Specifications

15 – Configuring Distributed Server Architecture
338 R400
Data Access
Distributed Server Architecture provides global access to point parameter data on
all servers in the system. Each server provides automatic dynamic caching of
remote data for all of its clients, so that clients access their local Server for all
data. Clients do not access remote Servers directly.
For example:
• Station accesses its local server to display alarms for points on local and
remote servers.
• Station accesses its local server to display data for points on local and
remote servers.
• An OPC client application can access a local OPC server on a PlantScape
server to obtain point data from that or any other PlantScape server.
• A Microsoft Excel spreadsheet connected only to its local server can display
data through Microsoft Excel Data Exchange from multiple servers.
• An application, or a network application, connected only to its local server
can access point data from multiple servers.
A server establishes a local cache point (see How Remote Points are Located) and
subscribes to a remote server when one of its clients requests data for a point that
is not already in the database. While the subscription is in effect the data owner
uses report by exception, only sending data to the caching server on change.
When the data is no longer referenced by any of its client Stations or applications,
the subscribing server cancels the subscription to the data owner. This
subscription mechanism ensures maximum efficiency both on the servers and
over the network.
How Remote Points are Located
Distributed Server Architecture provides a global name space for access to all
point information in the system (see “Point IDs” on page 339).
The first time a remote point is accessed, the local server asks the configured
servers in the system for the data owner of the point. When the data owner is
determined, the local server automatically creates a cache reference in the local
database. This cache reference is known as a “cache point”.
plscpcg.book Page 338 Monday, May 28, 2001 10:11 AM