2.4. Enabling Support for High-Availability Applications

Procedure 2.3. To Enable Support for High-Availability Applications:

1. To allow scalable applications to become highly available using the configured external router, edit the /etc/openshift/broker.conf file on the broker host and set the ALLOW_HA_APPLICATIONS parameter to "true":

   ALLOW_HA_APPLICATIONS="true"

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   Note that this parameter controls whether high-availability applications are allowed in general, but does not adjust user account capabilities. User account capabilities are discussed in a later step.
2. A scaled application that is not highly available uses the following URL form:

   http://${APP_NAME}-${DOMAIN_NAME}.${CLOUD_DOMAIN}

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   When high-availability is enabled, HAproxy instances are deployed in multiple gears of the application, which are spread across multiple node hosts. In order to load balance user requests, a high-availability application requires a new high-availability DNS name that points to the external routing layer rather than directly to the application head gear. The routing layer then forwards requests directly to the application's HAproxy instances, which are then distributed to the framework gears. In order to create DNS entries for high-availability applications that point to the routing layer, OpenShift Enterprise adds either a prefix or suffix, or both, to the regular application name:

   http://${HA_DNS_PREFIX}${APP_NAME}-${DOMAIN_NAME}${HA_DNS_SUFFIX}.${CLOUD_DOMAIN}

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   To change the prefix or suffix used in the high-availability URL, you can modify the HA_DNS_PREFIX or HA_DNS_SUFFIX parameters:

   HA_DNS_PREFIX="ha-"
   HA_DNS_SUFFIX=""

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   If you modify the HA_DNS_PREFIX parameter and are using the sample routing daemon, ensure this parameter and the HA_DNS_PREFIX parameter in the /etc/openshift/routing-daemon.conf file are set to the same value.
3. DNS entries for high-availability applications can either be managed by OpenShift Enterprise or externally. By default, this parameter is set to "false", which means the entries must be created externally; failure to do so could prevent the application from receiving traffic through the external routing layer. To allow OpenShift Enterprise to create and delete these entries when applications are created and deleted, set the MANAGE_HA_DNS parameter to "true":

   MANAGE_HA_DNS="true"

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   Then set the ROUTER_HOSTNAME parameter to the public hostname of the external routing layer, which the DNS entries for high-availability applications point to. Note that the routing layer host must be resolvable by the broker:

   ROUTER_HOSTNAME="www.example.com"

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4. For developers to enable high-availability support with their scalable applications, they must have the HA allowed capability enabled on their account. By default, the DEFAULT_ALLOW_HA parameter is set to "false", which means user accounts are created with the HA allowed capability initially disabled. To have this capability enabled by default for new user accounts, set DEFAULT_ALLOW_HA to "true":

   DEFAULT_ALLOW_HA="true"

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   You can also adjust the HA allowed capability per user account using the oo-admin-ctl-user command with the --allowha option:

   # oo-admin-ctl-user -l user --allowha trueoo-admin-ctl-user -l user --allowha trueoo-admin-ctl-user -l user --allowha trueoo-admin-ctl-user -l user --allowha true

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5. To make any changes made to the /etc/openshift/broker.conf file take effect, restart the broker service:

   # service openshift-broker restart

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