DBFSERVER

DBF Client/Server Suite for Xbase++

Contents

Copyright

This document as a whole is copyrighted © 2001 by Phil Ide. The software package (comprising dbfServer.exe, dbSocketc.dll, header [.ch] files, sample programs, source code) is copyright © Jack Duijf & Phil Ide, 2002. All Rights Reserved.

The package is released under the terms of the GNU GENERAL PUBLIC LICENCE

Disclaimer

This software is supplied free of charge, and so there is no warranty for the software or accompanying documentation (herinafter called the "package"), to the extent permitted by applicable law. Except when otherwise stated in writing the copyright holders and/or other parties provide the package "as is," without warranty of any kind, either expressed or implied, including, but not limited to, the implied warranties of merchantability and fitness for a particular purpose. The entire risk as to the quality of this package is with you.

In no event unless required by applicable law or agreed to in writing will any copyright holder, or any other party who may modify and/or redistribute the package as permitted above, be liable to you for damages, including any general, special, incidental or consequential damages arising out of the use or inability to use this package (including but not limited to loss of data or data being rendered inaccurate or losses sustained by you or third parties).

Any contributions made by others to this package have been acknowledged where possible. Commercial products, retailers, or distributors mentioned here are not necessarily endorsed by me or any other particular party.

Authors

Phil Ide    code and documentation
Jack Duijf    code, code, more code and lots of good ideas
Bjørn Kaarigstad concept, ideas, inspiration

Acknowledgements

Many thanks to those who tested and broke everything. Special thanks goes to the following people who made particularly important contributions.

(in no particular order)
Edgar Borger
Jan-Dirk Schuitmaker
Mike Evans
Mike Grace
Jose Luis Otermin
Markus Walter
Frans Vermeulen
An especial mention for Jack Duijf, who found more bugs than everyone else put together, and made some very shrewd suggestions, all of which have made dbfServer a far better product than it would otherwise have been.

Jack is now an author of dbfServer.

Thanks to everyone else who made comments, gave encouragement etc.

Documentation

Description

The package consists of a server program which accepts requests via TCP/IP to perform operations on .DBF database tables. All Xbase++ operations are supported with the exception of dacSession (which includes XbpQuickBrowse) and compound legacy UI superfunctions (e.g. dbEdit()). The results of these operations are returned to the client program.

There are many reasons why you might want to remove database operations to a remote program, but in Xbase++ probably the most compelling reason is the way that DBF access times deteriorate rapidly as more and more users connect to the database in a multi-user environment under Windows.

dbfServer still opens the table once for each client connection, spawning a new thread to service each connecting client. Within Xbase++, each thread maintains a unique workspace, which has it's own set of unique workareas. The thread remains active for as along as the server is running and the client is connected.

Client programs can be forced to use the server by linking in a DLL which handles all the communication between server and client. A #include file is supplied which translates all standard database operations to use the functions in the DLL. For many programs, particularly those generated by the Form Designer, this is sufficient. However, this is largely dependant upon coding style, as there are some elements the pre-processor cannot handle, and so some additional changes are likely to be required.

New code can be written to take advantage of the fact that operations are performed through the server. Converted legacy code will suffer from a certain amount of lag since the code was written in the standard 'local connection' format. Understanding how to reduce lag by caching commands is fundamental to getting the best performance from the server.

Using code optimised for best performance with regard to the server will produce fast responsive programs and reduce network traffic immensely thereby making the network itself faster and more responsive to the benefit of all users and applications regardless of whether or not they are utilising dbfServer.

There are several other distinct advantages to using this package:
  1. Want to lock everybody out of the system? Shut down the server program and everyone will be unable to access the tables!
  2. Using the server program console, you can see exactly who is connected, which machine they are using, which socket number they are connected through and which files they have open (both tables and indexes).
  3. Do you have a time-consuming process which you know would run much faster on the server? dbfServer has the ability to run such procedures for you. Not only will the process run much faster, but again network traffic will be vastly reduced to the benefit of all network users.
It should be pointed out that any DBF table can be accessed by any client program, provided it is visible to dbfServer.

