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========================
Commonly used technology
========================
High availability can be achieved only on system level, while both hardware and
software components can contribute to the system level availability.
This document lists the most common hardware and software technologies
that can be used to build a highly available system.
Hardware
~~~~~~~~
The following are the standard hardware requirements:
- Provider networks: See the *Overview -> Networking Option 1: Provider
networks* section of the
`Install Tutorials and Guides <https://docs.openstack.org/project-install-guide/ocata>`_
depending on your distribution.
- Self-service networks: See the *Overview -> Networking Option 2:
Self-service networks* section of the
`Install Tutorials and Guides <https://docs.openstack.org/project-install-guide/ocata>`_
depending on your distribution.
Load balancers
--------------
Using different technologies to enable high availability on the hardware
level provides a good basis to build a high available system. The next chapters
discuss the most common technologies used in this field.
Redundant switches
------------------
Network switches are single point of failures as networking is critical to
operate all other basic domains of the infrastructure, like compute and
storage. Network switches need to be able to forward the network traffic
and be able to forward the traffic to a working next hop.
For these reasons consider the following two factors when making a network
switch redundant:
#. The network switch itself should synchronize its internal state to a
redundant switch either in active/active or active/passive way.
#. The network topology should be designed in a way that the network router can
use at least two paths in every critical direction.
Bonded interfaces
-----------------
Bonded interfaces are two independent physical network interfaces handled as
one interface in active/passive or in active/active redundancy mode. In
active/passive mode, if an error happens in the active network interface or in
the remote end of the interface, the interfaces are switched over. In
active/active mode, when an error happens in an interface or in the remote end
of an interface, then the interface is marked as unavailable and ceases to be
used.
Load balancers
--------------
Physical load balancers are special routers which direct the traffic in
different directions based on a set of rules. Load balancers can be in
redundant mode similarly to the physical switches.
Load balancers are also important for distributing the traffic to the different
active/active components of the system.
Storage
-------
Physical storage high availability can be achieved with different scopes:
#. High availability within a hardware unit with redundant disks (mostly
organized into different RAID configurations), redundant control components,
redundant I/O interfaces and redundant power supply.
#. System level high availability with redundant hardware units with data
replication.
Software
~~~~~~~~