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DYNAMIC DATA EXCHANGE ( DDE )
The concepts in this section are key to understanding DDE and the DDEML.
Dynamic data exchange always takes place between a client application and a
server application. The client initiates the exchange by establishing a
conversation with the server so that it can send transactions to the server.
(A transaction is a request for data or services.)
The server responds to these transactions by providing data or services to
the client. A server can have many clients at the same time, and a client
can request data from multiple servers. Also, an application can be both a
client and a server.
A client terminates a conversation when it no longer needs a server's data
or services. For example, a graphics application might contain a bar graph
that represents a corporation's quarterly profits, and the data for the bar
graph might be contained in a spreadsheet application. To obtain the latest
profit figures, the graphics application (the client) establishes a
conversation with the spreadsheet application (the server). The graphics
application then sends a transaction to the spreadsheet application,
requesting the latest profit figures.
A DDE server uses a three-level hierarchy-service name (called "application
name" in previous DDE documentation), topic name, and item name-to uniquely
identify a unit of data that the server can exchange during a conversation.
A service name is a string that a server application responds to when a
client attempts to establish a conversation with the server. A client must
specify this service name to be able to establish a conversation with the
server. Although a server can respond to many service names, most servers
respond to only one name.
A topic name is a string that identifies a logical data context.
For servers that operate on file-based documents, topic names are
typically filenames; for other servers, they are other application-specific
strings. A client must specify a topic name along with a server's service
name when it attempts to establish a conversation with a server.
An item name is a string that identifies a unit of data that a server can
pass to a client during a transaction. For example, an item name might
identify an integer, a string, several paragraphs of text, or a bitmap.
To a client, these names are the keys that make it possible for the client
to establish a conversation with a server and to receive data from the
server.
The System topic provides a context for information that may be of general
interest to any DDE client. Server applications are encouraged to support
the System topic at all times
To find out which servers are present and the kinds of information they can
provide, a client can request a conversation on the System topic with the
service name set to NULL when the client application starts. Such wildcard
conversations should be kept to a minimum, because they are costly in terms
of system performance.
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