User's Guide

Part V. The SQL Anywhere Programming Interfaces

Chapter 37. The WSQL DDE Server

The following sections provide an introduction to dynamic data exchange (DDE).

What is DDE?

DDE is a method of interprocess communication, that is, a method for passing data back and forth between applications. DDE uses a client/server system: client applications such as Excel request data from server applications, such as WSQL DDE Server.

DDE conversations

An organized exchange of data using DDE is called a conversation. DDE conversations are distinguished by a three-level identification scheme: each conversation employs a service name, a topic name, and an item name.

At the top level is the service name. The WSQL DDE Server application uses the service name WSQLDDE. All conversations between a client application and WSQL DDE Server must use this service name.

The second level is the topic name. The topic name identifies the user and (optionally) specifies which SQL Anywhere database he wishes to query. The topic name must be of the form UserName,Password,Database or UserName,Password. (In the latter case, the default database environment is chosen.)

The third level is the item name. The item name further specifies the action to be performed by WSQL DDE Server. For instance, Connect and Clear are valid item names when sending data to WSQL DDE Server, while Column_Names_And_Data is a valid item name when requesting information from WSQL DDE Server.

Attempts to start a DDE conversation without a service name or a topic name are called wildconnects, and are not supported by WSQL DDE Server.

Sending and receiving data and commands using DDE

There are three main ways for a client to exchange information with a server: poking, executing, and requesting. These three operations are called DDE transactions (not to be confused with database transactions).

• Poke send information from the client to the server. Typically, the client will poke data specifying a query to the server, and set the item name to a command acting upon the data.
• Execute send the command directly to the database engine. No data is returned by a DDE execute transaction, and item names are ignored.
• Request lets WSQL DDE Server know that the client wants to receive a memory buffer containing the results of a query. (The query must have already been added to the query buffer.) WSQL DDE Server does not support warm or hot request links (see below).

DDE links

Methods of data exchange are often called links. There are three types of DDE links:

• A cold link occurs when the client requests data and the server immediately supplies the data.
• A warm link occurs when the server advises the client that some data that the client is interested in has changed. It is then the client's responsibility to request the data. WSQL DDE Server does not support warm links.
• A hot link occurs when the server sends a piece of data to the client as soon as the data changes. WSQL DDE Server does not support hot links.