3160:BDE and Database Desktop Locking Protocol
KEYWORDS: BDE, DataBase, Locking, pdoxusrs.net, pdoxusrs.lck, multiple net fil
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=<full drive, path, and file name> or
CONFIGFILE01=<full drive, path, and file NAME> in the [IDAPI]
group.
The locations of the Database Desktop "Working" and "Private"
directories are in the [DBD] group as WORKDIR=<full drive and
directory>, and PRIVDIR=<full drive and directory>.
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 <full drive, path,
and file name>.
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
<Full drive and directory>.
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.
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