# High Availability Features in Salt

Salt supports several features for high availability and fault tolerance. Brief documentation for these features is listed alongside their configuration parameters in Configuration file examples.

## Master Cluster

Added in version 3007.

Salt masters can be configured to act as a cluster. All masters in a cluster are peers. Job workloads are shared accross the cluster. Master clusters provide a way to scale masters horizontally. They do not require changes to the minions' configuration to add more resources. Cluster implementations are expected to use a load balancer, shared filesystem, and run on a reliable network.

Master Cluster Tutorial

## Multimaster

Salt minions can connect to multiple masters at one time by configuring the master configuration parameter as a YAML list of all the available masters. By default, all masters are "hot", meaning that any master can direct commands to the Salt infrastructure.

In a multimaster configuration, each master must have the same cryptographic keys, and minion keys must be accepted on all masters separately. The contents of file_roots and pillar_roots need to be kept in sync with processes external to Salt as well

A tutorial on setting up multimaster with "hot" masters is here:

Multimaster Tutorial

## Multimaster with Failover

Changing the `master_type` parameter from `str` to `failover` will cause minions to connect to the first responding master in the list of masters. Every `master_alive_interval` seconds the minions will check to make sure the current master is still responding. If the master does not respond, the minion will attempt to connect to the next master in the list. If the minion runs out of masters, the list will be recycled in case dead masters have been restored. Note that `master_alive_interval` must be present in the minion configuration, or else the recurring job to check master status will not get scheduled.

Failover can be combined with PKI-style encrypted keys, but PKI is NOT REQUIRED to use failover.

Multimaster with PKI and Failover is discussed in this tutorial

`master_type: failover` can be combined with `random_master: True` to spread minion connections across all masters (one master per minion, not each minion connecting to all masters). Adding Salt Syndics into the mix makes it possible to create a load-balanced Salt infrastructure. If a master fails, minions will notice and select another master from the available list.

## Syndic

Salt's Syndic feature is a way to create differing infrastructure topologies. It is not strictly an HA feature, but can be treated as such.

With the syndic, a Salt infrastructure can be partitioned in such a way that certain masters control certain segments of the infrastructure, and "Master of Masters" nodes can control multiple segments underneath them.

Syndics are covered in depth in Salt Syndic.

## Syndic with Multimaster

Added in version 2015.5.0.

Syndic with Multimaster lets you connect a syndic to multiple masters to provide an additional layer of redundancy in a syndic configuration.

Syndics are covered in depth in Salt Syndic.