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4.11 SCAN SYSTEM
The scan system is used to acquire data from one environment into another one. The receiving environment is always located on a WS. It provides an efficient mechanism to maintain local copies of remote attributes, allowing:
Faster and cheaper (in term of CPU usage) access to remote attributes in case they have to be regularly accessed.
An LCU shall be scanned from a single WS. Normally the reporting Workstation.
The relation between a remote attribute and its local copy is called a scan link.
Although scan links can be defined at any time, one should already prepare them during the design phase of the database. The scan links should be defined in configuration files to be handled later on with the scanLinks utility described in the CCS User Manual[1]. Provided the necessary information is supplied in the configuration file, the scan links can also be established off line.
The scan system provides two basic mechanism for updating the local copies: Polling and SRBX.
To optimize the traffic between environments one should use the most appropriate mechanism.
SRBX, or update on change shall be used whenever the values are not changing too often. If this is not the case, typically if the attribute changes value several times per second, one should limit the number of updates by introducing a deadband (no update until the value has changed more than the deadband) or use the polling mechanism.
Scan links should be established once at environment startup. If it is not necessary to actually scan all attributes, subsets can be predefined in configuration files, and enabled/disabled at any time using the scanLinksEnable, scanLinksDisable utilities.
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