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Chapter 28. Triggering scripts for cluster events

A Pacemaker cluster is an event-driven system, where an event might be a resource or node failure, a configuration change, or a resource starting or stopping. You can configure Pacemaker cluster alerts to take some external action when a cluster event occurs by means of alert agents, which are external programs that the cluster calls in the same manner as the cluster calls resource agents to handle resource configuration and operation.

The cluster passes information about the event to the agent by means of environment variables. Agents can do anything with this information, such as send an email message or log to a file or update a monitoring system.

  • Pacemaker provides several sample alert agents, which are installed in /usr/share/pacemaker/alerts by default. These sample scripts may be copied and used as is, or they may be used as templates to be edited to suit your purposes. Refer to the source code of the sample agents for the full set of attributes they support.
  • If the sample alert agents do not meet your needs, you can write your own alert agents for a Pacemaker alert to call.

When you use one of the sample alert agents, you should review the script to ensure that it suits your needs. These sample agents are provided as a starting point for custom scripts for specific cluster environments. Note that while Red Hat supports the interfaces that the alert agents scripts use to communicate with Pacemaker, Red Hat does not provide support for the custom agents themselves.

28.1.1. Prerequisites
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  • Install the agent on each node in the cluster 

    # install --mode=0755 /usr/share/pacemaker/alerts/alert_file.sh.sample /var/lib/pacemaker/alert_file.sh
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    After you have installed the script, you can create an alert that uses the script.

You can install the alert_file.sh alert agent and configure an alert to log events to a file. Alert agents run as the user hacluster, which has a minimal set of permissions.

Procedure

  1. On each node in the cluster, install the alert_file.sh.sample script as alert_file.sh. 

    # install --mode=0755 /usr/share/pacemaker/alerts/alert_file.sh.sample /var/lib/pacemaker/alert_file.sh
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  2. On each node in the cluster, create the log file that will be used to record the events. 

    # touch /var/log/pcmk_alert_file.log
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    1. Change the ownership of the file pcmk_alert_file.log to user hacluster. 

      # chown hacluster:haclient /var/log/pcmk_alert_file.log
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    2. Change the permissions for the file pcmk_alert_file.log to read and write for the user hacluster. 

      # chmod 600 /var/log/pcmk_alert_file.log
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  3. On one node in the cluster, create the alert. 

    # pcs alert create id=alert_file description="Log events to a file." path=/var/lib/pacemaker/alert_file.sh
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  4. Add the path to the log file as the recipient for the alert. 

    # pcs alert recipient add alert_file id=my-alert_logfile value=/var/log/pcmk_alert_file.log
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You can install the alert_snmp.sh.sample script as alert_snmp.sh and configure an alert that uses the installed alert_snmp.sh alert agent to send cluster events as SNMP traps. By default, the script will send all events except successful monitor calls to the SNMP server.

Procedure

  1. On each node in the cluster, install the alert_snmp.sh.sample script as alert_snmp.sh. 

    # install --mode=0755 /usr/share/pacemaker/alerts/alert_snmp.sh.sample /var/lib/pacemaker/alert_snmp.sh
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  2. On one node in the cluster, configure an alert that uses the alert_snmp.sh agent, configuring the timestamp format as a meta option. 

    # pcs alert create id=snmp_alert path=/var/lib/pacemaker/alert_snmp.sh meta timestamp-format="%Y-%m-%d,%H:%M:%S.%01N"
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  3. Configure a recipient for the alert 

    # pcs alert recipient add snmp_alert value=192.168.1.2
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  4. Display the alert configuration. 

    # pcs alert
Alerts:
 Alert: snmp_alert (path=/var/lib/pacemaker/alert_snmp.sh)
  Meta options: timestamp-format=%Y-%m-%d,%H:%M:%S.%01N.
  Recipients:
   Recipient: snmp_alert-recipient (value=192.168.1.2)
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You can install the alert_smtp.sh agent and then configure an alert that uses the installed alert agent to send cluster events as email messages.

Procedure

  1. On each node in the cluster, install the alert_smtp.sh.sample script as alert_smtp.sh. 

    # install --mode=0755 /usr/share/pacemaker/alerts/alert_smtp.sh.sample /var/lib/pacemaker/alert_smtp.sh
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  2. On one node in the cluster, configure an alert to send cluster events as email messages. 

    # pcs alert create id=smtp_alert path=/var/lib/pacemaker/alert_smtp.sh options email_sender=donotreply@example.com
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  3. Configure a recipient for the alert. 

    # pcs alert recipient add smtp_alert value=admin@example.com
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  4. Display the alert configuration. 

