the Administration Guide for more details.

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

Nova utilizes the RabbitMQ implementation of the AMQP messaging standard for performing communication between the various nova services. This message queuing service is used for both local and remote communication because Nova is designed so that there is no requirement that any of the services exist on the same physical machine. RabbitMQ in particular is very robust and provides the efficiency and reliability that Nova needs. More information about RabbitMQ can be found at http://www.rabbitmq.com/.

Concept: Fakes

• auth
• ldap

Concept: Scheduler

• simple
• random

Concept: Security Groups

Security groups

Concept: Certificate Authority

Nova does a small amount of certificate management. These certificates are used for project vpns <cloudpipe> and decrypting bundled images.

Concept: Images

• launching
• bundling