Table of Contents
In order to use transactions with your application, you must turn them on. To do this you must:
Use an environment (see Environments for details).
Turn on transactions for your environment.
You do this by providing the DB_INIT_TXN
flag to the
DB_ENV->open()
method.
Note that initializing the transactional subsystem implies that
the logging subsystem is also initialized. Also, note that
if you do not initialize transactions when you first create
your environment, then you cannot use transactions for that
environment after that. This is because DB
allocates certain structures needed for transactional
locking that are not available if the environment is
created without transactional support.
Initialize the in-memory cache by
passing the DB_INIT_MPOOL
flag to the
DB_ENV->open()
method.
Initialize the locking subsystem. This is what provides locking for concurrent applications. It also is used to perform deadlock detection. See Concurrency for more information.
You initialize the locking subsystem by
passing the DB_INIT_LOCK
flag to the
DB_ENV->open()
method.
Initialize the logging subsystem. While this is enabled by default for transactional applications, we suggest that you explicitly initialize it anyway for the purposes of code readability. The logging subsystem is what provides your transactional application its durability guarantee, and it is required for recoverability purposes. See Managing DB Files for more information.
You initialize the logging subsystem by
passing the DB_INIT_LOG
flag to the
DB_ENV->open()
method.
Transaction-enable your databases. If you are using the base API, transaction-enable your databases. You do this by encapsulating the database open in a transaction. Note that the common practice is for auto commit to be used to transaction-protect the database open. To use auto-commit, you must still enable transactions as described here, but you do not have to explicitly use a transaction when you open your database. An example of this is given in the next section.
For simple DB applications, environments are optional. However, in order to transaction protect your database operations, you must use an environment.
An environment, represents an encapsulation of one or more databases and any associated log and region files. They are used to support multi-threaded and multi-process applications by allowing different threads of control to share the in-memory cache, the locking tables, the logging subsystem, and the file namespace. By sharing these things, your concurrent application is more efficient than if each thread of control had to manage these resources on its own.
By default all DB databases are backed by files on disk. In addition to these files, transactional DB applications create logs that are also by default stored on disk (they can optionally be backed using shared memory). Finally, transactional DB applications also create and use shared-memory regions that are also typically backed by the filesystem. But like databases and logs, the regions can be maintained strictly in-memory if your application requires it. For an example of an application that manages all environment files in-memory, see In-Memory Transaction Example.
Using environments with some journaling filesystems might result in log file corruption. This can occur if the operating system experiences an unclean shutdown when a log file is being created. Please see Using Recovery on Journaling Filesystems in the Berkeley DB Programmer's Reference Guide for more information.
In order to operate, your DB application must be able to locate its database files, log files, and region files. If these are stored in the filesystem, then you must tell DB where they are located (a number of mechanisms exist that allow you to identify the location of these files – see below). Otherwise, by default they are located in the current working directory.
The environment home directory is used to determine where DB files are located. Its location is identified using one of the following mechanisms, in the following order of priority:
If no information is given as to where to put the environment home, then the current working directory is used.
If a home directory is specified on the
DB_ENV->open()
method,
then that location is always used for the environment
home.
If a home directory is not supplied to
DB_ENV->open(),
then the directory identified by the DB_HOME environment variable
is used if you specify
either the DB_USE_ENVIRON or
DB_USE_ENVIRON_ROOT flags to the
DB_ENV->open()
method. Both flags allow you to identify the
path to the environment's home directory
using the DB_HOME environment variable. However,
DB_USE_ENVIRON_ROOT is honored only if the
process is run with root or administrative privileges.
By default, all DB files are created relative to the environment
home directory. For example, suppose your environment home is in
/export/myAppHome. Also suppose you name your database
data/myDatabase.db.
Then in this case, the database is placed in:
/export/myAppHome/data/myDatabase.db.
That said, DB always defers to absolute pathnames. This means that if you provide an absolute filename when you name your database, then that file is not placed relative to the environment home directory. Instead, it is placed in the exact location that you specified for the filename.