Converting Existing Programs

To link to the dbfServer in your own application, you need to do the following:
  1. In each source file where database activity takes place, you need to add this directive:

    #include "dbfsocket.ch"


  2. In your project make file, you need to add the following statement to your executable's list of files:

    DBFSOCKETC.LIB

  3. In the same directory as your executable resides, and with the same name but with an .ini extension you need to create an ini file. This file should contain the following code:

    [SOCKETS]
    port=1042
    server=localhost


    You should change the port number to match the port dbfServer is listening on, and the server name should be the IP name (or dotted octet address) of the machine where dbfServer can be found.

  4. New in 1.5  You can now set the client to not automatically connect to the server, by adding to the ini file:
        autoconnect=0


    This is a boolean value which toggles the autoconnect functionality of the client. The default if this setting is not found is 1 (connect automatically).

  5. There are three types of code that dbfServer cannot handle:
    • Whilst it handles:
      (cExpr)->fname

      (cExpr)->(fname)


      it cannot handle:
      alias->fname

      alias->(fname)


      This is due to limitations in the preprocessor. You should change all such code to enclose the ALIAS in quotes and then parenthesis.
      alias->fname
      // before
      ("alias")->fname
      // after

    • Expressions generated as macro's from strings will not be parsed properly, and will still expect local workarea connectivity.

    • Fieldnames without aliased expressions cannot be pre- determined by the pre-processor. You must alias them.

      For example, if you have a table with the field NAME in it, you cannot do this:

      cName := NAME


      Instead, you must do this:

      cName := (Alias())->NAME
        // or
      cName := (cAlias)->NAME


  6. Remember that the path you supply to the program for opening tables is FROM THE SERVER not the client!
That's all there is to it!

Optimising

The Form Designer when used to connect to a database, adds database access controls to the
::editControls
 array, which is then evaluated when the dialog is created or the record pointer is moved (via dialog controls). In a 'local database connection' e.g. a
USE
  statement, this is ok. In a remote connection scenario, this becomes innefficient because to refresh 20 controls, you need to send at least 60 instructions to the server. In large data-intensive dialog's, you should consider another strategy.

One method is to perform
dbfScatter()/dbfGather()
 to/from an array, and point your dialog controls at the array rather than the database. dbfServer handles these read/write operations in a single instruction.

All database functions are mapped to the function
dbfSocket()
. This function accepts 3 parameters:
dbfSocket(<nFunction> [,<aParams> [, <lCache>] ] )


<nFunction>
  This is a numeric constant identifying the database operation to call. You should use the constants defined in dbfSocket2.ch

<aParams>
 This optional parameter is an array of parameters to be passed to the function defined in
<nFunction>


<lCache>
 When this parameter is TRUE, the command is automatically cached and not sent to the server until a non-cached command is issued.

Certain commands are automatically cached, but you can use
dbfSocket()
 with the auto-cache option to increase performance. The automatically cached commands are a subset of all commands which return NIL. Some NIL-returning commands are excluded from the subset because they are considered immediate, e.g. dbCommit() and dbCommitAll() should not be cached, nor unlock operations.

If you know that you are not interested in the return value from a command, then you should consider caching it. As an Example,
Select(cAlias)
 may be used to select a workarea prior to performing operations on it, and if you are not interested in saving the return value then you should cache the command.

dbfScatter()
 and
dbfGather()
 are special functions which request the server to return and process an array respectively. This is more efficient than a standard
Scatter()/Gather()
 which sends at least 3 requests to the server for each element of the array. As with the standard Alaska functions,
dbfScatter()
 and
dbfGather()
  handle arrays of simple values or objects, but also more complex data types.

dbfScatter()
 can accept an optional array. If no array is passed as a parameter, then it returns the entire record as an array. If the parameter is used, the elements can have various types, with each element handled individually. The type of the element dictates how it is interpreted:
Note that the O,B and C types can be aliased expressions, and the alias need not be pre-processed using dbfSocket.ch since it is evaluated on the server.

Further, type O, B and C may refer to other workareas, since they are expressions.

dbfGather()
 must be passed an array. The elements of the array are handled as follows:
 

Automatically Cached Commands

The following commands are automatically cached:

dbAppend()    dbGoTo()    dbSetFilter()
dbClearFilter() dbGoTop() dbSetIndex()
dbClearIndex() dbRefresh() dbSetOrder()
dbClearRelation() dbResumeSelect() dbSetRelation()
dbClearScope() dbRollBack() dbSetScope()
dbCloseRelation() dbRSuspendSelect() dbSkip()
dbGoBottom() dbSelectArea() OrdCondSet()
dbGoPosition() dbSetDescend()

Remote Procedures

Remote Procedures are functions which are called by the client program, but are run entirely on the server. The client tells the server to run the procedure, then waits until the process has completed. The return value of the process is the return value of the called procedure.

