Nova分析（1）——整体架构

Conceptual Diagram

Logical diagram

Nova is the most complicated and distributed component of OpenStack. A large number of processes cooperate to turn end user API requests into running virtual machines. Below is a list of these processes and their functions:

• nova-api accepts and responds to end user compute API calls. It supports OpenStack Compute API, Amazon's EC2 API and a special Admin API (for privileged users to perform administrative actions). It also initiates most of the orchestration activities (such as running an instance) as well as enforces some policy (mostly quota checks).

• The nova-compute process is primarily a worker daemon that creates and terminates virtual machine instances via hypervisor's APIs (XenAPI for XenServer/XCP, libvirt for KVM or QEMU, VMwareAPI for VMware, etc.). The process by which it does so is fairly complex but the basics are simple: accept actions from the queue and then perform a series of system commands (like launching a KVM instance) to carry them out while updating state in the database.

• nova-volume manages the creation, attaching and detaching of z volumes to compute instances (similar functionality to Amazon’s Elastic Block Storage). Cinder has eventually replace nova-volume functionality, nova-volume is already retired.

• The nova-network worker daemon is very similar to nova-compute and nova-volume. It accepts networking tasks from the queue and then performs tasks to manipulate the network (such as setting up bridging interfaces or changing iptables rules). This functionality has been migrated to Neutron, a separate OpenStack project.

• The nova-schedule process is conceptually the simplest piece of code in OpenStack Nova: it takes a virtual machine instance request from the queue and determines where it should run (specifically, which compute server host it should run on).

• The queue provides a central hub for passing messages between daemons. This is usually implemented with RabbitMQ today, but could be any AMQP message queue (such as Apache Qpid). New to the Folsom release is support for Zero MQ.

• The SQL database stores most of the build-time and runtime state for a cloud infrastructure. This includes the instance types that are available for use, instances in use, networks available and projects. Theoretically, OpenStack Nova can support any database supported by SQL-Alchemy but the only databases currently being widely used are SQLite3 (only appropriate for test and development work), MySQL and PostgreSQL.

• Nova also provides console services to allow end users to access their virtual instance's console through a proxy. This involves several daemons (nova-console, nova-novncproxy and nova-consoleauth).

Nova interacts with many other OpenStack services: Keystone for authentication, Glance for images and Horizon for web interface. The Glance interactions are central. The API process can upload and query Glance while nova-compute will download images for use in launching images.

setup.cfg

Python setuptools的Entry Points机制主要用途有两个：Automatic Script Creation和Dynamic Discovery of Services and Plugins。

Automatic Script Creation主要功能是封装了平台的差异，在不同平台上生成Python Package的入口脚本。如nova-api、nova-scheduler等入口脚本均是通过此机制生成的。

Dynamic Discovery of Services and Plugins主要是指本Package通过Entry Point声明自身的“扩展点”，然后第三方Package或者本Package的各个Module可以声明对此“扩展点”的实现。在Package中，可以通过某种方式获取相应“扩展点”当前所有的实现，并进行相应的处理。

Entry Point的声明和注册均是在各个Package的setup.cfg配置文件或者setup.py:setup method中完成的。

从Nova源码的setup.cfg可以看到nova在部署的时候都安装了哪些服务和配置，其中console_scripts部分为nova相关的服务：

console_scripts =
    nova-all = nova.cmd.all:main
    nova-api = nova.cmd.api:main
    nova-api-ec2 = nova.cmd.api_ec2:main
    nova-api-metadata = nova.cmd.api_metadata:main
    nova-api-os-compute = nova.cmd.api_os_compute:main
    nova-baremetal-deploy-helper = nova.cmd.baremetal_deploy_helper:main
    nova-baremetal-manage = nova.cmd.baremetal_manage:main
    nova-cells = nova.cmd.cells:main
    nova-cert = nova.cmd.cert:main
    nova-compute = nova.cmd.compute:main
    nova-conductor = nova.cmd.conductor:main
    nova-console = nova.cmd.console:main
    nova-consoleauth = nova.cmd.consoleauth:main
    nova-dhcpbridge = nova.cmd.dhcpbridge:main
    nova-manage = nova.cmd.manage:main
    nova-network = nova.cmd.network:main
    nova-novncproxy = nova.cmd.novncproxy:main
    nova-objectstore = nova.cmd.objectstore:main
    nova-rootwrap = oslo.rootwrap.cmd:main
    nova-scheduler = nova.cmd.scheduler:main
    nova-spicehtml5proxy = nova.cmd.spicehtml5proxy:main
    nova-xvpvncproxy = nova.cmd.xvpvncproxy:main

这些服务会根据nova的配置启动，其中nova-network/nova-dhcpbridge推荐用Neutron来代替。