    # pcs alert
Alerts:
 Alert: smtp_alert (path=/var/lib/pacemaker/alert_smtp.sh)
  Options: email_sender=donotreply@example.com
  Recipients:
   Recipient: smtp_alert-recipient (value=admin@example.com)
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28.2. Creating a cluster alert
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The following command creates a cluster alert. The options that you configure are agent-specific configuration values that are passed to the alert agent script at the path you specify as additional environment variables. If you do not specify a value for id, one will be generated. 

pcs alert create path=path [id=alert-id] [description=description] [options [option=value]...] [meta [meta-option=value]...]
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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.

The following example creates a simple alert that will call myscript.sh for each event. 

# pcs alert create id=my_alert path=/path/to/myscript.sh
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For a more complete example procedure to create and modify a full cluster alert, see Creating and modifying a cluster alert.

There are a variety of pcs commands you can use to display, modify, and remove cluster alerts.

Displaying configured cluster alerts

The following command shows all configured alerts along with the values of the configured options: 

# pcs alert [config]
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Modifying a configured cluster alert

The following command updates an existing alert with the specified alert-id value: 

# pcs alert update alert-id [path=path] [description=description] [options [option=value]...] [meta [meta-option=value]...]
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Removing a cluster alert

The following command removes an alert with the specified alert-id value: 

# pcs alert remove alert-id
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Alternately, you can run the pcs alert delete command, which is identical to the pcs alert remove command. Both the pcs alert delete and the pcs alert remove commands allow you to specify more than one alert to be deleted.

28.4. Configuring cluster alert recipients
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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.

The recipient may be anything the alert agent can recognize: an IP address, an email address, a file name, or whatever the particular agent supports.

Adding a recipient to a configured cluster alert

The following command adds a new recipient to the specified alert. 

# *pcs alert recipient add alert-id value=recipient-value [id=recipient-id] [description=description] [options [option=value]...] [meta [meta-option=value]...] *
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The following example command adds the alert recipient my-alert-recipient with a recipient ID of my-recipient-id to the alert my-alert. This will configure the cluster to call the alert script that has been configured for my-alert for each event, passing the recipient some-address as an environment variable. 

#  pcs alert recipient add my-alert value=my-alert-recipient id=my-recipient-id options value=some-address
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Updating an existing alert recipient

The following command updates an existing alert recipient. 

# pcs alert recipient update recipient-id [value=recipient-value] [description=description] [options [option=value]...] [meta [meta-option=value]...]
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Removing an alert recipient

The following command removes the specified alert recipient. 

# pcs alert recipient remove recipient-id
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Alternately, you can run the pcs alert recipient delete command, which is identical to the pcs alert recipient remove command. Both the pcs alert recipient remove and the pcs alert recipient delete commands allow you to remove more than one alert recipient.

28.5. Configuring cluster alert meta options
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As with resource agents, meta options can be configured for alert agents to affect how Pacemaker calls them. The following table describes the alert meta options. Meta options can be configured per alert agent as well as per recipient.

Expand
Table 28.1. Alert Meta Options
Meta-AttributeDefaultDescription

enabled

true

If set to false for an alert, the alert is not used. If set to true for an alert and false for a particular recipient of that alert, that recipient is not be used.

timestamp-format

%H:%M:%S.%06N

Format the cluster uses 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.

The following example procedure configures cluster alert meta options for both an alert agent and for the alert recipients. The procedure configures an alert that calls the script myscript.sh and then adds two recipients to the alert. The script gets called twice for each event.

Procedure

  1. Configure an alert that calls the script myscript.sh and that uses a 15-second timeout. 

    # pcs alert create id=my-alert path=/path/to/myscript.sh meta timeout=15s
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  2. Add an alert recipient that has an ID of my-alert-recipient1, passing the call to the recipient someuser@example.com with a timestamp in the format %D %H:%M. 

    # pcs alert recipient add my-alert value=someuser@example.com id=my-alert-recipient1 meta timestamp-format="%D %H:%M"
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  3. Add an alert recipient that has an ID of my-alert-recipient2, passing the call to the recipient otheruser@example.com with a timestamp in the format %c. 

    # pcs alert recipient add my-alert value=otheruser@example.com id=my-alert-recipient2 meta timestamp-format="%c"
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28.6. Creating and modifying a cluster alert
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The following example procedure creates alerts, adds recipients, modifies an alert, removes an alert, and displays the configured alerts at each step.

While you must install the alert agents themselves on each node in a cluster, you need to run the pcs commands only once.

Procedure

  1. Create a simple alert. Since no alert ID value is specified, the system creates an alert ID value of alert. 

    # pcs alert create path=/my/path
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  2. Add a recipient of rec_value to the alert. Since this command does not specify a recipient ID, the value of alert-recipient is used as the recipient ID. 

    # pcs alert recipient add alert value=rec_value
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  3. Add a second recipient of rec_value2 to the alert. This command specifies a recipient ID of my-recipient for the recipient. 

    # pcs alert recipient add alert value=rec_value2 id=my-recipient
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  4. Display the alert configuration. 