Alternatively, Remote Procedures can be called Asynchronously - which means the client instructs the server to perform the operation, but does not wait for it to finish. This can be used in two ways: A Remote Procedure is programmed as a function in a dynamically loadable DLL which must be visible to the server. If the function needs parameters, the function must accept at least one, possibly two parameters. Note that you cannot pass variables by reference to a remote procedure.

To call the remote procedure, use this command:

REMOTE CALL <func> IN <dll> [WITH <parms,...>] [BACKGROUND] [UNLOAD]


<func> is the name of the function to call
<dll> is the name (and path if required) of the dll where the function can be found
<parms> is a comma-seperated list of all parameters that need to be passed to the function
BACKGROUND causes the procedure to be called asynchronously
UNLOAD is an optional statement which causes the dll to be unloaded once the procedure has finished.

An example of a (totally senseless!) function that can be used as a remote procedure:

  Function Useless(argv)
    local bFilt := {|| TRUE }
    local i := 0
    local argc := len(argv)

    if argc > 0
       bFilt := argv[1]
    endif

    dbGoTop()
    While !Eof()
      if Eval(bFilt)
         i += RecNo() // this is silly...
      endif
      SKIP
    Enddo
    return (i)


To call this (assume it is linked into "RProc.DLL"):

REMOTE CALL Useless IN RProc WITH {|| GENDER == 'M' } UNLOAD


To perform the same task, but get status information back, call the procedure asynchronously. The procedure will have to be amended to accept the status code-block:

  Function Useless(argv, bStatus)
    local bFilt := {|| TRUE }
    local i := 0
    local argc := len(argv)
    local nCount := 0

    if argc > 0
       bFilt := argv[1]
    endif

    dbGoTop()
    While !Eof()
      if Eval(bFilt)
         i += RecNo() // this is silly...
         EVal(bStatus,++nCount) // record how many records we have processed
      endif
      SKIP
    Enddo
    return (i)

Then in the client program...

    uRet := TRUE
    lAbort := FALSE
    REMOTE CALL Useless IN RProc WITH {|| GENDER == 'M' } BACKGROUND UNLOAD
    while ValType(uRet) != "U" .and. !lAbort
       uRet  := dbfRemoteStatus(_DBF_REMOTEPROCSTATUS)
       @ 17,40 say as(uRet)
       if inkey(.5) = 27
          lAbort := TRUE
       endif
    enddo

Error Handling

If the client attempts to perform an operation which fails at the server, the server returns an error object to the client, which automaticaly passes it to the current error handler. Any error message displayed by the client will probably display the callstack of the client program, so you should be able to track logic errors in your code. In the event the error really belongs to a bug in the server (oh, horror!), you can find the callstack of the server embedded in the error object's cargo slot.

By passing the error back to the client to handle, the error is displayed where the problem most likely lays - if you attempt to write a record without locking it first, the problem is in the client code not the server. This also makes the server robust and resilient, allowing other connected clients to continue working without interruption.

Remote Disk, File & Environment Functions

There are a number of Disk, File & Environment functions which you can call. They have the same name as their 'local' counterparts with 'cs' prepended to their name. The calling convention is the same as for their local counterparts too, but some care should be taken when using these functions.

Server-Side Functions
csCreateDir() csFError() csFseek() csMemoWrit()
csDate() csFExists() csFSize() csNetErr()
csDirectory() csFile() csFWrite() csPutFile()
csFAttr() csFOpen() csGetDriveDescriptors() csRemoveDir()
csFClose() csFread() csGetEnv() csSeconds()
csFCreate() csFReadStr() csGetFile() csSetEnv()
csFErase() csFRename() csMemoRead() csTime()

Warnings

csFError() returns an OS error for a low-level file operation. The error returned is for the latest error, which in a multi-user application where multiple users are using the same program, may return someone else's error.

csNetErr() suffers from the same problems as FError(), only it is more likely to return an incorrect value.

csFRead(). This is now a direct replacement for FRead(), and should be coded in exactly the same way. The second parameter should be passed by reference, and the return value is the number of bytes read. This is incompatible with releases prior to 1.0.

