Control plane architecture

Control plane architecture

OpenStack is designed to be massively horizontally scalable, which allows all services to be distributed widely. However, to simplify this guide, we have decided to discuss services of a more central nature, using the concept of a cloud controller. A cloud controller is a conceptual simplification. In the real world, you design an architecture for your cloud controller that enables high availability so that if any node fails, another can take over the required tasks. In reality, cloud controller tasks are spread out across more than a single node.

The cloud controller provides the central management system for OpenStack deployments. Typically, the cloud controller manages authentication and sends messaging to all the systems through a message queue.

For many deployments, the cloud controller is a single node. However, to have high availability, you have to take a few considerations into account, which we’ll cover in this chapter.

The cloud controller manages the following services for the cloud:

Tracks current information about users and instances, for example, in a database, typically one database instance managed per service
Message queue services
All Advanced Message Queuing Protocol (AMQP) messages for services are received and sent according to the queue broker
Conductor services
Proxy requests to a database
Authentication and authorization for identity management
Indicates which users can do what actions on certain cloud resources; quota management is spread out among services, howeverauthentication
Image-management services
Stores and serves images with metadata on each, for launching in the cloud
Scheduling services
Indicates which resources to use first; for example, spreading out where instances are launched based on an algorithm
User dashboard
Provides a web-based front end for users to consume OpenStack cloud services
API endpoints
Offers each service’s REST API access, where the API endpoint catalog is managed by the Identity service

For our example, the cloud controller has a collection of nova-* components that represent the global state of the cloud; talks to services such as authentication; maintains information about the cloud in a database; communicates to all compute nodes and storage workers through a queue; and provides API access. Each service running on a designated cloud controller may be broken out into separate nodes for scalability or availability.

As another example, you could use pairs of servers for a collective cloud controller—one active, one standby—for redundant nodes providing a given set of related services, such as:

  • Front end web for API requests, the scheduler for choosing which compute node to boot an instance on, Identity services, and the dashboard
  • Database and message queue server (such as MySQL, RabbitMQ)
  • Image service for the image management

Now that you see the myriad designs for controlling your cloud, read more about the further considerations to help with your design decisions.

Hardware Considerations

A cloud controller’s hardware can be the same as a compute node, though you may want to further specify based on the size and type of cloud that you run.

It’s also possible to use virtual machines for all or some of the services that the cloud controller manages, such as the message queuing. In this guide, we assume that all services are running directly on the cloud controller.

Table. Cloud controller hardware sizing considerations contains common considerations to review when sizing hardware for the cloud controller design.

Table. Cloud controller hardware sizing considerations
Consideration Ramification
How many instances will run at once? Size your database server accordingly, and scale out beyond one cloud controller if many instances will report status at the same time and scheduling where a new instance starts up needs computing power.
How many compute nodes will run at once? Ensure that your messaging queue handles requests successfully and size accordingly.
How many users will access the API? If many users will make multiple requests, make sure that the CPU load for the cloud controller can handle it.
How many users will access the dashboard versus the REST API directly? The dashboard makes many requests, even more than the API access, so add even more CPU if your dashboard is the main interface for your users.
How many nova-api services do you run at once for your cloud? You need to size the controller with a core per service.
How long does a single instance run? Starting instances and deleting instances is demanding on the compute node but also demanding on the controller node because of all the API queries and scheduling needs.
Does your authentication system also verify externally? External systems such as LDAP or Active Directory require network connectivity between the cloud controller and an external authentication system. Also ensure that the cloud controller has the CPU power to keep up with requests.

Separation of Services

While our example contains all central services in a single location, it is possible and indeed often a good idea to separate services onto different physical servers. Table. Deployment scenarios is a list of deployment scenarios we’ve seen and their justifications.

