by Yi Kun Lin - GUI Computing

Visual Basic's ability to use ODBC to connect to databases other than Access, has allowed Database vendors to enter into the rapidly expanding GUI Market. One such vendor is Information Resources Inc. who have developed a backend called pcEXPRESS.

pcEXPRESS is a multi-dimensional database which can have up to seven dimensions organised in tables much like a standard spreadsheet. pcEXPRESS allows you to create complicated queries, or 'Rotations', simply by changing the order of the table dimensions.

IRI have now released an API suite which allows current users to retain their database and move to the GUI interface. The API's work exactly like any other Windows API and developers that are currently using pcEXPRESS will be happy to learn that the syntax they are using currently has changed little in the new standard.

A component called the PIPE handles the communications link between the Windows client side and pcEXPRESS server side at a low level. The retrieved large block of pcEXPRESS data is then stored in binary form. The API has two basic properties: Message and Data. Before starting the conversation with the pcEXPRESS server, the front-end must open a pipe connection by setting CCXAPI’S message-handling property XAPIMode to True. If pcEXPRESS is not running, the pipe connection fails and a trappable error is returned. In this case, the Visual Basic shell function can be used to start pcEXPRESS, after which a connection can be established.

  Sub StartUp (ByVal PATH As String)
    dim Temp
    gXAPIErrorCode = PR_SUCCESS          ' to initialise to CCXAPI error code
    gPipeOpened = False                  ' and pipe connection flag
  If .XAPI1.XAPIReady = False Then       ' if the pipe connection is not open
    .XAPI1.XAPIMode = True               ' try to open pipe connection
  End If                                 ' wait til pipe connection 
                                           open or error occurs
  Do While gXAPIErrorCode = PR_SUCCESS And gPipeOpened = False
    temp = DoEvents()
  Loop
  If XAPIErrorCode = PR_PCX_UNCON Then   ' pcEXPRESS is not running
    Temp = Shell ( PATH, WIN_MINIMIZED)  ' launching pcEXPRESS 
                                           with shell function
    XAPI1.XAPIMode = False               ' stop opening pipe connection
    gXAPIErrorCode = PR_SUCCESS          ' initialise the error code again
    If gPCXStarted = True Then           ' pcEXPRESS started successfully
        XAPI1.XAPIMode = True            ' reconnect pipe
      End If
    Else                                 ' pcEXPRESS is already running
      XAPI1.XAPIMode = True              ' reconnect pipe
    End If
  End Sub

In above code, the global error variable gXAPIErrorCode is assigned a value according to the error type in XAPIError events; global Boolean variable gPipeOpened is set to True in XAPIOpen event triggered when the pipe connection is successfully opened; global Boolean variable gPCXStarted is set to True in XAPIPoke event trigged when a response message is received from pcEXPRESS.

Once a Pipe connection has been established, the application can send its requests, typically a pcEXPRESS command, via the API. For example;

  XAPI1.XAPICommand = ExpressCommand$
  XAPI1.XAPISend=True

It is important to synchronise the client’s requests and server’s responses as the API control does not support multiple transactions. If this is not catered for, some previous server’s response may be lost. This problem can be prevented by using modal forms or disabling buttons to avoid users from sending further requests.

To retrieve a block of data from pcEXPRESS, you can assign a pcEXPRESS FETCH command to XAPICommand and set XAPISend to True. For example:

  XAPI1.XAPICommand ="FETCH SALES LABELED"
  XAPI1.XAPISend=True

Again a trappable error will be returned if the operation fails. Otherwise the data block is successfully returned into an array.

From my viewpoint, although you still have to write a few lines of code to process the retrieved data in the array, you can probably benefit from saving the code of handling queries with pcEXPRESS's powerful multi-dimension feature. On the other hand, Visual Basic’s GUI capability can certainly make pcEXPRESS more user-friendly.

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