GetDriveDescriptors() / csGetDriveDescriptors()
These functions return a list of available local and mapped drives on the machine where the function is performed. In the case of GetDriveDescriptors() it is performed on the client machine, and csGetDriveDescriptors() is performed on the server.

The return value is an array, where each element is an array which describes a logical drive. The format of the sub-array is:
    {cDriveName, nDescriptor}
cDriveName is 'A:\', 'C:\', 'D:\' etc.
nDescriptor is a numeric, which matches one of the constants defined in dbfSocket.ch

Use the function DriveToText(<n>) passing nDescriptor to get a textual description of the drive, as indicated in the table below.

Constants for Drive Descriptors
Constant Explanation DriveToText()
DRIVE_UNKNOWN Unidentified "Unknown"
DRIVE_NO_ROOT Problem with drive "No Root"
DRIVE_REMOVEABLE Non-Floppy, Non-CD Removable "Removeable"
DRIVE_FIXED Local Hard drive "Fixed"
DRIVE_REMOTE Network Drive "Network"
DRIVE_CDROM CD-ROM drive "CD-ROM"
DRIVE_RAMDISK Ramdisk "Ramdisk"

csPutFile(cLocalFile,cRemoteFile) is used to transfer a file to the server.

csGetFile(cRemoteFile,cLocalFile) is used to transfer a file from the server.

Using Remote XPF files

The commands SAVE TO & RESTORE FROM (and their variants) can be forced to save/restore from files held on the server by prepending the command with the keyword REMOTE
  SAVE TO temp              // save all memvars to local xpf
  RESTORE FROM temp         // restore from local xpf

  REMOTE SAVE TO temp       // save all memvars to remote xpf
  REMOTE RESTORE FROM temp  // restore from remote xpf
An alternative to using the REMOTE keyword, is to define the constant
_DBFSERVER_MEM_REMOTE_ before the #include "dbfSocket.ch" statement.
This will cause existing code to save/restore from the server.

Connecting New Threads

If you spawn a new thread in your application, it is necessary for that thread to have it's own connection to the server, since each thread has it's own workspace.

The easiest way to do this, is to subclass Thread() and overload with these methods: 
    #define SOCK_STREAM 1
    METHOD myThread:atStart()
      local aSockParms := dbfSocketGetParameters()

      if dbfSocketOpen( SOCK_STREAM, ;
                        aSockParms[DBFCS_GP_SERVER],;
                        aSockParms[DBFCS_GP_PORT] ) <> 0
         // error - want to quit the thread?
      endif
      return self

    METHOD myThread:atEnd()
      dbfSocketClose()
      return self

Rebuilding the Server and dbfSocket.dll

To rebuild either the server or dbfSocketC.dll, you will require ASINET1C.LIB, which is part of the Professional Subscription.

ASINET1C.DLL, which is part of the Professional Subscription redistributable libraries, is supplied with the package, and should be placed in your RUNTIME directory or somewhere in your PATH.

About Scopes

Scopes implemented through dbfServer behave slightly differently from the standard Xbase++ implementation. In Xbase++, after setting a scope, the following is true:
(assume scope does not include logical top and bottom records) 
    dbGoTop()
    nTop := RecNo()
    dbSkip(-1)
    ? Bof()         // TRUE
    ? Eof()         // FALSE
    ? RecNo()       // == nTop - 1

    dbGoBottom()
    nBottom := RecNo()
    dbSkip()
    ? Bof()         // FALSE
    ? Eof()         // TRUE
    ? RecNo()       // nBottom+1
With dbfServer, the following is true: 
    dbGoTop()
    nTop := RecNo()
    dbSkip(-1)
    ? Bof()         // TRUE
    ? Eof()         // FALSE
    ? RecNo()       // == nTop

    dbGoBottom()
    dbSkip()
    ? Bof()         // FALSE
    ? Eof()         // TRUE
    ? RecNo()       // LastRec()+1 (ghost record)
Whilst this is incompatible with Xbase++, we believe this is the correct behaviour.

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Version 2, June 1991

Copyright (C) 1989, 1991 Free Software Foundation, Inc.
59 Temple Place - Suite 330, Boston, MA 02111-1307, USA

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