7. Alerts

Alerts may be configured to take some external action when a cluster event occurs (node failure, resource starting or stopping, etc.).

7.1. Alert Agents

As with resource agents, the cluster calls an external program (an alert agent) to handle alerts. The cluster passes information about the event to the agent via environment variables. Agents can do anything desired with this information (send an e-mail, log to a file, update a monitoring system, etc.).

Simple alert configuration

<configuration>
   <alerts>
      <alert id="my-alert" path="/path/to/my-script.sh" />
   </alerts>
</configuration>

In the example above, the cluster will call my-script.sh for each event.

Multiple alert agents may be configured; the cluster will call all of them for each event.

Alert agents will be called only on cluster nodes. They will be called for events involving Pacemaker Remote nodes, but they will never be called on those nodes.

7.2. Alert Recipients

Usually, alerts are directed towards a recipient. Thus, each alert may be additionally configured with one or more recipients. The cluster will call the agent separately for each recipient.

Alert configuration with recipient

<configuration>
   <alerts>
      <alert id="my-alert" path="/path/to/my-script.sh">
          <recipient id="my-alert-recipient" value="some-address"/>
      </alert>
   </alerts>
</configuration>

In the above example, the cluster will call my-script.sh for each event, passing the recipient some-address as an environment variable.

The recipient may be anything the alert agent can recognize – an IP address, an e-mail address, a file name, whatever the particular agent supports.

7.3. Alert Meta-Attributes

As with resource agents, meta-attributes can be configured for alert agents to affect how Pacemaker calls them.

Meta-Attributes of an Alert

Meta-Attribute	Default	Description
timestamp-format	%H:%M:%S.%06N	Format the cluster will use when sending the event’s timestamp to the agent. This is a string as used with the date(1) command.
timeout	30s	If the alert agent does not complete within this amount of time, it will be terminated.

Meta-attributes can be configured per alert agent and/or per recipient.

Alert configuration with meta-attributes

<configuration>
   <alerts>
      <alert id="my-alert" path="/path/to/my-script.sh">
         <meta_attributes id="my-alert-attributes">
            <nvpair id="my-alert-attributes-timeout" name="timeout"
                    value="15s"/>
         </meta_attributes>
         <recipient id="my-alert-recipient1" value="someuser@example.com">
            <meta_attributes id="my-alert-recipient1-attributes">
               <nvpair id="my-alert-recipient1-timestamp-format"
                       name="timestamp-format" value="%D %H:%M"/>
            </meta_attributes>
         </recipient>
         <recipient id="my-alert-recipient2" value="otheruser@example.com">
            <meta_attributes id="my-alert-recipient2-attributes">
               <nvpair id="my-alert-recipient2-timestamp-format"
                       name="timestamp-format" value="%c"/>
            </meta_attributes>
         </recipient>
      </alert>
   </alerts>
</configuration>

In the above example, the my-script.sh will get called twice for each event, with each call using a 15-second timeout. One call will be passed the recipient someuser@example.com and a timestamp in the format %D %H:%M, while the other call will be passed the recipient otheruser@example.com and a timestamp in the format %c.

7.4. Alert Instance Attributes

As with resource agents, agent-specific configuration values may be configured as instance attributes. These will be passed to the agent as additional environment variables. The number, names and allowed values of these instance attributes are completely up to the particular agent.

Alert configuration with instance attributes

<configuration>
   <alerts>
      <alert id="my-alert" path="/path/to/my-script.sh">
         <meta_attributes id="my-alert-attributes">
            <nvpair id="my-alert-attributes-timeout" name="timeout"
                    value="15s"/>
         </meta_attributes>
         <instance_attributes id="my-alert-options">
             <nvpair id="my-alert-options-debug" name="debug"
                     value="false"/>
         </instance_attributes>
         <recipient id="my-alert-recipient1"
                    value="someuser@example.com"/>
      </alert>
   </alerts>
</configuration>

7.5. Alert Filters

By default, an alert agent will be called for node events, fencing events, and resource events. An agent may choose to ignore certain types of events, but there is still the overhead of calling it for those events. To eliminate that overhead, you may select which types of events the agent should receive.

Alert configuration to receive only node events and fencing events

<configuration>
   <alerts>
      <alert id="my-alert" path="/path/to/my-script.sh">
         <select>
            <select_nodes />
            <select_fencing />
         </select>
         <recipient id="my-alert-recipient1"
                    value="someuser@example.com"/>
      </alert>
   </alerts>
</configuration>

The possible options within <select> are <select_nodes>, <select_fencing>, <select_resources>, and <select_attributes>.

With <select_attributes> (the only event type not enabled by default), the agent will receive alerts when a node attribute changes. If you wish the agent to be called only when certain attributes change, you can configure that as well.

Alert configuration to be called when certain node attributes change

<configuration>
   <alerts>
      <alert id="my-alert" path="/path/to/my-script.sh">
         <select>
            <select_attributes>
               <attribute id="alert-standby" name="standby" />
               <attribute id="alert-shutdown" name="shutdown" />
            </select_attributes>
         </select>
         <recipient id="my-alert-recipient1" value="someuser@example.com"/>
      </alert>
   </alerts>
</configuration>

Node attribute alerts are currently considered experimental. Alerts may be limited to attributes set via attrd_updater, and agents may be called multiple times with the same attribute value.

7.6. Using the Sample Alert Agents

Pacemaker provides several sample alert agents, installed in /usr/share/pacemaker/alerts by default.

