Writing Monitors for the Event Monitoring Service (December 1999)

Chapter 3 63

Creating a Resource Monitor

Writing the Monitor

Log File Recommendations

To log error messages, log to the api.log using the rm_log() function.

Startup and Initialization Considerations

This section describes the startup and initialization sequence performed

by EMS. For proper operation of your resource monitor, ensure that your

monitor does not conﬂict with the actions of the Registrar.

Registrar Considerations

The registrar is started via inetd(1M). To allow this to happen, the

registrar needs a known port number assignment.

At installation, the Registrar’s reserved port number is added to the

/etc/services ﬁle. The entry is as follows:

registrar 1712/tcp # resource monitoring service

registrar 1712/udp # resource monitoring service

NOTE The registrar does not actually use its UDP listening port. It is included

only because the Internet Assigned Numbers Authority (IANA) prefers

to assign ports in pairs

If the appropriate resource monitor process is not found, the registrar

starts the process and waits until the resource monitor can communicate

with the registrar. The resource monitor is ready to communicatewhen it

has called rm_monitor_start() and rm_get_next_event().

In order to minimize potential timeouts when communicating with a

monitor, call rm_monitor_start and rm_get_next_event (in that order)

as soon as possible in your monitor code.

EMS starts a new instance of a resource monitor if no running processes

exactly match the path name and arguments speciﬁed after the MONITOR

keyword in the resource dictionary. EMS hashes the complete command

line into a unique number. A named pipe is created in

/etc/opt/resmon/pipe with this number as its name. When the

registrar needs to communicate with a monitor, it checks to see whether

any process is reading from the corresponding named pipe. If so, it begins

communicating with that process. Otherwise, it starts the monitor in a

way that causes the monitor to open a named pipe. For this reason,