# Infrastructure high availability¶

## Overview¶

Prior to deploying your OpenStack cloud you will need to consider the critical aspect of high availability (HA) as this will dictate the cloud’s topology. This guide discusses the basics of high availability, how Charmed OpenStack delivers HA, and any ramifications for the operator once the cloud is deployed.

In addition, any known issues affecting highly available OpenStack applications are documented. Although they are presented last, it is recommended to review them prior to attempting to apply any of the information shown here.

Important

This document assumes that the OpenStack cloud is backed by MAAS.

## Cloud topology guidelines¶

Deploying applications under HA will involve multiple application units being distributed amongst the available cloud nodes, thereby dramatically influencing the cloud topology. The output of the juju status command is a natural way for Juju operators to view their cloud’s topology.

### MAAS zones and failure domains¶

A fundamental part of achieving an optimal cloud topology is to have the underlying cloud nodes organised in terms of hardware failure domains. This means assigning MAAS nodes to MAAS zones so that each zone comprises a failure domain.

Note

A failure domain is often defined as a collection of physical devices that use common power, cooling, and networking hardware. This often corresponds to a data centre machine rack.

MAAS guarantees at least one zone to exist (the ‘default’ zone) but the minimum for HA is three. A MAAS zone is exposed in the juju status command output (see the AZ column) and is to be interpreted as an “HA zone” which should be associated with a complete set of HA cloud services.

### Unit distribution¶

Juju distributes an application’s units across zones by default. For example, if a three-unit application is deployed within a three-zone MAAS environment, each unit will be assigned to a different zone, and thereby to a different failure domain.

This behaviour can be overridden by assigning units to specific machines. This is done through the use of a placement directive (the --to option available to the juju deploy or juju add-unit commands) or the ‘zones’ machine constraint (e.g. --constraints zones=zone2). This can be done at deploy or scale out time and is particularly useful for hyperconverged cloud nodes. See Deploying to specific machines and Bundle reference in the Juju documentation.

Note

When a LXD container is the targeted machine, its zone is inherited from its host machine.

Whether using the default behaviour or the manual placement method, the resulting units will show in juju status command output as being in the corresponding MAAS node’s zone (AZ column).

Note

Charmed OpenStack cloud deployments generally use hyperconverged nodes and make extensive use of the placement directive in combination with bundles (and possibly bundle overlays).

### Availability zones¶

The term “availability zone” can take on different meanings depending on the context and/or the software implementation. Below we’ll see mention of the following terms:

• Juju AZ (a general HA zone, or failure domain)

• Neutron AZ

• Nova AZ

• Ceph AZ

Note

The term “zone” used in the swift-proxy and swift-storage charms refers specifically to internal functionality of Swift. Similarly, terms “zone” and “zonegroup” used in the ceph-radosgw charm refer specifically to the internal functionality of RADOS Gateway.

#### JUJU_AVAILABILITY_ZONE¶

The internal Juju variable JUJU_AVAILABILITY_ZONE provides further availability zone awareness to a unit, however both the backing cloud and the charm need to support it. For the purposes of this document, this variable associates a MAAS zone with a unit at deploy time.

This feature is available to the neutron-gateway, nova-compute, and ceph-osd charms and is enabled via configuration option customize-failure-domain (details below).

#### Neutron AZ¶

This section pertains to the neutron-gateway charm.

Configuration option default-availability-zone sets a single default Neutron availability zone to use for Neutron agents (DHCP and L3) when a network or router is defined with multiple sets of these agents. The default value is ‘nova’.

When option customize-failure-domain is set to ‘true’ then all MAAS-defined zones will become available as Neutron availability zones. In the absence of a client-specified AZ during router/network creation, the Neutron agents will be distributed amongst the zones. When ‘true’, and MAAS is the backing cloud, this option overrides option default-availability-zone.

#### Nova AZ¶

This section pertains to the nova-compute charm.

Configuration option default-availability-zone sets a single default Nova availability zone. It is used when an OpenStack instance is created without a Nova AZ being specified. The default value is ‘nova’. Note that such a Nova AZ must be created manually (i.e. command openstack aggregate create).

When option customize-failure-domain is set to ‘true’ then all MAAS-defined zones will become available as Nova availability zones. In the absence of a client-specified AZ during instance creation, one of these zones will be scheduled. When ‘true’, and MAAS is the backing cloud, this option overrides option default-availability-zone.

