Technical Information Database

TI1437D.txt - BDE and Database Desktop Locking Protocol

Category   :Database Programming
Platform   :All Windows
Product    :

Description:

 Intended Audience
  This information will be of benefit to anyone considering
 designing a database application using Delphi and the BDE.

 Prerequisites
  A basic knowledge or interest in Paradox locking protocols
 and table formats.

 Purpose
  To give users a better understanding of the table locking
  protocol.

 Table and field types and features supported
  Each major release of Paradox has implemented improvements
  to  table structures since version 2.0.  All Paradox table
  types from Paradox 1.0 through Paradox 3.5 are compatible
  with each other.

  Paradox 4.0 adds a new data type to the table format: Binary
  Large Objects, commonly known as BLObs and new types of
  Secondary Indices.  Paradox 4.0 supports two types of BLOb
  fields: Memo, and BLOb.  Versions of Paradox prior to 4.0,
  or the Engine prior to 3.0, cannot read, write, or create
  this new table format.  If you attempt to read or write a
  Paradox 4.0 table type in an earlier version of Paradox,
  it will return an error that the table is password protected.

  Paradox 5.0 added several new data types to the table format:
  Long Integer, Time, TimeStamp, Logical, Autoincrement, BCD,
  Bytes.  Paradox 7.0 added a descending secondary index.
  Any table created or modified to include any of these newer
  features will default to the respective table level. The
  default table type created using the Database Desktop and the 
  BDE (Borland Database Engine) is a Paradox 4.0.  Although the 
  default can be changed in the BDE configuration utility or
  the Database desktop configuration utility to default to
  Paradox 3, 4, 5 and 7 for the BDE.

  Paradox 4.0 can read, write, and create Paradox table types that
  are compatible with Paradox 1.0 through Paradox 4.0.  So a table
  created in Paradox 1.0 is compatible with Paradox 4.0.  A table
  created in Engine 1.0 or 2.0 can be read by or written to by
  Paradox 4.0.

  Paradox and the Engine do not change the table type when reading
  or writing.  The table type is only changed when a  Restructure is 
  performed on the table.

 Paradox Locking Protocols
  There are two different Paradox locking protocols: the protocol
  introduced with Paradox 2.0 and the protocol introduced with
  Paradox 4.0.  These two protocols are not compatible with each
  other.  The locking protocol has no bearing on which type of
  table a program can work with.  There are a few programs that can
  support either locking protocol; however, these programs can only
  support one protocol at a time.  We will only focus on the 4.0
  locking protocol.

 Database Desktop/ Paradox 4.0 Locking Protocol
  The Paradox 4.0 locking protocol is the only protocol available
  for Paradox 4.0 and the IDAPI Engine.   The designation "Paradox 4.0
  locking protocol" represents this style of locking.  

 Directory Locks
  Paradox 4.0 places a locking file, PDOXUSRS.LCK, in each
  directory where tables are being accessed.  This locking file
  regulates concurrent access to files in the directory.  The lock
  file references PDOXUSRS.NET, so every user must map to 
  the data directory the same way.  It also places an exclusive 
  PARADOX.LCK file in the directory as well.  It does this to 
  prevent versions of Paradox or the Engine using the older
  locking system from inadvertently accessing tables.

 Working/Shareable Directories
  When Paradox or Database Desktop needs to access tables in a
  directory, they place a "Shareable" PDOXUSRS.LCK file in that
  directory and an "Exclusive" PARADOX.LCK file in that directory.
  This designation means that other Paradox 4.0 users can access
  tables in that directory.  The exclusive PARADOX.LCK file is
  placed in this directory to keep the older, incompatible locking
  protocol from putting data at risk.  In Paradox, this is known as
  a "Working" directory.

 Private/Exclusive Directories
  Paradox and Database Desktop also need a directory to store
  temporary files, such as the Answer table from a query.  When
  Paradox or Paradox Runtime start, they also place an "Exclusive"
  PDOXUSRS.LCK file in a directory and an "Exclusive" PARADOX.LCK
  file in the same directory, designating that directory as the
  location for temporary files.  This designation means that other
  Paradox users cannot access tables in that directory.  In Paradox
  this is known as a "Private" directory.

 Table Locks
  Paradox 4.0 places each table lock in the directory locking file:
  PDOXUSRS.LCK.  It no longer uses the separate table lock file of
  previous versions.  For example, if three users are viewing the
  CUSTOMER.DB table and one user is restructuring the ORDERS.DB
  table, the PDOXUSRS.LCK file will have a shareable lock listed
  for each of those three users who are viewing the CUSTOMER.DB
  table, and an exclusive lock on ORDERS.DB for the user who is
  restructuring that table.

