Using the Data Access Manager

There are at least three different ways in which you can use the

Data Access Manager to communicate with a data source.

• You can use low-level interface routines to send queries and retrieve

data from the data source. In this case, your application must be

capable of preparing a query in a language appropriate for the data

server.

• You can use high-level interface routines to send queries and retrieve

data from the data source. In this case, you must have one or more

query documents provided by another application.

• You can create your own query documents and use high-level interface

routines to send queries and retrieve data from the data source. In this

case, your application must be capable of preparing a query, but it can

use the same query repeatedly once it has been prepared.

This section describes how to call Data Access Manager functions

Using the Low-Level Interface describe using each of the interfaces to

the Data Access Manager to send queries to a data server. ;

determine the status of the high-level functions at various points in their

execution (and cancel execution if you so desire);

Getting Information about Sessions in Progress explains how to

obtain information about Data Access Manager sessions that are in

progress; and Processing Query Results describes how to retrieve query

results and convert them to text.

Executing Routines Asychronously

All of the Data Access Manager low-level routines and some of the

high-level routines can execute asynchronously-that is, the routine returns

control to your application before the routine has completed execution. Your

application must call the WaitNextEvent function periodically to allow an

asynchronous routine to complete execution.

Note: The database extension is responsible for implementing

asynchronous execution of Data Access Manager routines. For

example, if you call the DBSend function to send a query to a data

server, and the database extension calls a device driver, the database

extension can return control to your application as soon as the device

driver has placed its routine in the driver I/O queue. If you attempt

to execute a routine asynchronously and the database extension that

the user has selected does not support asynchronous execution, the

routine returns a result code of rcDBAsyncNotSupp and terminates

execution.

All Data Access Manager routines that can execute asynchronously take as

a parameter a pointer to a parameter block known as the asynchronous

parameter block. If this pointer is NIL, the function is executed

synchronously-that is, the routine does not return control to your application

until execution is complete.

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