#### Ceph AZ¶

This section pertains to the ceph-osd charm.

Configuration option availability_zone sets a single availability zone for OSD location. The use of this option implies a very manual approach to constructing a Ceph CRUSH map and is therefore not recommended.

When option customize-failure-domain is set to ‘false’ (the default) a Ceph CRUSH map will be generated that will replicate data across hosts (implemented as Ceph bucket type ‘host’).

When option customize-failure-domain is set to ‘true’ then all MAAS-defined zones will be used to generate a Ceph CRUSH map that will replicate data across Ceph availability zones (implemented as bucket type ‘rack’). This option is also supported by the ceph-mon charm and both charms must give it the same value. When ‘true’, this option overrides option availability_zone.

### Containerisation¶

Generally speaking, every major OpenStack application can be placed into a LXD container with the following exceptions:

• ceph-osd

• neutron-gateway

• nova-compute

• swift-storage

Containerisation is effective for scaling out and it renders complex cloud upgrades manageable. Mapping applications to machines is exceptionally convenient.

Applications that have been configured to utilise another storage solution as their backend, such as Ceph, are often containerised. Common applications in this category include:

• cinder

• glance

## HA applications¶

This section provides an overview of HA applications. Deployment details are provided in the section following.

An HA-enabled application is resistant to disruptions affecting its other cluster members. That is, such a disruption would have no impact on both client requests to the application and the application itself.

Note

Highly available applications may require attention if subjected to a power event (see the Managing power events appendix).

Cloud applications are typically made highly available through the use of techniques applied externally to the application itself (e.g. using a subordinate charm). Some applications, though, achieve HA via the application’s built-in capabilities, and can be called natively HA.

Important

The nova-compute application cannot be made highly available. However, see Charmed Masakari for an implementation of cloud instance HA.

### Native HA¶

OpenStack service and applications that support native HA are listed here:

Service

Application/Charm

Ceph

ceph-mon, ceph-osd

MySQL

percona-cluster

MySQL 5.x; external HA technique required for client access; available prior to Ubuntu 20.04 LTS

MySQL

mysql-innodb-cluster

MySQL 8.x; used starting with Ubuntu 20.04 LTS

OVN

ovn-central, ovn-chassis

OVN is HA by design; available starting with Ubuntu 18.04 LTS and Ubuntu 20.04 LTS on OpenStack Ussuri

RabbitMQ

rabbitmq-server

Swift

swift-storage

### Non-native HA¶

There are two mutually exclusive strategies when implementing high availability for applications that do not support it natively:

• virtual IP(s)

• DNS

In both cases, the hacluster subordinate charm is required. It provides the Corosync/Pacemaker backend HA functionality.

Note

The virtual IP (VIP) method is intended for use in MAAS managed environments.

#### virtual IP(s)¶

To use virtual IP(s) the clustered nodes and the VIP must be on the same subnet. That is, the VIP must be a valid IP on the subnet for one of the node’s interfaces and each node has an interface in that subnet.

The VIP therefore becomes a highly-available API endpoint and is defined via the principle charm configuration option vip. Its value can take on space-separated IP addresses if multiple networks are in use.

Generic deployment commands for a three-unit cluster are provided below.

juju deploy -n 3 --config vip=<ip-address> <charm-name>
juju deploy --config cluster_count=3 hacluster <charm-name>-hacluster


The hacluster application name was chosen as ‘<charm-name>-hacluster’. This is the recommended notation.

Note

The default value of option cluster_count is ‘3’, but it is best practice to provide a value explicitly.

#### DNS¶

DNS high availability does not require the clustered nodes to be on the same subnet, and as such is suitable for use in routed network design where L2 broadcast domains terminate at the “top-of-rack” switch.

It does require:

• an environment with MAAS 2.0 and Juju 2.0 (as minimum versions)

• clustered nodes with static or “reserved” IP addresses registered in MAAS

• DNS hostnames pre-registered in MAAS (if MAAS < 2.3)

At a minimum, the configuration option dns-ha must be set to ‘true’ and at least one of os-admin-hostname, os-internal-hostname, or os-public-hostname must be set.

An error will occur if:

• neither vip nor dns-ha is set and the charm has a relation added to hacluster

• both vip and dns-ha are set

• dns-ha is set and none of os-admin-hostname, os-internal-hostname, or os-public-hostname are set

Caution

DNS HA has been reported to not work on the focal series. See LP #1882508 for more information.