 Paradox 4.0 Locking Protocol Concurrency
  In a multi-user environment, the Paradox 4.0 locking protocol
  maintains concurrency, the simultaneous use of applications,
  through the PDOXUSRS.NET file.  All users who want to share 
  Paradox tables must map to the same PDOXUSRS.NET file in the 
  same way using the same path, but not necessarily the same 
  drive letter.  Paradox places a PDOXUSRS.LCK and an exclusive 
  PARADOX.LCK file in each directory where tables are being 
  accessed preventing previous versions of Paradox from accessing
  files in the same directory.  Each user who wants to share tables
  in that directory must map that directory the same way using 
  the same drive letter and path.  Paradox then places all of the 
  locking information for that table in the PDOXUSRS.LCK file, 
  reducing the number of files needed.

 Network Control File
  The Paradox network control file, PDOXUSRS.NET, serves as the
  reference point for all lock files created by Paradox.  The
  net file contains the users currently using the BDE and which
  table they're accessing.  Each lock file references the network
  control file and contains information regarding the locks on the
  table and by which user, so each user must map to the same 
  network control file in the same way, but not necessarily with 
  the same drive letter.

  For example, if you are using volume DATA on server SERVER_1 and
  the network control file is in the directory \PDOXDATA each user
  must map \\SERVER_1\DATA:\PDOXDATA the same way, however, each
  user should, but is not required to use the same drive letter.
  If the network you are using does not have volumes, then DATA
  would be a directory off the root of SERVER_1.

  If you are mapping \\SERVER_1\DATA to the root of drive P: then
  each Paradox system would specify the location of PARADOX.NET as
  P:\PDOXDATA\.  However, other users could map \\SERVER_1\DATA to
  the root of drive O: and specify O:\PDOXDATA\ as the location of
  the network control file.

 Configuring 16 bit Database Engine / IDAPI.CFG
  The Database Engine configuration file holds the network specific 
  information and the list of database aliases, as well as other
  information. You can configure IDAPI using the Database Engine
  configuration program, BDECFG.EXE, to  set the location of the
  network control file. Also add, delete, modify database aliases
  including which driver or type of alias used and whether IDAPI
  will share local tables with other programs using the Paradox
  4.0 locking protocol as well as some other specifics regarding the 
  tables and how data is displayed.  

 Local Settings 16 bit
  The WIN.INI file holds the locations of the IDAPI.CFG file, the
  Database Desktop "Working" directory, and the Database Desktop
  "Private" directory.  You can use any text editor to change these
  designations in the WIN.INI file.  The location of the IDAPI.CFG
  file is CONFIGFILE= or
  CONFIGFILE01= in the [IDAPI]
  group.

  The locations of the Database Desktop "Working" and "Private"
  directories are in the [DBD] group as WORKDIR=, and PRIVDIR=. 

 Configuring 32 bit Database Engine / IDAPI32.CFG
  The BDE configuration file holds the same information as the
  Database Engine configuration file.  Use the BDE Configuration,
  BDECFG32.EXE, to configure IDAPI32.CFG.  Optionally you can store
  the information in the registry or in both the registry and
  IDAPI32.CFG.

 Local Settings 32 bit
  The registry holds the locations of the IDAPI32.CFG, the "Working"
  directory, and the "Private" directory.  The location of the
  IDAPI32.CFG file is stored in
  HKEY_LOCAL_MACHINE\Software\Borland\Database Engine.
  The value CONFIGFILE01 holds the data containing .
  
  The location of the BDE "Working" and "Private" directories are
  stored in
  HKEY_CURRENT_USER\Software\Borland\DBD\7.0\Configuration\WorkDir and
  HKEY_CURRENT_USER\Software\Borland\DBD\7.0\Configuration\PrivDir
  respectively. Each directory default value stores the data containing
  .

 Accessing a Paradox Table
  The BDE will first try to access the PDOXUSRS.NET file.
  If a PDOXUSRS.NET file is not found, Paradox will create a 
  new PDOXUSRS.NET file and continue with the startup procedure. 
  If the PDOXUSRS.NET file is found but the owner of this net file 
  used a different path, i.e. mapped to the server differently, an
  exception of "Multiple net files in use" will be raised and the BDE
  will shutdown. After the net is successfully opened an exclusive
  lock, PARADOX.LCK, is placed in the temporary, private, directory.
  If it fails to place the lock the BDE will shut down.  This can fail
  if some other user has an exclusive lock in this directory or the
  lock files are using different net files. After it secures a
  directory for private use it will place a shareable PARADOX.LCK in
  the working directory and now Initialization is complete.
 

Reference:
 

3/31/99 10:21:50 AM