On UNIX systems, an absolute pathname is a name that begins with a forward slash ('/'). On Windows systems, an absolute pathname is a name that begins with one of the following:
A backslash ('\').
Any alphabetic letter, followed by a colon (':'), followed by a backslash ('\').
Try not to use absolute path names for your environment's files. Under certain recovery scenarios, absolute path names can render your environment unrecoverable. This occurs if you are attempting to recover your environment on a system that does not support the absolute path name that you used.
As described in the previous sections, DB will place all its files in or relative to the environment home directory. You can also cause a specific database file to be placed in a particular location by using an absolute path name for its name. In this situation, the environment's home directory is not considered when naming the file.
It is frequently desirable to place database, log, and region files on separate disk drives. By spreading I/O across multiple drives, you can increase parallelism and improve throughput. Additionally, by placing log files and database files on separate drives, you improve your application's reliability by providing your application with a greater chance of surviving a disk failure.
You can cause DB's files to be placed in specific locations using the following mechanisms:
| File Type | To Override |
|---|---|
| database files |
You can cause database files to be created
in a directory other than the
environment home by using the
This method modifies the directory used for database files created and managed by a single environment handle; it does not configure the entire environment. This method may not be called after the environment has been opened.
You can also set a default data location that is used by
the entire environment by using the
|
| Log files |
You can cause log files to be created
in a directory other than the environment home
directory by using the
This method modifies the directory used for database files created and managed by a single environment handle; it does not configure the entire environment. This method may not be called after the environment has been opened.
You can also set a default log file location that is used by
the entire environment by using the
|
| Region files | If backed by the filesystem, region files are always placed in the environment home directory. |
Note that the DB_CONFIG must reside in the
environment home directory. Parameters are specified in it one
parameter to a line. Each parameter is followed by a space,
which is followed by the parameter value. For example:
set_data_dir /export1/db/env_data_files
To simplify error handling and to aid in application debugging, environments offer several useful methods. Note that many of these methods are identical to the error handling methods available for the DB structure. They are:
set_errcall()
Defines the function that is called when an error message is issued by DB. The error prefix and message are passed to this callback. It is up to the application to display this information correctly.
This is the recommended way to get error messages from DB.
set_errfile()
Sets the C library FILE * to be used for
displaying error messages issued by the DB library.
set_errpfx()
Sets the prefix used to for any error messages issued by the DB library.
err()
Issues an error message based upon a DB error code a message text that you supply.
The error message is sent to the
callback function as defined by set_errcall().
If that method has not been used, then the error message is sent to the
file defined by
set_errfile().
If none of these methods have been used, then the error message is sent to
standard error.
The error message consists of the prefix string
(as defined by set_errprefix()),
an optional printf-style formatted message,
the DB error message associated with the supplied error code,
and a trailing newline.
errx()
Behaves identically to err() except
that you do not provide the DB error code and so
the DB message text is not displayed.
In addition, you can use the db_strerror()
function to directly return the error string that corresponds to a
particular error number. For more information on the
db_strerror() function, see the Error Returns
section of the Getting Started with Berkeley DB guide.
The subsystems that you enable for an environment (in our case, transaction, logging, locking, and the memory pool) are described by one or more regions. The regions contain all of the state information that needs to be shared among threads and/or processes using the environment.
Regions may be backed by the file system, by heap memory, or by system shared memory.
By default, shared memory regions are created as files in the environment's
home directory (not the environment's data
directory). If it is available, the POSIX mmap
interface is used to map these files into your application's
address space. If mmap
is not available, then the UNIX shmget interfaces
are used instead (again, if they are available).
In this default case, the region files are named
__db.###
(for example, __db.001, __db.002,
and so on).
If heap memory is used to back your shared memory regions, then
you can only open a single handle for the environment. This
means that the environment cannot be accessed by multiple
processes. In this case, the regions are managed only in
memory, and they are not written to the filesystem. You
indicate that heap memory is to be used for the region files by
specifying
DB_PRIVATE to the
DB_ENV->open()
method.