While these sample scripts may be copied and used as-is, they are provided mainly as templates to be edited to suit your purposes. See their source code for the full set of instance attributes they support.

Sending cluster events as SNMP traps

<configuration>
   <alerts>
      <alert id="snmp_alert" path="/path/to/alert_snmp.sh">
         <instance_attributes id="config_for_alert_snmp">
            <nvpair id="trap_node_states" name="trap_node_states"
                    value="all"/>
         </instance_attributes>
         <meta_attributes id="config_for_timestamp">
            <nvpair id="ts_fmt" name="timestamp-format"
                    value="%Y-%m-%d,%H:%M:%S.%01N"/>
         </meta_attributes>
         <recipient id="snmp_destination" value="192.168.1.2"/>
      </alert>
   </alerts>
</configuration>

Sending cluster events as e-mails

<configuration>
   <alerts>
      <alert id="smtp_alert" path="/path/to/alert_smtp.sh">
         <instance_attributes id="config_for_alert_smtp">
            <nvpair id="email_sender" name="email_sender"
                    value="donotreply@example.com"/>
         </instance_attributes>
         <recipient id="smtp_destination" value="admin@example.com"/>
      </alert>
   </alerts>
</configuration>

7.7. Writing an Alert Agent

Environment variables passed to alert agents

Environment Variable	Description
CRM_alert_kind	The type of alert (node, fencing, resource, or attribute)
CRM_alert_node	Name of affected node
CRM_alert_node_sequence	A sequence number increased whenever an alert is being issued on the local node, which can be used to reference the order in which alerts have been issued by Pacemaker. An alert for an event that happened later in time reliably has a higher sequence number than alerts for earlier events. Be aware that this number has no cluster-wide meaning.
CRM_alert_recipient	The configured recipient
CRM_alert_timestamp	A timestamp created prior to executing the agent, in the format specified by the timestamp-format meta-attribute. This allows the agent to have a reliable, high-precision time of when the event occurred, regardless of when the agent itself was invoked (which could potentially be delayed due to system load, etc.).
CRM_alert_timestamp_epoch	The same time as CRM_alert_timestamp, expressed as the integer number of seconds since January 1, 1970. This (along with CRM_alert_timestamp_usec) can be useful for alert agents that need to format time in a specific way rather than let the user configure it.
CRM_alert_timestamp_usec	The same time as CRM_alert_timestamp, expressed as the integer number of microseconds since CRM_alert_timestamp_epoch.
CRM_alert_version	The version of Pacemaker sending the alert
CRM_alert_desc	Detail about event. For node alerts, this is the node’s current state (member or lost). For fencing alerts, this is a summary of the requested fencing operation, including origin, target, and fencing operation error code, if any. For resource alerts, this is a readable string equivalent of CRM_alert_status.
CRM_alert_nodeid	ID of node whose status changed (provided with node alerts only)
CRM_alert_rc	The numerical return code of the fencing or resource operation (provided with fencing and resource alerts only)
CRM_alert_task	The requested fencing or resource operation (provided with fencing and resource alerts only)
CRM_alert_exec_time	The (wall-clock) time, in milliseconds, that it took to execute the action. If the action timed out, CRM_alert_status will be 2, CRM_alert_desc will be “Timed Out”, and this value will be the action timeout. May not be supported on all platforms. (resource alerts only) (since 2.0.1)
CRM_alert_interval	The interval of the resource operation (resource alerts only)
CRM_alert_rsc	The name of the affected resource (resource alerts only)
CRM_alert_status	A numerical code used by Pacemaker to represent the operation result (resource alerts only)
CRM_alert_target_rc	The expected numerical return code of the operation (resource alerts only)
CRM_alert_attribute_name	The name of the node attribute that changed (attribute alerts only)
CRM_alert_attribute_value	The new value of the node attribute that changed (attribute alerts only)

Special concerns when writing alert agents:

• Alert agents may be called with no recipient (if none is configured), so the agent must be able to handle this situation, even if it only exits in that case. (Users may modify the configuration in stages, and add a recipient later.)
• If more than one recipient is configured for an alert, the alert agent will be called once per recipient. If an agent is not able to run concurrently, it should be configured with only a single recipient. The agent is free, however, to interpret the recipient as a list.
• When a cluster event occurs, all alerts are fired off at the same time as separate processes. Depending on how many alerts and recipients are configured, and on what is done within the alert agents, a significant load burst may occur. The agent could be written to take this into consideration, for example by queueing resource-intensive actions into some other instance, instead of directly executing them.
• Alert agents are run as the hacluster user, which has a minimal set of permissions. If an agent requires additional privileges, it is recommended to configure sudo to allow the agent to run the necessary commands as another user with the appropriate privileges.
• As always, take care to validate and sanitize user-configured parameters, such as CRM_alert_timestamp (whose content is specified by the user-configured timestamp-format), CRM_alert_recipient, and all instance attributes. Mostly this is needed simply to protect against configuration errors, but if some user can modify the CIB without having hacluster-level access to the cluster nodes, it is a potential security concern as well, to avoid the possibility of code injection.

Note

ocf:pacemaker:ClusterMon compatibility

The alerts interface is designed to be backward compatible with the external scripts interface used by the ocf:pacemaker:ClusterMon resource, which is now deprecated. To preserve this compatibility, the environment variables passed to alert agents are available prepended with CRM_notify_ as well as CRM_alert_. One break in compatibility is that ClusterMon ran external scripts as the root user, while alert agents are run as the hacluster user.