Controller high availability

To ensure the high availability of deployed applications, the controller must itself be highly available. This necessitates the creation of additional controllers, all of which naturally reside within the ‘controller’ model. The initial controller becomes known as the master and automatic failover occurs should it lose connectivity with its cluster peers.

See Application high availability for the application side of things.

Overview

Controller HA is managed with the juju enable-ha command. It does this by ensuring that the cluster has the requisite number of controllers present. By default, this number is three but the -n switch can be used to change that. Therefore, this command is used to both enable HA as well as compensate for any missing controllers, as is the case if you enable HA and then remove one or more controllers.

When a controller is provisioned, API server code is installed along with a MongoDB database.

The number of controllers must be an odd number in order for a master to be “voted in” amongst its peers. A cluster with an even number of members will cause a random member to become inactive. This latter system will become a “hot standby” and automatically become active should some other member fail. Furthermore, due to limitations of the underlying database in an HA context, that number cannot exceed seven. All this means that a cluster can only have three, five, or seven active members.

Juju clients and agents talk to any of the controllers in the cluster. This means that processing at the controller (API) level is distributed. However, there is only one primary database at any given time and all controllers write to it. The “master”, therefore, actually refers to the underlying database.

Enabling controller HA

To enable controller HA simply invoke the enable-ha command:

juju enable-ha

Since a specific number of cluster machines were not requested, the default of three is used. We would therefore expect two new controllers to appear. Indeed, the output to the above command reflects this:

maintaining machines: 0
adding machines: 1, 2

We can also query for machines in the ‘controller’ model:

juju machines -m controller

The output should show two new machines being provisioned:

Machine  State    DNS            Inst id              Series  AZ          Message
0        started  54.166.164.0   i-04790c2414e4c8e80  xenial  us-east-1a  running
1        pending  54.145.192.13  i-071660e9ce3c3cee5  xenial  us-east-1c  running
2        pending  54.80.176.66   i-0b36284d1ebb816cf  xenial  us-east-1a  running

Invoking juju enable-ha again would have no effect since three controllers are already present.

Refreshing the list of controllers with juju controllers --refresh displays an HA level of 3:

Controller  Model    User   Access     Cloud/Region         Models  Machines    HA  Version
aws-ha*     default  admin  superuser  aws/us-east-1             2         3     3  2.4-beta2

Removing machines from the cluster

It is possible to remove a machine from the cluster at any time. Typical reasons for doing this are:

  • A controller is misbehaving and your intention is to replace it with another one.
  • You’ve decided that your current level of HA is not necessary and you wish to decrease it.
    • If the removal of a controller will result in an even number of systems then one will act as a “hot standby”.
    • If the removal of a controller will result in an odd number of systems then each one will actively participate in the cluster.

A controller is removed from the cluster by removing its machine from the model (juju remove-machine).

Using the example in the previous section, this is how we would remove machine ‘1’:

juju remove-machine -m controller 1

The output to juju controllers --refresh now becomes:

Controller  Model    User   Access     Cloud/Region         Models  Machines    HA  Version
aws-ha*     default  admin  superuser  aws/us-east-1             2         2   1/2  2.4-beta2

There is now only a single active controller (and one standby) in this cluster (i.e. one out of two are active). Note that this situation should be rectified as soon as possible.

The enable-ha command cannot be used to remove machines from the cluster.

Adding machines to the cluster

Use the enable-ha command to achieve the desired number of controllers (i.e. HA level).

In our ongoing example, our original 3-member cluster now has two machines. We can bring it back to three by issuing juju enable-ha again but we would do the following if we decided to make it a 5-member cluster instead:

juju enable-ha -n 5

This would cause three controllers (2 + 3 = 5) to be spawned.

The output to juju controllers --refresh now becomes:

Controller  Model    User   Access     Cloud/Region         Models  Machines    HA  Version
aws-ha*     default  admin  superuser  aws/us-east-1             2         5     5  2.4-beta2

Viewing extended controller HA information

Extended information on controller HA can be obtained from the show-controller command:

juju show-controller

Partial output for our 5-member cluster is listed here:

 ...
 ...
 controller-machines:
    "0":
      instance-id: i-04790c2414e4c8e80
      ha-status: ha-enabled
    "2":
      instance-id: i-0b36284d1ebb816cf
      ha-status: ha-enabled
    "3":
      instance-id: i-09ff42ba5fb9429b0
      ha-status: ha-enabled
    "4":
      instance-id: i-098222dad56cbe9a0
      ha-status: ha-enabled
    "5":
      instance-id: i-0613fb1fa8346de8a
      ha-status: ha-enabled
  models:
    controller:
      uuid: e8c4d910-8818-4a8a-8839-25766a1875d3
      machine-count: 5
      core-count: 5
  ...
  ...

Here, machine-count is the total number of machines in model ‘controller’ and core-count is the number of controller machines. The key ha-status shows ‘ha-enabled’ if a member is active and ‘ha-pending’ if it is in hot standby mode.

Recovering from controller failure

In the advent of failed controllers, new controllers are not automatically re-spawned nor are failed ones removed. However, as long as more than half of the original number of cluster members remain available manual recovery is straightforward:

  1. Remove bad controllers (as described above).
  2. Add new controllers (as described above).

Controllers must be removed prior to the addition of new ones because the enable-ha command does not check for failure. It simply ensures the total number of members are present.

You must restore from backups in the unfortunate case where there are an insufficient number of working controllers present. See Backing up and restoring Juju for how that works.

A controller is considered failed if it enters the ‘down’ state. This can be monitored by applying the status command to the ‘controller’ model:

juju status -m controller

This output shows that the controller running on machine ‘3’ has lost connectivity with the rest of the cluster:

Machine  State    DNS             Inst id              Series  AZ          Message
0        started  54.166.164.0    i-04790c2414e4c8e80  xenial  us-east-1a  running
2        started  54.80.176.66    i-0b36284d1ebb816cf  xenial  us-east-1a  running
3        down     54.157.161.147  i-09ff42ba5fb9429b0  xenial  us-east-1e  running
4        started  54.227.91.241   i-098222dad56cbe9a0  xenial  us-east-1d  running
5        started  174.129.90.47   i-0613fb1fa8346de8a  xenial  us-east-1c  running

However, the ‘HA’ column in the output to juju controllers --refresh still says ‘5’, as before:

Controller  Model    User   Access     Cloud/Region         Models  Machines    HA  Version
aws-ha*     default  admin  superuser  aws/us-east-1             2         5     5  2.4-beta2

To recover from this degraded cluster you would do:

juju remove-machine -m controller 3
juju enable-ha -n 5

Controller HA and logging

All Juju machines send their logs to a controller in the HA cluster. Each controller, in turn, sends those logs to a MongoDB database which is synchronized across controllers. The user reads logging information with the juju debug-log command as normal. See Juju logs.

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