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Because OpenStack is highly configurable, with many different back-ends and network configuration options, it is difficult to write documentation that covers all possible OpenStack deployments. Therefore, this guide defines an example architecture to simplify the task of documenting, as well as to scope this guide. You can find additional architectures in the Use Cases appendix, the OpenStack Installation Guides, or the OpenStack User Stories page.

The simplest architecture you can build upon for Compute has a single cloud controller and multiple compute nodes. The simplest architecture for Object Storage has five nodes: one for identifying users and proxying requests to the API, then four for storage itself to provide enough replication for eventual consistency. This example architecture does not dictate a particular number of nodes, but shows the thinking and considerations that went into choosing this architecture including the features offered.

OpenStack release Havana Host operating system Ubuntu 12.04 LTS or Red Hat Enterprise Linux 6.5 including derivatives such as CentOS and Scientific Linux OpenStack package repository Ubuntu Cloud Archive (https://wiki.ubuntu.com/ServerTeam/CloudArchive) or RDO (http://openstack.redhat.com/Frequently_Asked_Questions) * Hypervisor KVM Database MySQL* Message queue RabbitMQ for Ubuntu, Qpid for Red Hat Enterprise Linux and derivatives Networking service nova-network Network manager FlatDHCP Single nova-network or multi-host? multi-host* Image Service (glance) back-end file Identity Service (keystone) driver SQL Block Storage Service (cinder) back-end LVM/iSCSI Live Migration back-end shared storage using NFS * Object storage OpenStack Object Storage (swift) An asterisk (*) indicates when the example architecture deviates from the settings of a default installation. The following features of OpenStack are supported by the example architecture documented in this guide, but are optional: dashboard block storage floating IP addresses live migration object storage

This example architecture has been selected based on the current default feature set of OpenStack Havana, with an emphasis on stability. We believe that many clouds that currently run OpenStack in production have made similar choices. You must first choose the operating system that runs on all of the physical nodes. While OpenStack is supported on several distributions of Linux, we used Ubuntu 12.04 LTS (Long Term Support), which is used by the majority of the development community, has feature completeness compared with other distributions, and has clear future support plans. We recommend that you do not use the default Ubuntu OpenStack install packages and instead use the Ubuntu Cloud Archive (https://wiki.ubuntu.com/ServerTeam/CloudArchive). The Cloud Archive is a package repository supported by Canonical that allows you to upgrade to future OpenStack releases while remaining on Ubuntu 12.04. KVM as a hypervisor complements the choice of Ubuntu - being a matched pair in terms of support, and also because of the significant degree of attention it garners from the OpenStack development community (including the authors, who mostly use KVM). It is also feature complete, free from licensing charges and restrictions. MySQL follows a similar trend. Despite its recent change of ownership, this database is the most tested for use with OpenStack and is heavily documented. We deviate from the default database, SQLite, because SQLite is not an appropriate database for production usage. The choice of RabbitMQ over other AMQP compatible options that are gaining support in OpenStack, such as ZeroMQ and Qpid is due to its ease of use and significant testing in production. It also is the only option which supports features such as Compute cells. We recommend clustering with RabbitMQ, as it is an integral component of the system, and fairly simple to implement due to its inbuilt nature. As discussed in previous chapters, there are several options for networking in OpenStack Compute. We recommend FlatDHCP and to use Multi-Host networking mode for high availability, running one nova-network daemon per OpenStack Compute host. This provides a robust mechanism for ensuring network interruptions are isolated to individual compute hosts, and allows for the direct use of hardware network gateways. Live Migration is supported by way of shared storage, with NFS as the distributed file system. Acknowledging that many small-scale deployments see running an Object Storage service just for the storage of virtual machine images as too costly, we opted for the file back-end in the OpenStack Image Catalog and Delivery Service (Glance). If the cloud you are designing also intends to run Object Storage, it is trivial to enable this as the back-end instead, and a recommended approach. We chose the SQL back-end for Identity Service (keystone) over others, such as LDAP. This back-end is simple to install and is robust. The authors acknowledge that many installations want to bind with existing directory services, and caution careful understanding of the array of options available (http://docs.openstack.org/trunk/config-reference/content/ch_configuring-openstack-identity.html#configuring-keystone-for-ldap-backend) The Block Storage service (cinder) is installed natively on external storage nodes and uses the LVM/iSCSI plugin. Most Block Storage Service plugins are tied to particular vendor products and implementations limiting their use to consumers of those hardware platforms, but LVM/iSCSI is robust and stable on commodity hardware. While the cloud can be run without the OpenStack Dashboard, we consider it to be indispensable, not just for user interaction with the cloud, but also as a tool for operators. Additionally, the dashboard's use of Django makes it a flexible framework for extension.

This example architecture does not use the OpenStack Network Service (neutron), because it does not yet support multi-host networking and our organizations (university, government) have access to a large range of publicly-accessible IPv4 addresses.

In a default OpenStack deployment, there is a single nova-network service that runs within the cloud (usually on the cloud controller) that provides services such as network address translation (NAT), DHCP, and DNS to the guest instances. If the single node that runs the nova-network service goes down, you cannot access your instances and the instances cannot access the Internet. The single node that runs the nova-network service can become a bottleneck if excessive network traffic comes in and goes out of the cloud. Multi-host (http://docs.openstack.org/havana/install-guide/install/apt/content/nova-network.html) is a high-availability option for the network configuration where the nova-network service is run on every compute node instead of running on only a single node.

The reference architecture consists of multiple compute nodes, a cloud controller, an external NFS storage server for instance storage and an OpenStack Block Storage server for volume storage. A network time service (Network Time Protocol, NTP) synchronizes time for all the nodes. FlatDHCPManager in multi-host mode is used for the networking. The cloud controller runs: the dashboard, the API services, the database (MySQL), a message queue server (RabbitMQ), the scheduler for choosing compute resources (nova-scheduler), Identity services (keystone, nova-consoleauth), Image services (glance-api, glance-registry), services for console access of guests, and block storage services including the scheduler for storage resources (cinder-api and cinder-scheduler). Compute nodes are where the computing resources are held, and in our example architecture they run the hypervisor (KVM), libvirt (the driver for the hypervisor, which enables live migration node to node), nova-compute, nova-api-metadata (generally only used when running in multi-host mode, it retrieves instance-specific metadata), nova-vncproxy, and nova-network. The network consists of two switches, one for the management or private traffic, and one which covers public access including Floating IPs. To support this, the cloud controller and the compute nodes have two network cards. The OpenStack Block Storage and NFS storage servers only need to access the private network and therefore only need one network card, but multiple cards run in a bonded configuration are recommended if possible. Floating IP access is direct to the internet, whereas Flat IP access goes through a NAT.

You can extend this reference architecture as follows: Add additional cloud controllers (see ). Add an OpenStack Storage service (see the Object Storage chapter in the OpenStack Installation Guide for your distribution. Add additional OpenStack Block Storage hosts (see ).