DB_ENV->open()

#include <db.h>

int
DB_ENV->open(DB_ENV *dbenv, char *db_home, u_int32_t flags, int mode);  

The DB_ENV->open() method opens a Berkeley DB environment. It provides a structure for creating a consistent environment for processes using one or more of the features of Berkeley DB.

The DB_ENV->open() method method returns a non-zero error value on failure and 0 on success. If DB_ENV->open() fails, the DB_ENV->close() method must be called to discard the DB_ENV handle.

Warning

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.

Parameters

db_home

The db_home parameter is the database environment's home directory. For more information on db_home, and filename resolution in general, see Berkeley DB File Naming. The environment variable DB_HOME may be used as the path of the database home, as described in Berkeley DB File Naming.

When using a Unicode build on Windows (the default), the db_home argument will be interpreted as a UTF-8 string, which is equivalent to ASCII for Latin characters.

flags

The flags parameter specifies the subsystems that are initialized and how the application's environment affects Berkeley DB file naming, among other things. The flags parameter must be set to 0 or by bitwise inclusively OR'ing together one or more of the values described in this section.

Because there are a large number of flags that can be specified, they have been grouped together by functionality. The first group of flags indicates which of the Berkeley DB subsystems should be initialized.

The choice of subsystems initialized for a Berkeley DB database environment is specified by the thread of control initially creating the environment. Any subsequent thread of control joining the environment will automatically be configured to use the same subsystems as were created in the environment (unless the thread of control requests a subsystem not available in the environment, which will fail). Applications joining an environment, able to adapt to whatever subsystems have been configured in the environment, should open the environment without specifying any subsystem flags. Applications joining an environment, requiring specific subsystems from their environments, should open the environment specifying those specific subsystem flags.

  • DB_INIT_CDB

    Initialize locking for the Berkeley DB Concurrent Data Store product. In this mode, Berkeley DB provides multiple reader/single writer access. The only other subsystem that should be specified with the DB_INIT_CDB flag is DB_INIT_MPOOL.

  • DB_INIT_LOCK

    Initialize the locking subsystem. This subsystem should be used when multiple processes or threads are going to be reading and writing a Berkeley DB database, so that they do not interfere with each other. If all threads are accessing the database(s) read-only, locking is unnecessary. When the DB_INIT_LOCK flag is specified, it is usually necessary to run a deadlock detector, as well. See db_deadlock and DB_ENV->lock_detect() for more information.

  • DB_INIT_LOG

    Initialize the logging subsystem. This subsystem should be used when recovery from application or system failure is necessary. If the log region is being created and log files are already present, the log files are reviewed; subsequent log writes are appended to the end of the log, rather than overwriting current log entries.

  • DB_INIT_MPOOL

    Initialize the shared memory buffer pool subsystem. This subsystem should be used whenever an application is using any Berkeley DB access method.

  • DB_INIT_REP

    Initialize the replication subsystem. This subsystem should be used whenever an application plans on using replication. The DB_INIT_REP flag requires the DB_INIT_TXN and DB_INIT_LOCK flags also be configured.

    You can also specify this flag in the DB_CONFIG configuration file. The syntax is a single line with the string "set_open_flags", one or more whitespace characters, the string "DB_INIT_REP", optionally one or more whitespace characters and the string "on" or "off". If the optional string is omitted, the default is "on"; for example, "set_open_flags DB_INIT_REP" or "set_open_flags DB_INIT_REP on". Because the DB_CONFIG file is read when the database environment is opened, it will silently overrule configuration done before that time.

  • DB_INIT_TXN

    Initialize the transaction subsystem. This subsystem should be used when recovery and atomicity of multiple operations are important. The DB_INIT_TXN flag implies the DB_INIT_LOG flag.

The second group of flags govern what recovery, if any, is performed when the environment is initialized:

  • DB_RECOVER

    Run normal recovery on this environment before opening it for normal use. If this flag is set, the DB_CREATE and DB_INIT_TXN flags must also be set, because the regions will be removed and re-created, and transactions are required for application recovery.

  • DB_RECOVER_FATAL

    Run catastrophic recovery on this environment before opening it for normal use. If this flag is set, the DB_CREATE and DB_INIT_TXN flags must also be set, because the regions will be removed and re-created, and transactions are required for application recovery.