Note that you can also set this flag by using the
set_open_flags parameter in the
DB_CONFIG file. See the
Berkeley DB C API Reference Guide for more information.
(For an example of an entirely in-memory transactional application, see In-Memory Transaction Example.)
Finally, you can cause system memory to be used for your
regions instead of memory-mapped files. You do this by providing
DB_SYSTEM_MEM to the
DB_ENV->open()
method.
When region files are backed by system memory, DB creates a single file in the environment's home directory. This file contains information necessary to identify the system shared memory in use by the environment. By creating this file, DB enables multiple processes to share the environment.
The system memory that is used is architecture-dependent. For
example, on systems supporting X/Open-style shared memory
interfaces, such as UNIX systems, the shmget(2)
and related System V IPC interfaces are used.
Additionally, VxWorks systems use system memory. In these cases,
an initial segment ID must be specified by the application to
ensure that applications do not overwrite each other's
environments, so that the number of segments created does not
grow without bounds. See the
DB_ENV->set_shm_key()
method for more information.
On Windows platforms, the use of system memory for the region files is problematic because the operating system uses reference counting to clean up shared objects in the paging file automatically. In addition, the default access permissions for shared objects are different from files, which may cause problems when an environment is accessed by multiple processes running as different users. See Windows notes or more information.
When using environments, there are some security considerations to keep in mind:
Database environment permissions
The directory used for the environment
should have its permissions set to ensure that files in the
environment are not accessible to users without appropriate
permissions. Applications that add to the user's permissions
(for example, UNIX setuid or
setgid applications), must be
carefully checked to not permit illegal use of those
permissions such as general file access in the environment
directory.
Environment variables
Setting
the DB_USE_ENVIRON or
DB_USE_ENVIRON_ROOT flags
so that environment variables can be used during file naming
can be dangerous. Setting those flags in DB
applications with additional permissions (for example, UNIX
setuid or setgid
applications) could potentially allow users
to read and write databases to which they would not normally
have access.
For example, suppose you write a DB application
that runs setuid. This means that
when the application runs, it does so under a
userid different than that of the application's caller.
This is especially problematic if the application is
granting stronger privileges to a user than the user
might ordinarily have.
Now, if
the DB_USE_ENVIRON or
DB_USE_ENVIRON_ROOT flags
are set for the environment,
then the environment that the application is
using is modifiable using the
DB_HOME environment variable. In
this scenario, if the uid used by the application has
sufficiently broad privileges, then the application's caller
can read and/or write databases owned by another user
simply by setting his
DB_HOME environment variable to the
environment used by that other user.
Note that this scenario need not be malicious; the
wrong environment could be used by the application
simply by inadvertently specifying the wrong path to
DB_HOME.
As always, you should use setuid
sparingly, if at all. But if you do use
setuid, then you should refrain from
specifying
the DB_USE_ENVIRON or
DB_USE_ENVIRON_ROOT flags
for the environment open. And, of course, if you must
use setuid, then make sure you use
the weakest uid possible – preferably one that is
used only by the application itself.
File permissions
By default, DB always creates database and log files readable and
writable by the owner and the group (that is,
S_IRUSR,
S_IWUSR, S_IRGRP and
S_IWGRP; or octal mode 0660 on historic
UNIX systems). The group ownership of created files is based
on the system and directory defaults, and is not further
specified by DB.
Temporary backing files
If an unnamed database is created and the cache is too small
to hold the database in memory, Berkeley DB will create a
temporary physical file to enable it to page the database to
disk as needed. In this case, environment variables such as
TMPDIR may be used to specify the
location of that temporary file. Although temporary backing
files are created readable and writable by the owner only
(S_IRUSR and S_IWUSR,
or octal mode 0600 on historic UNIX systems), some
filesystems may not sufficiently protect temporary files
created in random directories from improper access. To be
absolutely safe, applications storing sensitive data in
unnamed databases should use the
DB_ENV->set_tmp_dir()
method to specify a temporary directory with known permissions.