• 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).

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 =http://www.mamicode.com/ 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