## Deployment of HA applications¶

This section provides instructions for deploying common native HA and non-native HA applications. Keystone will be used to demonstrate how to deploy a non-native HA application using the hacluster subordinate charm.

The sub-sections are not meant to be followed as a guide on how to deploy a cloud. They are a collection of examples only.

Any relations needed in order for other applications to work with the deployed HA applications are not considered unless they aid in demonstrating an exceptional aspect of the HA application’s deployment.

### Keystone - hacluster¶

Keystone is not natively HA so the hacluster method is used. Many OpenStack applications are made highly available in this way.

In this example the VIP approach is taken. These commands will deploy a three-node Keystone HA cluster, with a VIP of 10.246.114.11. Each will reside in a container on existing machines 0, 1, and 2:

juju deploy -n 3 --to lxd:0,lxd:1,lxd:2 --config vip=10.246.114.11 keystone
juju deploy --config cluster_count=3 hacluster keystone-hacluster


Here is sample output from the juju status command resulting from such a deployment:

Unit                     Workload  Agent  Machine  Public address  Ports     Message
keystone/0*              active    idle   0/lxd/0  10.246.114.59   5000/tcp  Unit is ready
keystone-hacluster/0   active    idle            10.246.114.59             Unit is ready and clustered
keystone/1               active    idle   1/lxd/0  10.246.114.60   5000/tcp  Unit is ready
keystone-hacluster/2*  active    idle            10.246.114.60             Unit is ready and clustered
keystone/2               active    idle   2/lxd/0  10.246.114.61   5000/tcp  Unit is ready
keystone-hacluster/1   active    idle            10.246.114.61             Unit is ready and clustered


The VIP is not exposed in this output.

Note

The unit numbers of the hacluster subordinate and its parent do not necessarily coincide. In the above example, only for keystone/0 does this occur. That is, keystone-hacluster/0 is the subordinate unit of keystone/0.

To add a relation between an hacluster-enabled application and another OpenStack application proceed as if hacluster was not involved. For Cinder:

juju add-relation keystone:identity-service cinder:identity-service


### MySQL 5¶

The percona-cluster charm is used for OpenStack clouds that leverage MySQL 5 software. There is a hybrid aspect to MySQL 5 HA: although the backend is natively HA, client access demands an external technique be used.

Important

MySQL 5 is used on cloud nodes whose operating system is older than Ubuntu 20.04 LTS. Percona XtraDB Cluster, based on MySQL 5, is the actual upstream source used.

This example will also use the VIP method. These commands will deploy a three-node MySQL 5 HA active/active cluster, with a VIP of 10.244.40.22. Each node will reside in a container on existing machines 4, 5, and 6. It is common to use an application name of ‘mysql’:

juju deploy -n 3 --to lxd:4,lxd:5,lxd:6 --config min-cluster-size=3 --config vip=10.244.40.22 percona-cluster mysql
juju deploy --config cluster_count=3 hacluster mysql-hacluster


### MySQL 8¶

MySQL 8 is purely and natively HA; no external technique is necessary.

MySQL 8 always requires at least three database units via the mysql-innodb-cluster charm. In addition, every OpenStack application requiring a connection to the database will need its own subordinate mysql-router application. The latter should be named accordingly at deploy time (e.g. ‘<application-name>-mysql-router’). Finally, to connect an OpenStack application to the database a relation is added between it and the mysql-router application.

Here is an example of deploying a three-node MySQL 8 cluster. Each node will reside in a container on existing machines 0, 1, and 2. The cluster will then be connected to an existing highly available keystone application:

juju deploy -n 3 --to lxd:0,lxd:1,lxd:2 mysql-innodb-cluster
juju deploy mysql-router keystone-mysql-router


Below is resulting output from the juju status command for such a scenario:

Unit                        Workload  Agent  Machine  Public address  Ports     Message
keystone/6                  active    idle   0/lxd/4  10.246.114.71   5000/tcp  Unit is ready
keystone-hacluster/0*     active    idle            10.246.114.71             Unit is ready and clustered
keystone-mysql-router/2   active    idle            10.246.114.71             Unit is ready
keystone/7*                 active    idle   1/lxd/4  10.246.114.61   5000/tcp  Unit is ready
keystone-hacluster/1      active    idle            10.246.114.61             Unit is ready and clustered
keystone-mysql-router/0*  active    idle            10.246.114.61             Unit is ready
keystone/8                  active    idle   2/lxd/4  10.246.114.72   5000/tcp  Unit is ready
keystone-hacluster/2      active    idle            10.246.114.72             Unit is ready and clustered
keystone-mysql-router/1   active    idle            10.246.114.72             Unit is ready
mysql-innodb-cluster/6*     active    idle   0/lxd/5  10.246.114.58             Unit is ready: Mode: R/W
mysql-innodb-cluster/7      active    idle   1/lxd/5  10.246.114.59             Unit is ready: Mode: R/O
mysql-innodb-cluster/8      active    idle   2/lxd/5  10.246.114.60             Unit is ready: Mode: R/O


