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Logical Setup
An OpenStack HA cluster involves, at a minimum, three types of nodes: controller nodes, compute nodes, and storage nodes.
Controller Nodes
The first order of business in achieving high availability (HA) is redundancy, so the first step is to provide multiple controller nodes. You must keep in mind, however, that the database uses Galera to achieve HA, and Galera is a quorum-based system. That means that you must provide at least 3 controller nodes.
Every OpenStack controller runs HAProxy, which manages a single External Virtual IP (VIP) for all controller nodes and provides HTTP and TCP load balancing of requests going to OpenStack API services, RabbitMQ, and MySQL.
When an end user accesses the OpenStack cloud using Horizon or makes a request to the REST API for services such as nova-api, glance-api, keystone-api, quantum-api, nova-scheduler, MySQL or RabbitMQ, the request goes to the live controller node currently holding the External VIP, and the connection gets terminated by HAProxy. When the next request comes in, HAProxy handles it, and may send it to the original controller or another in the cluster, depending on load conditions.
Each of the services housed on the controller nodes has its own mechanism for achieving HA:
- nova-api, glance-api, keystone-api, quantum-api and nova-scheduler are stateless services that do not require any special attention besides load balancing.
- Horizon, as a typical web application, requires sticky sessions to be enabled at the load balancer.
- RabbitMQ provides active/active high availability using mirrored queues.
- MySQL high availability is achieved through Galera active/active multi-master deployment and Pacemaker.
- Quantum agents are managed by Pacemaker.
Compute Nodes
OpenStack compute nodes are, in many ways, the foundation of your cluster; they are the servers on which your users will create their Virtual Machines (VMs) and host their applications. Compute nodes need to talk to controller nodes and reach out to essential services such as RabbitMQ and MySQL. They use the same approach that provides redundancy to the end-users of Horizon and REST APIs, reaching out to controller nodes using the VIP and going through HAProxy.
Storage Nodes
In this OpenStack cluster reference architecture, shared storage acts as a backend for Glance, so that multiple Glance instances running on controller nodes can store images and retrieve images from it. To achieve this, you are going to deploy Swift. This enables you to use it not only for storing VM images, but also for any other objects such as user files.
