HP-UX I/O Subsystem EVM Events

3 EVM Eventing Mechanism

EVM

Daemon

Event Viewer

dev/kevm

Logger

Subscribers

Posters

Logs

Application

rocesses

Application

rocesses

sockets

Kernel

Posters

Kernel

Subscribers

kevm

Kernel Q

User Q

! KERNEL_ONLY

An EVM subsystem is a means through which any system component or application program can report the status

of an operation. The EVM subsystem consists of an EVM daemon which is a resident process responsible for

distributing the events to the subscribing processes. The component that makes this information available to the

EVM daemon is known as an Event poster. The component that is interested in the posted information is known as

the Event Subscriber.

An Event Poster must declare an EVM template before posting the event. An EVM template consists of the event

name and other data items whose values are consistent across the instances. The EVM daemon matches the

posted event with the event template. If the EVM template is not available it returns an error. If the EVM template is

available, the Event poster appends the variable data such as the class, instance, and so forth, and the posting

API appends the environmental information such as the username and timestamp. This whole package of data

consisting of the standard data from the template, the variable data and the environmental information is known

as the Event. This event is distributed to the subscribing clients.

Once the Event Poster posts the event to the EVM daemon, the EVM daemon notifies the subscribed clients of the

occurrence of the event. For example, subscribers can be system administrators or other kernel subsystem

components. An event posted in the kernel space can be subscribed by kernel as well as user processes. User

space events cannot be subscribed by the kernel processes. The EVM API library provides the event name

matching mechanisms that can be used by the Event subscribers. EVM filters can be used to filter events based on

the interest. The Event Subscriber then determines what action to take with the notified event.