Table. Deployment scenarios
Scenario Justification
Run glance-* servers on the swift-proxy server. This deployment felt that the spare I/O on the Object Storage proxy server was sufficient and that the Image Delivery portion of glance benefited from being on physical hardware and having good connectivity to the Object Storage back end it was using.
Run a central dedicated database server. This deployment used a central dedicated server to provide the databases for all services. This approach simplified operations by isolating database server updates and allowed for the simple creation of slave database servers for failover.
Run one VM per service. This deployment ran central services on a set of servers running KVM. A dedicated VM was created for each service (nova-scheduler, rabbitmq, database, etc). This assisted the deployment with scaling because administrators could tune the resources given to each virtual machine based on the load it received (something that was not well understood during installation).
Use an external load balancer. This deployment had an expensive hardware load balancer in its organization. It ran multiple nova-api and swift-proxy servers on different physical servers and used the load balancer to switch between them.

One choice that always comes up is whether to virtualize. Some services, such as nova-compute, swift-proxy and swift-object servers, should not be virtualized. However, control servers can often be happily virtualized—the performance penalty can usually be offset by simply running more of the service.


OpenStack Compute uses an SQL database to store and retrieve stateful information. MySQL is the popular database choice in the OpenStack community.

Loss of the database leads to errors. As a result, we recommend that you cluster your database to make it failure tolerant. Configuring and maintaining a database cluster is done outside OpenStack and is determined by the database software you choose to use in your cloud environment. MySQL/Galera is a popular option for MySQL-based databases.

Message Queue

Most OpenStack services communicate with each other using the message queue. For example, Compute communicates to block storage services and networking services through the message queue. Also, you can optionally enable notifications for any service. RabbitMQ, Qpid, and Zeromq are all popular choices for a message-queue service. In general, if the message queue fails or becomes inaccessible, the cluster grinds to a halt and ends up in a read-only state, with information stuck at the point where the last message was sent. Accordingly, we recommend that you cluster the message queue. Be aware that clustered message queues can be a pain point for many OpenStack deployments. While RabbitMQ has native clustering support, there have been reports of issues when running it at a large scale. While other queuing solutions are available, such as Zeromq and Qpid, Zeromq does not offer stateful queues. Qpid is the messaging system of choice for Red Hat and its derivatives. Qpid does not have native clustering capabilities and requires a supplemental service, such as Pacemaker or Corsync. For your message queue, you need to determine what level of data loss you are comfortable with and whether to use an OpenStack project’s ability to retry multiple MQ hosts in the event of a failure, such as using Compute’s ability to do so.

Conductor Services

In the previous version of OpenStack, all nova-compute services required direct access to the database hosted on the cloud controller. This was problematic for two reasons: security and performance. With regard to security, if a compute node is compromised, the attacker inherently has access to the database. With regard to performance, nova-compute calls to the database are single-threaded and blocking. This creates a performance bottleneck because database requests are fulfilled serially rather than in parallel.

The conductor service resolves both of these issues by acting as a proxy for the nova-compute service. Now, instead of nova-compute directly accessing the database, it contacts the nova-conductor service, and nova-conductor accesses the database on nova-compute’s behalf. Since nova-compute no longer has direct access to the database, the security issue is resolved. Additionally, nova-conductor is a nonblocking service, so requests from all compute nodes are fulfilled in parallel.


If you are using nova-network and multi-host networking in your cloud environment, nova-compute still requires direct access to the database.

The nova-conductor service is horizontally scalable. To make nova-conductor highly available and fault tolerant, just launch more instances of the nova-conductor process, either on the same server or across multiple servers.

Application Programming Interface (API)

All public access, whether direct, through a command-line client, or through the web-based dashboard, uses the API service. Find the API reference at Development resources for OpenStack clouds.

You must choose whether you want to support the Amazon EC2 compatibility APIs, or just the OpenStack APIs. One issue you might encounter when running both APIs is an inconsistent experience when referring to images and instances.

For example, the EC2 API refers to instances using IDs that contain hexadecimal, whereas the OpenStack API uses names and digits. Similarly, the EC2 API tends to rely on DNS aliases for contacting virtual machines, as opposed to OpenStack, which typically lists IP addresses.

If OpenStack is not set up in the right way, it is simple to have scenarios in which users are unable to contact their instances due to having only an incorrect DNS alias. Despite this, EC2 compatibility can assist users migrating to your cloud.

As with databases and message queues, having more than one API server is a good thing. Traditional HTTP load-balancing techniques can be used to achieve a highly available nova-api service.


