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SELECT statements and their associated clauses are evaluated in the order they
appear in the CDS file. If all the clauses of a SELECT statement are evaluated
successfully, the incoming event matches the corresponding class.
After an event is matched with a class because of successful SELECT statement
evaluation, processing continues with the FETCH statement, unless the class is
*DISCARD*, in which case the event is discarded. If the evaluation of a SELECT
statement fails, the kernel tries to match the event with the SELECT statement of
the next class. If the incoming event cannot be matched with any class, it is
discarded.
Each time a SELECT statement is evaluated successfully, the adapter kernel layer
creates three temporary pseudo-variables: $Nn, $Kn, $Vn (where n is the
identification number of a clause in the SELECT statement). These variables
contain the name, key, and value of the attribute specified in the clause,
respectively. The pseudo-variables can then be used in any following SELECT,
FETCH,orMAP statement.
By default, the attribute name specified in an ATTR() expression is a string, and
the attribute matching this name is searched for sequentially in the incoming event.
For most adapters, every incoming event contains a minimum set of mandatory
fields. For this reason, each adapter supports built-in keywords that can be used to
reference these mandatory attributes and thereby directly access their values. These
keywords have the format $attribute_name. Examples of keywords supported by the
SNMP adapter are: $AGENT_ADDRESS, $COMMUNITY, $ENTERPRISE,
$TYPE, and $SPECIFIC. These keywords refer to the mandatory fields of an
SNMP Trap-PDU. Each adapter can also define global variables, such as
RECEPTION TIME, SVARBIND, and so forth.
Using the $ notation, a clause for SNMP authentication failure traps can be written
as follows:
1: ATTR(=,$TYPE),VALUE(=,4);
This notation is not simpler than the format shown in the previous example,
ATTR(=, ″type″), but evaluation will be faster since it results in direct access to the
variable instead of a linear search.
The syntax shown in the preceding example is generic, and as such, it can be
rather verbose for commonly used criteria. Several shortcuts are provided in order
to alleviate the notation. For example, the previous example can be written as
follows:
1:$TYPE=4;
Output from the class selection process is the name of the event class, a table of
pseudo-variables $Nn, $Kn, $Vn, and all adapter-specific variables (for example,
$TYPE, $VARBIND, and so forth).
FETCH Statement
The FETCH statement of a class definition statement allows manipulation and
modification to the attribute names, keys, and values retrieved by the SELECT
statement for the incoming event. Sometimes it is necessary to perform tasks such
as extracting a substring from an attribute value, adding two values, and so forth.
158 IBM Tivoli Enterprise Console: Adapters Guide