    # pcs alert config
Alerts:
 Alert: alert (path=/my/path)
  Recipients:
   Recipient: alert-recipient (value=rec_value)
   Recipient: my-recipient (value=rec_value2)
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  5. Add a second alert, with an alert ID of my-alert. 

    # pcs alert create id=my-alert path=/path/to/script description=alert_description options option1=value1 opt=val meta timeout=50s timestamp-format="%H%B%S"
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  6. Add a recipient for that alert for the second alert with a recipient value of my-other-recipient. Since no recipient ID is specified, the system provides a recipient id of my-alert-recipient. 

    # pcs alert recipient add my-alert value=my-other-recipient
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  7. Display the alert configuration. 

    # pcs alert
Alerts:
 Alert: alert (path=/my/path)
  Recipients:
   Recipient: alert-recipient (value=rec_value)
   Recipient: my-recipient (value=rec_value2)
 Alert: my-alert (path=/path/to/script)
  Description: alert_description
  Options: opt=val option1=value1
  Meta options: timestamp-format=%H%B%S timeout=50s
  Recipients:
   Recipient: my-alert-recipient (value=my-other-recipient)
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  8. Modify the alert values for the alert my-alert. 

    # pcs alert update my-alert options option1=newvalue1 meta timestamp-format="%H%M%S"
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  9. Modify the alert values for the recipient my-alert-recipient. 

    # pcs alert recipient update my-alert-recipient options option1=new meta timeout=60s
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  10. Display the alert configuration. 

    # pcs alert
Alerts:
 Alert: alert (path=/my/path)
  Recipients:
   Recipient: alert-recipient (value=rec_value)
   Recipient: my-recipient (value=rec_value2)
 Alert: my-alert (path=/path/to/script)
  Description: alert_description
  Options: opt=val option1=newvalue1
  Meta options: timestamp-format=%H%M%S timeout=50s
  Recipients:
   Recipient: my-alert-recipient (value=my-other-recipient)
    Options: option1=new
    Meta options: timeout=60s
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  11. Remove the recipient my-alert-recipient from alert. 

    # pcs alert recipient remove my-recipient
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  12. Display the alert configuration. 

    # pcs alert
Alerts:
 Alert: alert (path=/my/path)
  Recipients:
   Recipient: alert-recipient (value=rec_value)
 Alert: my-alert (path=/path/to/script)
  Description: alert_description
  Options: opt=val option1=newvalue1
  Meta options: timestamp-format="%M%B%S" timeout=50s
  Recipients:
   Recipient: my-alert-recipient (value=my-other-recipient)
    Options: option1=new
    Meta options: timeout=60s
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  13. Remove the alert myalert from the configuration. 

    # pcs alert remove myalert
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  14. Display the alert configuration. 

    # pcs alert
Alerts:
 Alert: alert (path=/my/path)
  Recipients:
   Recipient: alert-recipient (value=rec_value)
    Copy to Clipboard Toggle word wrap

28.7. Writing a cluster alert agent
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There are three types of Pacemaker cluster alerts: node alerts, fencing alerts, and resource alerts. The environment variables that are passed to the alert agents can differ, depending on the type of alert. The following table describes the environment variables that are passed to alert agents and specifies when the environment variable is associated with a specific alert type.

Expand
Table 28.2. Environment Variables Passed to Alert Agents
Environment VariableDescription

CRM_alert_kind

The type of alert (node, fencing, or resource)

CRM_alert_version

The version of Pacemaker sending the alert

CRM_alert_recipient

The configured recipient

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_timestamp

A timestamp created prior to executing the agent, in the format specified by the timestamp-format meta option. 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 or other circumstances).

CRM_alert_node

Name of affected node

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_task

The requested fencing or resource operation (provided with fencing and resource alerts only)

CRM_alert_rc

The numerical return code of the fencing or resource operation (provided with fencing and resource alerts only)

CRM_alert_rsc

The name of the affected resource (resource alerts only)

CRM_alert_interval

The interval of the resource operation (resource alerts only)

CRM_alert_target_rc

The expected numerical return code of the operation (resource alerts only)

CRM_alert_status

A numerical code used by Pacemaker to represent the operation result (resource alerts only)

When writing an alert agent, you must take the following concerns into account.

  • 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.
  • 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 alert options. This is necessary to protect against configuration errors. In addition, if some user can modify the CIB without having hacluster-level access to the cluster nodes, this is a potential security concern as well, and you should avoid the possibility of code injection.
  • If a cluster contains resources with operations for which the on-fail parameter is set to fence, there will be multiple fence notifications on failure, one for each resource for which this parameter is set plus one additional notification. Both the pacemaker-fenced and pacemaker-controld will send notifications. Pacemaker performs only one actual fence operation in this case, however, no matter how many notifications are sent.
Note

The alerts interface is designed to be backward compatible with the external scripts interface used by the ocf:pacemaker:ClusterMon resource. 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 the ClusterMon resource ran external scripts as the root user, while alert agents are run as the hacluster user.

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