Compute service overview¶

Use OpenStack Compute to host and manage cloud computing systems. OpenStack Compute is a major part of an Infrastructure-as-a-Service (IaaS) system. The main modules are implemented in Python.

OpenStack Compute interacts with OpenStack Identity for authentication; OpenStack Image service for disk and server images; and OpenStack Dashboard for the user and administrative interface. Image access is limited by projects, and by users; quotas are limited per project (the number of instances, for example). OpenStack Compute can scale horizontally on standard hardware, and download images to launch instances.

OpenStack Compute consists of the following areas and their components:

nova-api service

Accepts and responds to end user compute API calls. The service supports the OpenStack Compute API, the Amazon EC2 API, and a special Admin API for privileged users to perform administrative actions. It enforces some policies and initiates most orchestration activities, such as running an instance.

nova-api-metadata service

Accepts metadata requests from instances. The nova-api-metadata service is generally used when you run in multi-host mode with nova-network installations. For details, see Metadata service in the Compute Administrator Guide.

nova-compute service

A worker daemon that creates and terminates virtual machine instances through hypervisor APIs. For example:

• XenAPI for XenServer/XCP
• libvirt for KVM or QEMU
• VMwareAPI for VMware

Processing is fairly complex. Basically, the daemon accepts actions from the queue and performs a series of system commands such as launching a KVM instance and updating its state in the database.

nova-placement-api service

Tracks the inventory and usage of each provider. For details, see Placement API.

nova-scheduler service

Takes a virtual machine instance request from the queue and determines on which compute server host it runs.

nova-conductor module

Mediates interactions between the nova-compute service and the database. It eliminates direct accesses to the cloud database made by the nova-compute service. The nova-conductor module scales horizontally. However, do not deploy it on nodes where the nova-compute service runs. For more information, see the conductor section in the Configuration Options.

nova-consoleauth daemon

Authorizes tokens for users that console proxies provide. See nova-novncproxy and nova-xvpvncproxy. This service must be running for console proxies to work. You can run proxies of either type against a single nova-consoleauth service in a cluster configuration. For information, see About nova-consoleauth.

nova-novncproxy daemon

Provides a proxy for accessing running instances through a VNC connection. Supports browser-based novnc clients.

nova-spicehtml5proxy daemon

Provides a proxy for accessing running instances through a SPICE connection. Supports browser-based HTML5 client.

nova-xvpvncproxy daemon

Provides a proxy for accessing running instances through a VNC connection. Supports an OpenStack-specific Java client.

The queue

A central hub for passing messages between daemons. Usually implemented with RabbitMQ, also can be implemented with another AMQP message queue, such as ZeroMQ.

SQL database

Stores most build-time and run-time states for a cloud infrastructure, including:

• Available instance types
• Instances in use
• Available networks
• Projects

Theoretically, OpenStack Compute can support any database that SQLAlchemy supports. Common databases are SQLite3 for test and development work, MySQL, MariaDB, and PostgreSQL.