### Ceph¶

High availability in Ceph is achieved by means of a storage node cluster and a monitor node cluster. As opposed to Swift, Ceph clients connect to storage nodes (OSD) directly. This is made possible by updated “maps” that are retrieved from the monitor (MON) cluster.

A three MON node cluster is a typical design whereas a three OSD node cluster is considered the minimum. Below is one way how such a topology can be created. Each OSD is deployed to existing machines 7, 8, and 9 and a containerised MON is placed alongside each OSD:

juju deploy -n 3 --to 7,8,9 --config osd-devices=/dev/sdb ceph-osd
juju deploy -n 3 --to lxd:7,lxd:8,lxd:9 --config monitor-count=3 ceph-mon


The monitor cluster will not be complete until the specified number of ceph-mon units (monitor-count) have been fully deployed. This is to ensure that a quorum has been met prior to the initialisation of storage nodes.

Note

The default value of option monitor-count is ‘3’, but it is best practice to provide a value explicitly.

Ceph can support data resilience at the host level or the AZ level (i.e. racks or groups of racks). Host is the default but the charms can use the Juju provided AZ information to build a more complex CRUSH map.

### RabbitMQ¶

RabbitMQ has native broker clustering; clients can be configured with knowledge of all units of the cluster and will failover to an alternative unit in the event that the current selected unit fails. Message queues are also mirrored between cluster nodes.

A cluster is created simply by deploying multiple application units. This command will deploy a three-node RabbitMQ HA active/active cluster where the nodes will be containerised within their respective newly deployed machines.

juju deploy -n 3 --to lxd,lxd,lxd --config min-cluster-size=3 rabbitmq-server


Note

The default value of option cluster-partition-handling is ‘ignore’ as it has proven to be the most effective method for dealing with RabbitMQ network partitions.

### Swift¶

Swift is implemented by having storage nodes fronted by a proxy node. Unlike with Ceph, Swift clients do not communicate directly with the storage nodes but with the proxy instead. Multiple storage nodes ensure write and read storage high availability while a cluster of proxy nodes provides HA at the proxy level. Spanning clusters across geographical regions adds resiliency (multi-region clusters).

The below example shows one way to deploy a two-node proxy cluster and a three-node storage cluster, all within a single OpenStack region. The proxy nodes will be deployed to containers on existing machines 3 and 7 whereas the storage nodes will be deployed to new machines:

juju deploy -n 2 --to lxd:3,lxd:7 --config zone-assignment=manual --config replicas=3 swift-proxy
juju deploy --config zone=1 --config block-device=/dev/sdc swift-storage swift-storage-zone1
juju deploy --config zone=2 --config block-device=/dev/sdc swift-storage swift-storage-zone2
juju deploy --config zone=3 --config block-device=/dev/sdc swift-storage swift-storage-zone3


This will result in three storage zones with each zone consisting of a single storage node, thereby satisfying the replica requirement of three.

Note

The default values for options zone-assignment and replicas are ‘manual’ and ‘3’ respectively.

Refer to appendix Swift usage for more information on how to deploy Swift.

### Vault¶

An HA Vault deployment requires both the etcd and easyrsa applications in addition to hacluster and MySQL. Also, every vault unit in the cluster must have its own instance of Vault unsealed.

