| To be used for the index to a table. Allows an application
to download only those rows changed since a particular time.
Note that this is not a history mechanism. Only current values
of underlying objects are returned; saved instance values
associated with particular values of sysUpTime are not.
An entry is considered changed if the value of any object in the
entry changes, if the row is created, or if any object in the
entry is created or deleted. Note that deleted entries cannot
be detected or downloaded.
A time-filtered conceptual table is created by inserting a
single object of SYNTAX TimeFilter as the first INDEX component
in a copy of an existing basic conceptual table (i.e., any
SEQUENCE without a TimeFilter INDEX component). Thus, for
each conceptual entry 'I' in the basic table, there exists N
conceptual entries in the time-filtered version, indexed N.I,
where 'N' is equal to the value of sysUpTime.
When an application retrieves conceptual instances from a
time-filtered table, and an INDEX value is provided for the
TimeFilter INDEX component 'N', the agent will only consider
returning basic conceptual entries (e.g., 'fooColumn.N.I') if
any column within the basic conceptual entry has changed since
sysUpTime 'N'. If not, the basic conceptual entry will
be ignored for the particular retrieval operation.
When sysUpTime is equal to zero, this table shall be empty.
One conceptual entry exists for each past value of sysUpTime,
except that the whole table is purged should sysUpTime wrap.
As an entry in a time-filtered table is updated (i.e., one of
the columns in the basic conceptual table is changed), new
conceptual entries are also created in the time-filtered version
(which still shares the now updated object values with all other
instances). The number of unique time-filtered instances that
are created is determined by the value of sysUpTime at which the
basic entry was last updated. One unique instance will exist
for each value of sysUpTime at the last update time for the row.
However, a new TimeFilter index instance is created for each new
sysUpTime value. The TimeFilter index values not associated
with entry updates are called duplicate time-filtered instances.
After some deployment experience, it has been determined that
a time-filtered table is more efficient if the agent
stops a MIB walk operation by skipping over rows with a
TimeFilter index value higher than the value in the received
GetNext/GetBulk request. That is, instead of incrementing a
TimeFilter index value, the agent will continue to the next
object or table. As a consequence, GetNext or GetBulk
operations will provide only one pass through a time-filtered
table.
It is suggested that an agent implement a time-filtered table
in this manner to improve performance and avoid a MIB walk
getting stuck in time-filtered tables. It is, however, still
acceptable for an agent to implement a time-filtered table in
the traditional manner (i.e., every conceptual time-filtered
instance is returned in GetNext and GetBulk PDU responses), and
management applications must be able to deal with such
traditional implementations.
See the appendix for further discussion of this textual
convention.
The following example is provided to demonstrate TimeFilter
behavior:
Consider the following basic conceptual table, basicFooTable.
(Note that the basic version of a time-filtered table may not
actually be defined.)
basicFooTable:
basicFooTable ...
INDEX { fooIndex }
BasicFooEntry {
fooIndex Integer32,
fooCounts Counter32
}
For this example, the basicFooTable contains two static
conceptual entries (fooIndex equals '1' and '2'), created at
time zero. It also contains one dynamic conceptual entry
(fooIndex equals '3'), which is created at time '3' and deleted
at time '7'.
The time-filtered version of the basicFooTable could be defined
as follows:
FooTable:
fooTable ...
INDEX { fooTimeMark, fooIndex }
FooEntry {
fooTimeMark TimeFilter,
fooIndex Integer32,
fooCounts Counter32
}
Note that entries exist in the time-filtered conceptual table
only if they actually exist in the underlying (basic) table.
For this example, the fooTable will have three underlying
basic entries (fooIndex == 1, 2, and 3), with the following
activity (for sysUpTime equal 0 to 9):
- fooEntry.N.1 is created at time '0' and most recently
updated at time '6' to the value '5'.
- fooEntry.N.2 is created at time '0' and most recently
updated at time '8' to the value '9'.
- fooEntry.N.3 is created at time '3', updated at time '5'
to the value '17', and deleted at time '7'.
The following tables show the values that would be returned for
MIB walk operations with various TimeFilter values, done at
different times. An application issues a retrieval request at
time 'T', with a TimeFilter value, 'N' (typically set to a lower
value, such as the value of sysUpTime at the last polling cycle).
The following values would be returned in a MIB walk of
fooCounts.N if T equals '0' and N equals '0':
fooCounts.N.I Value
==========================
fooCounts.0.1 0
fooCounts.0.2 0
Note that nothing is returned for fooCounts.0.3, since that
entry does not exist at sysUpTime equals '0'.
The following values would be returned in a full (traditional) MIB
walk of fooCounts.N if T equals '3' and N equals '0':
fooCounts.N.I Value
=======================
fooCounts.0.1 0
fooCounts.0.2 0
fooCounts.0.3 0
fooCounts.1.3 0
fooCounts.2.3 0
fooCounts.3.3 0
Note that there are no instances for T equals 1 or 2 for the
first two values of N, as these entries did not change
since they were created at time '0'.
Note that the current value for 'fooCounts.N.3' is returned
here, even for values of N less than '3' (when the entry was
created). The agent only considers the current existence of an
entry in the TimeFilter algorithm, not the time when the entry
was created.
Note that the instances 'fooCounts.0.3', 'fooCounts.1.3',
and 'fooCounts.2.3' are duplicates and can be suppressed by the
agent in a MIB walk.
The following values would be returned in a full (traditional)
MIB walk of fooCounts.N if T equals '6' and N equals '3':
fooCounts.N.I Value
=======================
fooCounts.3.1 5
fooCounts.3.3 17
fooCounts.4.1 5
fooCounts.4.3 17
fooCounts.5.1 5
fooCounts.5.3 17
fooCounts.6.1 5
Note that no instances for entry 'fooCounts.N.2' are returned,
since it has not changed since time '3'.
Note that all instances except 'fooCounts.5.3' and
'fooCounts.6.1' are duplicates and can be suppressed by the
agent in a MIB walk.
The following values would be returned in a full (traditional)
MIB walk of fooCounts.N if T equals '9' and N equals '6':
fooCounts.N.I Value
=======================
fooCounts.6.1 5
fooCounts.6.2 9
fooCounts.7.2 9
fooCounts.8.2 9
Note that no instances for entry 'fooCounts.N.3' are returned,
since it was deleted at time '7'.
Note that instances 'fooCounts.6.2' and 'fooCounts.7.2'
are duplicates and can be suppressed by the agent in a MIB
walk. |