The API Specifications define the core actions, capabilities, and mediatypes of the OpenStack API. A client can always depend on the availability of this core API, and implementers are always required to support it in its entirety. Requiring strict adherence to the core API allows clients to rely upon a minimal level of functionality when interacting with multiple implementations of the same API.

The OpenStack Compute API is extensible. An extension adds capabilities to an API beyond those defined in the core. The introduction of new features, MIME types, actions, states, headers, parameters, and resources can all be accomplished by means of extensions to the core API. This allows the introduction of new features in the API without requiring a version change and allows the introduction of vendor-specific niche functionality.


The scheduling services are responsible for determining the compute or storage node where a virtual machine or block storage volume should be created. The scheduling services receive creation requests for these resources from the message queue and then begin the process of determining the appropriate node where the resource should reside. This process is done by applying a series of user-configurable filters against the available collection of nodes.

There are currently two schedulers: nova-scheduler for virtual machines and cinder-scheduler for block storage volumes. Both schedulers are able to scale horizontally, so for high-availability purposes, or for very large or high-schedule-frequency installations, you should consider running multiple instances of each scheduler. The schedulers all listen to the shared message queue, so no special load balancing is required.


The OpenStack Image service consists of two parts: glance-api and glance-registry. The former is responsible for the delivery of images; the compute node uses it to download images from the back end. The latter maintains the metadata information associated with virtual machine images and requires a database.

The glance-api part is an abstraction layer that allows a choice of back end. Currently, it supports:

OpenStack Object Storage
Allows you to store images as objects.
File system
Uses any traditional file system to store the images as files.
Allows you to fetch images from Amazon S3.
Allows you to fetch images from a web server. You cannot write images by using this mode.

If you have an OpenStack Object Storage service, we recommend using this as a scalable place to store your images. You can also use a file system with sufficient performance or Amazon S3—unless you do not need the ability to upload new images through OpenStack.


The OpenStack dashboard (horizon) provides a web-based user interface to the various OpenStack components. The dashboard includes an end-user area for users to manage their virtual infrastructure and an admin area for cloud operators to manage the OpenStack environment as a whole.

The dashboard is implemented as a Python web application that normally runs in Apache httpd. Therefore, you may treat it the same as any other web application, provided it can reach the API servers (including their admin endpoints) over the network.

Authentication and Authorization

The concepts supporting OpenStack’s authentication and authorization are derived from well-understood and widely used systems of a similar nature. Users have credentials they can use to authenticate, and they can be a member of one or more groups (known as projects or tenants, interchangeably).

For example, a cloud administrator might be able to list all instances in the cloud, whereas a user can see only those in his current group. Resources quotas, such as the number of cores that can be used, disk space, and so on, are associated with a project.

OpenStack Identity provides authentication decisions and user attribute information, which is then used by the other OpenStack services to perform authorization. The policy is set in the policy.json file. For information on how to configure these, see Managing Projects and Users in the OpenStack Operations Guide.

OpenStack Identity supports different plug-ins for authentication decisions and identity storage. Examples of these plug-ins include:

  • In-memory key-value Store (a simplified internal storage structure)
  • SQL database (such as MySQL or PostgreSQL)
  • Memcached (a distributed memory object caching system)
  • LDAP (such as OpenLDAP or Microsoft’s Active Directory)

Many deployments use the SQL database; however, LDAP is also a popular choice for those with existing authentication infrastructure that needs to be integrated.

Network Considerations

Because the cloud controller handles so many different services, it must be able to handle the amount of traffic that hits it. For example, if you choose to host the OpenStack Image service on the cloud controller, the cloud controller should be able to support the transferring of the images at an acceptable speed.

As another example, if you choose to use single-host networking where the cloud controller is the network gateway for all instances, then the cloud controller must support the total amount of traffic that travels between your cloud and the public Internet.

We recommend that you use a fast NIC, such as 10 GB. You can also choose to use two 10 GB NICs and bond them together. While you might not be able to get a full bonded 20 GB speed, different transmission streams use different NICs. For example, if the cloud controller transfers two images, each image uses a different NIC and gets a full 10 GB of bandwidth.

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