In these example commands, for simplicity, a single percona-cluster unit is used:

juju deploy --to lxd:1 percona-cluster mysql
juju deploy -n 3 --to lxd:0,lxd:1,lxd:2 --config vip=10.246.114.11 vault
juju deploy --config cluster_count=3 hacluster vault-hacluster
juju deploy -n 3 --to lxd:0,lxd:1,lxd:2 etcd
juju deploy --to lxd:0 cs:~containers/easyrsa


Initialise Vault to obtain the master key shards (KEY-N) and initial root token (VAULT_TOKEN). Work from an external host that has access to the vault units and has the vault snap installed. Do so by referring to any unit (VAULT_ADDR):

export VAULT_ADDR="http://10.246.114.58:8200"
vault operator init -key-shares=5 -key-threshold=3
export VAULT_TOKEN=s.vhlAKHfkHBvOvRRIE6KIkwRp


Repeat the below command block for each unit. The unit’s temporary token used below is generated by the token create subcommand:

export VAULT_ADDR="http://10.246.114.??:8200"
vault operator unseal KEY-1
vault operator unseal KEY-2
vault operator unseal KEY-3
vault token create -ttl=10m
juju run-action --wait vault/leader authorize-charm token=s.ROnC91Y3ByWDDncoZJ3YMtaY


Here is output from the juju status command for this deployment:

Unit                  Workload  Agent  Machine  Public address  Ports     Message
easyrsa/0*            active    idle   0/lxd/2  10.246.114.71             Certificate Authority connected.
etcd/0                active    idle   0/lxd/1  10.246.114.69   2379/tcp  Healthy with 3 known peers
etcd/1*               active    idle   1/lxd/1  10.246.114.61   2379/tcp  Healthy with 3 known peers
etcd/2                active    idle   2/lxd/1  10.246.114.70   2379/tcp  Healthy with 3 known peers
mysql/0*              active    idle   1/lxd/2  10.246.114.72   3306/tcp  Unit is ready
vault/0               active    idle   0/lxd/0  10.246.114.58   8200/tcp  Unit is ready (active: true, mlock: disabled)
vault-hacluster/1   active    idle            10.246.114.58             Unit is ready and clustered
vault/1*              active    idle   1/lxd/0  10.246.114.59   8200/tcp  Unit is ready (active: false, mlock: disabled)
vault-hacluster/0*  active    idle            10.246.114.59             Unit is ready and clustered
vault/2               active    idle   2/lxd/0  10.246.114.60   8200/tcp  Unit is ready (active: false, mlock: disabled)
vault-hacluster/2   active    idle            10.246.114.60             Unit is ready and clustered


Only a single vault unit is active at any given time (reflected in the above output). The other units will proxy incoming API requests to the active unit over a secure cluster connection.

### Neutron OVS/DVR (legacy)¶

Neutron OVS/DVR refers to the traditional functionality of Open vSwitch (OVS). It may optionally use Distributed Virtual Routing (DVR) as an alternate method for creating virtual router topologies. With the advent of OVN (see below) this framework is regarded as legacy OpenStack networking.

#### Control plane HA¶

Control plane HA is implemented by the neutron-api and hacluster charms.

Neutron OVS/DVR is configured via the Neutron APIs and maintains its state in the cloud’s database, which has its own HA implementation (see MySQL 5 or MySQL 8). Workers on the Neutron API nodes respond to requests through message queues hosted by RabbitMQ, which also has its own HA implementation (see RabbitMQ).

#### Data plane HA¶

Data plane HA is implemented by the neutron-gateway and neutron-openvswitch charms and the post-install network configuration of the cloud.

East/West traffic failures are akin to hypervisor failures: events that cannot be resolved by HA (but can be mitigated by “instance HA” solutions such as Masakari). A disruption to North/South traffic however will adversely affect the entire cloud and can well be prevented through HA.

Data plane HA involves the scheduling of each virtual router to dedicated gateway nodes (for non-DVR mode) or hypervisors (for DVR mode). Liveness detection between routers uses a combination of AMQP messaging and the Virtual Router Redundancy Protocol (VRRP).

In the DVR case, when Floating IPs are used, traffic is handled by the instance’s respective hypervisor. When FIPs are not used a hypervisor is randomly selected. DVR can therefore render every hypervisor self-sufficient in terms of routing traffic for its instances. This is a form of HA and is therefore recommended for clouds that employ Floating IPs. See High availability using DVR in the Neutron documentation for more information.

Note

A set of Neutron agents runs on each hypervisor: Open vSwitch agent, DHCP agent, and L3 agent. These agents communicate over the RabbitMQ message queue with Neutron API workers and any interruption to their services affect only their respective hypervisor. There is no HA for these agents but note that the components needed for their operation are all HA (RabbitMQ, Neutron API, and MySQL).

### OVN¶

Open Virtual Network (OVN) complements the existing capabilities of OVS by adding native support for virtual network abstractions, such as virtual L2 and L3 overlays and security groups.

