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Overview of highly-available controllers
OpenStack is a set of multiple services exposed to the end users as HTTP(s) APIs. Additionally, for own internal usage OpenStack requires SQL database server and AMQP broker. The physical servers, where all the components are running are often called controllers. This modular OpenStack architecture allows to duplicate all the components and run them on different controllers. By making all the components redundant it is possible to make OpenStack highly-available.
In general we can divide all the OpenStack components into three categories:
- OpenStack APIs, these are HTTP(s) stateless services written in python, easy to duplicate and mostly easy to load balance.
- SQL relational database server provides stateful type consumed by other components. Supported databases are MySQL, MariaDB, and PostgreSQL. Making SQL database redundant is complex.
Advanced Message Queuing Protocol (AMQP)provides OpenStack internal stateful communication service.
Network components
[TODO Need discussion of network hardware, bonding interfaces, intelligent Layer 2 switches, routers and Layer 3 switches.]
The configuration uses static routing without Virtual Router Redundancy Protocol (VRRP) or similar techniques implemented.
[TODO Need description of VIP failover inside Linux namespaces and expected SLA.]
See [TODO link] for more information about configuring networking for high availability.
Common deployement architectures
There are primarily two HA architectures in use today.
One uses a cluster manager such as Pacemaker or Veritas to co-ordinate the actions of the various services across a set of machines. Since we are focused on FOSS, we will refer to this as the Pacemaker architecture.
The other is optimized for Active/Active services that do not require any inter-machine coordination. In this setup, services are started by your init system (systemd in most modern distributions) and a tool is used to move IP addresses between the hosts. The most common package for doing this is keepalived.
intro-ha-arch-pacemaker.rst intro-ha-arch-keepalived.rst