Rockwell Automation FactoryTalk Transaction Manager User Guide - Manual (Page 137)

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controller by the FactoryTalk Live Data server once it receives the read
request from the Transaction Control Manager service. Another exception is
that the FactoryTalk Transaction Manager service never requests the current
value of an unscheduled data point from the FactoryTalk Live Data server.
B
UFFERING
I
N THE
F
ACTORY
T
ALK
T
RANSACTION
M
ANAGER
S
ERVICE
The FactoryTalk Transaction Manager service has a local data point cache
that contains the current values for every data point and the time the value
was collected. If the Data Valid option that you set on the appropriate
connector’s Data Point Definition dialog box is set to zero, data is requested
from the control connector every time it is needed. Using this data, the
FactoryTalk Transaction Manager service knows which data points are still
fresh (still in the data valid timeout range), and which data points need to be
requested from the control connector. Note that the data is not read from the
controller at this time but from the control connector’s buffers. The
exception to this rule is device scheduled data points, which are read from
the controller by the data server once it receives the read request from the
control connector. The other exception is that the FactoryTalk Transaction
Manager service never requests the current value of an unscheduled data
point from the control connector.
Individual transaction buffers are maintained for each transaction that is
running; if two copies of the same transaction are running at the same time,
the data values from the second transaction do not overwrite the data values
from the first. Only unscheduled transactions can execute more than one
copy of the same transaction at the same time.
B UFFERING I N C ACHED T RANSACTION F ILES
Completed transactions that are not configured for real-time storage are
buffered in cached transaction (*.rsl) files prior to storage in the database.
Therefore, data will not be available from a query until it has been removed
from the cache file and written to the database.
The rate that the cache transaction files are applied is also controllable. In
the Transaction Definition dialog box, set the number of completed