Important

OVN is available as an option starting with Ubuntu 20.04 LTS on OpenStack Ussuri. The use of OVN obviates the need for the neutron-gateway and neutron-openvswitch charms.

#### Control plane HA¶

The OVN control plane is implemented by the ovn-central charm.

Like Neutron OVS/DVR, the desired state of the system is configured via the Neutron APIs whose HA is implemented by the hacluster charm. Neutron maintains its state in the cloud’s database, which has its own HA implementation (see MySQL 5 or MySQL 8). The neutron-api application is made aware of OVN by means of the neutron-api-plugin-ovn subordinate charm.

The desired state is transferred to an OVN database by Neutron API workers. The run-time state is the product of having that data translated into a second OVN database by the ovn-northd daemon. The daemon, of which there are multiple copies running and thereby forms its own active/standby cluster, and its databases are deployed by the ovn-central application. The databases are configured to use the OVSDB protocol along with the Clustered Database Service Model.

The recommended topology is a three-node cluster with the resulting database cluster uses the Raft algorithm to ensure consistency. These units, along with their corresponding ovn-northd services and database cluster, constitute OVN control plane HA.

#### Data plane HA¶

The OVN data plane is implemented by the ovn-chassis subordinate charm.

An OVS switch runs on each hypervisor (chassis) and is programmed by the ovn-controller daemon, which has access to the second (translated) OVN database.

East/West traffic flows directly from the source chassis to the destination chassis. North/South traffic passes through gateway chassis that are either dynamically selected by algorithms or statically configured by the operator; Floating IPs don’t play a special role in that determination.

HA applies to North/South traffic and involves the scheduling of each virtual router to up to five gateway chassis. Liveness detection between routers is done using the BFD protocol. East/West traffic disruptions are localised to individual hypervisors and can be aided by instance HA solutions (e.g. Masakari).

The recommended topology is to have one ovn-chassis unit placed on each hypervisor. These units, along with their corresponding ovn-controller daemon, comprise OVN data plane HA.

#### Deployment¶

A set of deployment steps for OVN is given below. Specific requisite components are working nova-compute and vault applications.

juju deploy neutron-api
juju deploy neutron-api-plugin-ovn
juju deploy -n 3 ovn-central
juju deploy ovn-chassis



Finally, you will need to provide an SSL certificate. This can be done by having Vault use a self-signed certificate or by using a certificate chain. We’ll do the former here for simplicity but see Managing TLS certificates for how to use a chain.

juju run-action --wait vault/leader generate-root-ca


Here is select output from the juju status command for a minimal deployment of OVN with MySQL 8:

Unit                           Workload  Agent  Machine  Public address  Ports              Message
mysql-innodb-cluster/0*        active    idle   0/lxd/0  10.246.114.61                      Unit is ready: Mode: R/W
mysql-innodb-cluster/1         active    idle   1/lxd/0  10.246.114.69                      Unit is ready: Mode: R/O
mysql-innodb-cluster/2         active    idle   2/lxd/0  10.246.114.72                      Unit is ready: Mode: R/O
neutron-api/0*                 active    idle   3/lxd/1  10.246.114.75   9696/tcp           Unit is ready
neutron-api-mysql-router/0*  active    idle            10.246.114.75                      Unit is ready
neutron-api-plugin-ovn/0*    active    idle            10.246.114.75                      Unit is ready
nova-compute/0*                active    idle   4        10.246.114.58                      Unit is ready
ovn-chassis/0*               active    idle            10.246.114.58                      Unit is ready
ovn-central/2                  active    idle   2/lxd/1  10.246.114.71   6641/tcp,6642/tcp  Unit is ready
vault/0*                       active    idle   3/lxd/2  10.246.114.74   8200/tcp           Unit is ready (active: true, mlock: disabled)
vault-mysql-router/0*        active    idle            10.246.114.74                      Unit is ready


Refer to appendix Open Virtual Network for more information on how to deploy OVN.

## Other items of interest¶

Various HA related topics are covered in this section.

The detection and alerting of service outages occurring in applications under HA is especially important. This can take the shape of a full LMA stack but the essence is the integration of a service application (e.g. keystone) with a nagios application. These two are joined by means of the nrpe subordinate charm. Configuration options available to the service application and to the nrpe application are used to enable the checks.

## Known issues¶

No major issues at this time.

Consult each charm’s bug tracker for full bug listings. See the OpenStack Charms project group.