A set of RMC subroutines enables an application to register
with the RMC subsystem for event notifications.
An event is a change in an attribute that is of interest to the
application.
To register for event notifications, the application calls a particular
RMC subroutine and provides it with an event expression. In
most cases, an event expression consists of one or more attribute
names (usually dynamic attribute names), one or more mathematical
comparison symbols, and one or more constants that together describe
the event of interest. The RMC subsystem will evaluate the event expression
when the attribute values are observed. The RMC subsystem observes,
or obtains, the values of attributes specified in event expressions
either at periodic intervals or whenever a new value is supplied by
the associated resource manager, depending on the variable type of
the attribute.
When the RMC subsystem evaluates an event expression, it will trigger
an event if the expression evaluates to True. For example, resources
of the
IBM.FileSystem resource class use the dynamic attribute
PercentTotUsed to
represent the percentage of space used in a file system. The following
event expression, once registered with the RMC subsystem for a resource,
would cause the RMC subsystem to notify the application if the particular
file system resource is over 90 percent full.
PercentTotUsed > 90
As already stated, each event expression refers to a particular
attribute value. This is usually a dynamic attribute, since such attributes
represent changing characteristics of a resource. Once the application
registers with the RMC subsystem for event notifications based on
a particular event expression, the RMC subsystem will observe the
value of the attribute and evaluate the event expression. If the event
expression evaluates to True, the RMC subsystem will notify the application.
When observing attribute values to evaluate event expressions,
RMC remembers the previously observed value of the attribute. If the
event expression suffixes the attribute name with
@P, this
represents the previously observed value of the attribute. For example,
resources of the
IBM.Host resource class have the dynamic attribute
ProcRunQueue,
which indicates the average number of processes that are waiting for
the processor. The following event expression, once registered with
the RMC subsystem for an
IBM.Host resource, would cause the
RMC subsystem to notify the application if the average number of processes
in the run queue has increased by 50 percent or more between observations.
(ProcRunQueue - ProcRunQueue@P) >= (ProcRunQueue@P * 0.5)
Although
most event expressions consist of attribute names, one or more
mathematical comparison symbols, and one or more constants, this is
not True if the attribute type is of variable type
Quantum.
An attribute of variable type
Quantum signifies a change, but
has no value associated with it. To register for events involving
a single attribute of type
Quantum, the event expression
must consist of the attribute name only.
For example, the
IBM.FileSystem resource class has the dynamic
attribute
ResourceDefined that is of type
Quantum. Although
it has no value, it is asserted whenever a resource of the class is
created. The following event expression, once registered with the
RMC subsystem for the
IBM.FileSystem resource class, would
cause the RMC subsystem to notify the application when a file system
is created.
ResourceDefined
To register for events involving more than one attribute name,
where at least one attribute name is an attribute of type
Quantum,
the attributes of type
Quantum can only be used as operands
of boolean operators. For example, the
IBM.FileSystem resource
class has the dynamic resource attributes
ConfigChanged and
PercentTotused. The attribute
ConfigChanged is of
type
Quantum, while
PercentTotused is not. The following
event expression would result in an event notification if the file
system is more than 90% full or the persistent attributes of the
resource are modified:
PercentTotUsed > 90 || ConfigChanged
When registering for event notifications, a cluster applicaiton
can optionally specify a rearm event expression. Like most
event expressions, a rearm event expression consists of one or
more attribute names, one or more mathematical comparison symbol,
and one or more constants. The attributes in the rearm event expression
do not need to be the same as the attributes that are used in the
event expression. If a rearm event expression is specified, then
the RMC will stop evaluating the event expression once it tests True,
and instead will evaluate the rearm event expression until it tests
True. Once the rearm event expression tests True, the RMC subsystem
will once again evaluate the event expression.
The following diagram illustrates the cycle of event expression
/ rearm event expression evaluation.
To understand why the application might want to specify a rearm
event expression when registering for event notifications, consider
the event registration that specifies the following event expression
on an IBM.FileSystem resource.
PercentTotUsed > 90
For this event expression, the RMC subsystem will periodically
observe the value of the PercentTotUsed attribute for the resource
and will evaluate the event expression. If the event expression evaluates
to True, the RMC subsystem will notify the application. If there is
no rearm event expression, and the event expression still tests True
the next time the RMC subsystem evaluates it, the RMC subsystem will
again notify the application. The RMC subsystem will continue to notify
the application each time it evaluates the event expression until
it tests false. In some cases, the application may want these repeated
notifications. In other situations, however, it might be preferable
to be notified once until the problem has been solved and only then
"rearm" the event expression. The rearm event expression is a way
of telling the RMC subsystem when the event expression should be evaluated
again. In our example, say the application also provides the following
rearm event expression when registering for the event notifications.
PercentTotUsed < 75
If this rearm event expression is specified, then once the event
expression PercentTotUsed > 90 evaluates to True,
the RMC subsystem will stop evaluating it, and will instead evaluate
the rearm event expression until it tests True. When calling the RMC
subroutine to register for event notifications, the application can
specify whether or not it also wants to be notified when the rearm
event expression evaluates to True. Whether the application chooses
to be notified or not, once the rearm event expression tests True,
the RMC subsystem will toggle to once again evaluate the event expression.
