# Multi Master Tutorial

As of Salt 0.16.0, the ability to connect minions to multiple masters has been made available. The multi-master system allows for redundancy of Salt masters and facilitates multiple points of communication out to minions. When using a multi-master setup, all masters are running hot, and any active master can be used to send commands out to the minions.

Note

If you need failover capabilities with multiple masters, there is also a MultiMaster-PKI setup available, that uses a different topology MultiMaster-PKI with Failover Tutorial

In 0.16.0, the masters do not share any information, keys need to be accepted on both masters, and shared files need to be shared manually or use tools like the git fileserver backend to ensure that the `file_roots` are kept consistent.

Beginning with Salt 2016.11.0, the Pluggable Minion Data Cache was introduced. The minion data cache contains the Salt Mine data, minion grains, and minion pillar information cached on the Salt Master. By default, Salt uses the `localfs` cache module, but other external data stores can be used instead.

Using a pluggable minion cache modules allows for the data stored on a Salt Master about Salt Minions to be replicated on other Salt Masters the Minion is connected to. Please see the Minion Data Cache documentation for more information and configuration examples.

## Summary of Steps

1. Create a redundant master server

2. Copy primary master key to redundant master

3. Start redundant master

4. Configure minions to connect to redundant master

5. Restart minions

6. Accept keys on redundant master

## Prepping a Redundant Master

The first task is to prepare the redundant master. If the redundant master is already running, stop it. There is only one requirement when preparing a redundant master, which is that masters share the same private key. When the first master was created, the master's identifying key pair was generated and placed in the master's `pki_dir`. The default location of the master's key pair is `/etc/salt/pki/master/`. Take the private key, `master.pem`, and copy it to the same location on the redundant master. Do the same for the master's public key, `master.pub`. Assuming that no minions have yet been connected to the new redundant master, it is safe to delete any existing key in this location and replace it.

Note

There is no logical limit to the number of redundant masters that can be used.

Once the new key is in place, the redundant master can be safely started.

## Configure Minions

Since minions need to be master-aware, the new master needs to be added to the minion configurations. Simply update the minion configurations to list all connected masters:

    master:
      - saltmaster1.example.com
      - saltmaster2.example.com

Now the minion can be safely restarted.

Note

If the ipc_mode for the minion is set to TCP (default in Windows), then each minion in the multi-minion setup (one per master) needs its own tcp_pub_port and tcp_pull_port.

If these settings are left as the default 4510/4511, each minion object will receive a port 2 higher than the previous. Thus the first minion will get 4510/4511, the second will get 4512/4513, and so on. If these port decisions are unacceptable, you must configure tcp_pub_port and tcp_pull_port with lists of ports for each master. The length of these lists should match the number of masters, and there should not be overlap in the lists.

Now the minions will check into the original master and also check into the new redundant master. Both masters are first-class and have rights to the minions.

Note

Minions can automatically detect failed masters and attempt to reconnect to them quickly. To enable this functionality, set master_alive_interval in the minion config and specify a number of seconds to poll the masters for connection status.

If this option is not set, minions will still reconnect to failed masters but the first command sent after a master comes back up may be lost while the minion authenticates.

## Sharing Files Between Masters

Salt does not automatically share files between multiple masters. A number of files should be shared or sharing of these files should be strongly considered.

### Minion Keys

Minion keys can be accepted the normal way using salt-key on both masters. Keys accepted, deleted, or rejected on one master will NOT be automatically managed on redundant masters; this needs to be taken care of by running salt-key on both masters or sharing the `/etc/salt/pki/master/{minions,minions_pre,minions_rejected}` directories between masters.

Note

While sharing the /etc/salt/pki/master directory will work, it is strongly discouraged, since allowing access to the master.pem key outside of Salt creates a SERIOUS security risk.

### File_Roots

The `file_roots` contents should be kept consistent between masters. Otherwise state runs will not always be consistent on minions since instructions managed by one master will not agree with other masters.

The recommended way to sync these is to use a fileserver backend like gitfs or to keep these files on shared storage.

Important

If using gitfs/git_pillar with the cachedir shared between masters using GlusterFS, nfs, or another network filesystem, and the masters are running Salt 2015.5.9 or later, it is strongly recommended not to turn off `gitfs_global_lock`/`git_pillar_global_lock` as doing so will cause lock files to be removed if they were created by a different master.

### Pillar_Roots

Pillar roots should be given the same considerations as `file_roots`.

### Master Configurations

While reasons may exist to maintain separate master configurations, it is wise to remember that each master maintains independent control over minions. Therefore, access controls should be in sync between masters unless a valid reason otherwise exists to keep them inconsistent.

These access control options include but are not limited to:

- external_auth

- publisher_acl

- peer

- peer_run