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WatchdogApp

Watchdog Application Configuration

The N2SVCD Watchdog Application is a special case application.

A WatchdogApp will always be created, even if not present in applications.
The WatchdogApp always runs within the n2svcd process, it never forks.

Note that the WatchdogApp is quite distinct from the ManageApp:

The WatchdogApp is mandatory, while ManageApp is entirely optional.
The WatchdogApp is never forked, but ManageApp may be forked.
The WatchdogApp is responsible for Shutdown on Signals and Heartbeats.
The ManageApp supports query, trace, and management of applications and instances.

The following configuration is used to create a Watchdog Application instance. If you omit this section, a default Watchdog Application will be created.

<?xml version="1.0" encoding="utf-8"?>
<n2svcd>
  ...
  <applications>
    ...
    <application name="Watchdog" module="WatchdogApp">
      <include><lib>../apps/watchdog/lib</lib></include>
      <parameters>
        <parameter name="shutdown_notice_secs" value="4"/>
        <parameter name="termination_notice_ms" value="500"/>
        <parameter name="heartbeat_interval_secs" value="120"/>
        <parameter name="heartbeat_response_ms" value="900"/>
      </parameters>
      <config>
        <heartbeats>
            <heartbeat app_name="App1" heartbeat_interval_secs="300" heartbeat_response_ms="300000"/>
        </heartbeats>
      </config>
    </application>
    ...
  </application>
  ...
</n2svcd>

Configuration Details

The application element attributes for a Watchdog Application instance may include the below.

For details of the various parameter types used, refer to Common Configuration.

Parameter Name	Type	XML Type	Description
See: Common Application configuration
`module`	String	Attribute	[Required] `WatchdogApp`
`include.lib`	String	Attribute	[Required] `../apps/watchdog/lib`
`parameters`	Array	Element	[Required] As per Common Configuration Application `parameters`.
`"edr_enabled"`	-	-	This value is ignored; the Watchdog Application does not support writing EDRs.
`"shutdown_notice_secs"`	Integer (1..60)	Attribute	Number of seconds from Shutdown Notice until actual Shutdown. (Default = `2`)
`"termination_notice_ms"`	Integer (100..5000)	Attribute	Number of milliseconds from Shutdown until process Termination. (Default = `500`)
`"heartbeat_interval_secs"`	Integer (0..60)	Attribute	Number of seconds between application heartbeats. Set to `0` to disable heartbeats. (Default = `30`)
`"heartbeat_startup_secs"`	Integer (1..60)	Attribute	Number of seconds after Watchdog startup to send initial application heartbeats. Only applicable if `heartbeat_interval_secs` is greater than `0`. (Default = `3`)
`"heartbeat_response_ms"`	Integer (100..5000)	Attribute	Number of milliseconds before an application Heartbeat Response is considered overdue. (Default = `1000`)
`"overdue_to_terminate_secs"`	Integer (0..60)	Attribute	When a heartbeat response is overdue, the watchdog immediately generates a warning message and then begins checking to see if the application sub-process has died. If, after this number of seconds, the overdue application sub-process has not died, then watchdog will actively terminate the application sub-process using the `TERM` signal. (Default = `3`)
`"terminated_to_killed_secs"`	Integer	Attribute	After sending the `TERM` signal, the watching process will continue to monitor to check to see if the application sub-process has died. If, after this number of seconds, the overdue application sub-process has not died, then watchdog will actively kill the application subprocess using the `KILL` signal, and then will wait indefinitely until the sub-process dies. (Default = `3`)
`"restart_retry_secs"`	Integer	Attribute	After the overdue application sub-process has died, then the watchdog process will immediately restart a new application sub-process. If this restart fails, the watchdog process will repeatedly attempt to restart the process each this number of seconds. Will be set to the maximum of `1` or the configured value. (Default = `30`)
`"application_availability_secs"`	Integer (0..60)	Attribute	Number of seconds between reporting application availability. Set to `0` to disable checking. Note that this is not affected by application heartbeats. (Default = `10`)

Application-Specific Heartbeats

Individual applications may have explicit heartbeat configuration values set, overriding the heartbeat_interval_secs, heartbeat_startup_secs, and heartbeat_response_ms parameters for that application only.

Application-specific heartbeats are configured in the heartbeats section of the extended configuration config block:

<?xml version="1.0" encoding="utf-8"?>
<n2svcd>
  ...
  <applications>
    ...
    <application name="Watchdog" module="WatchdogApp">
      ...
      <parameters>
        ...
      </parameters>
      <config>
        <heartbeats>
            <heartbeat app_name="App1" heartbeat_interval_secs="300" heartbeat_response_ms="300000"/>
        </heartbeats>
      </config>
    </application>
    ...
  </application>
  ...
</n2svcd>

The following parameters are supported for each heartbeat configured:

Field	Type	Details
`app_name`	String	[Required] The configured application name that the heartbeat applies for.
`heartbeat_interval_secs`	Integer	The number of seconds between sending application heartbeats. (Default: `30`)
`heartbeat_startup_secs`	Integer	The number of seconds before sending first heartbeat. (Default: `3`)
`heartbeat_response_ms`	Integer	The number of milliseconds before an application heartbeat response is considered overdue. _(Default: `1000`)

Message Handling

In addition to the common Application management messages, the WatchdogApp uses the following messages:

MANAGEMENT Messages (!MANAGEMENT-*-REQUEST outbound).