A standard part of the recovery process is to remove the existing Berkeley DB environment and create a new one in which to perform recovery. If the thread of control performing recovery does not specify the correct region initialization information (for example, the correct memory pool cache size), the result can be an application running in an environment with incorrect cache and other subsystem sizes. For this reason, the thread of control performing recovery should specify correct configuration information before calling the DB_ENV->open() method; or it should remove the environment after recovery is completed, leaving creation of the correctly sized environment to a subsequent call to the DB_ENV->open() method.

All Berkeley DB recovery processing must be single-threaded; that is, only a single thread of control may perform recovery or access a Berkeley DB environment while recovery is being performed. Because it is not an error to specify DB_RECOVER for an environment for which no recovery is required, it is reasonable programming practice for the thread of control responsible for performing recovery and creating the environment to always specify the DB_CREATE and DB_RECOVER flags during startup.

The third group of flags govern file-naming extensions in the environment:

  • DB_USE_ENVIRON

    The Berkeley DB process' environment may be permitted to specify information to be used when naming files; see Berkeley DB File Naming. Because permitting users to specify which files are used can create security problems, environment information will be used in file naming for all users only if the DB_USE_ENVIRON flag is set.

  • DB_USE_ENVIRON_ROOT

    The Berkeley DB process' environment may be permitted to specify information to be used when naming files; see Berkeley DB File Naming. Because permitting users to specify which files are used can create security problems, if the DB_USE_ENVIRON_ROOT flag is set, environment information will be used in file naming only for users with appropriate permissions (for example, users with a user-ID of 0 on UNIX systems).

Finally, there are a few additional unrelated flags:

  • DB_CREATE

    Cause Berkeley DB subsystems to create any underlying files, as necessary.

  • DB_LOCKDOWN

    Lock shared Berkeley DB environment files and memory-mapped databases into memory.

  • DB_FAILCHK

    Internally call the DB_ENV->failchk() method as part of opening the environment. When DB_FAILCHK is specified, a check is made to ensure all DB_ENV->failchk() prerequisites are meet.

    If the DB_FAILCHK flag is used in conjunction with the DB_REGISTER flag, then a check will be made to see if the environment needs recovery. If recovery is needed, a call will be made to the DB_ENV->failchk() method to release any database reads locks held by the thread of control that exited and, if needed, to abort the unresolved transaction. If DB_ENV->failchk() determines environment recovery is still required, the recovery actions for DB_REGISTER will be followed.

    If the DB_FAILCHK flag is not used in conjunction with the DB_REGISTER flag, then make an internal call to DB_ENV->failchk() as the last step of opening the environment. If DB_ENV->failchk() determines database environment recovery is required, DB_RUNRECOVERY will be returned.

  • DB_PRIVATE

    Allocate region memory from the heap instead of from memory backed by the filesystem or system shared memory.

    Note

    Use of this flag means that the environment can only be accessed by one environment handle. The environment cannot be accessed by multiple processes. This is true even if one of those processes is one of the the Berkeley DB utilities. (For example, db_archive, db_checkpoint or db_stat.) Nor can a single process open multiple handles to the environment.

    This flag has two effects on the Berkeley DB environment. First, all underlying data structures are allocated from per-process memory instead of from shared memory that is accessible to more than a single process. Second, mutexes are only configured to work between threads.

    See Shared Memory Regions for more information.

    You can also specify this flag in the DB_CONFIG configuration file. The syntax is a single line with the string "set_open_flags", one or more whitespace characters, the string "DB_PRIVATE", optionally one or more whitespace characters and the string "on" or "off". If the optional string is omitted, the default is "on"; for example, "set_open_flags DB_PRIVATE" or "set_open_flags DB_PRIVATE on". Because the DB_CONFIG file is read when the database environment is opened, it will silently overrule configuration done before that time.

  • DB_REGISTER

    Check to see if recovery needs to be performed before opening the database environment. (For this check to be accurate, all processes using the environment must specify DB_REGISTER when opening the environment.) If recovery needs to be performed for any reason (including the initial use of the DB_REGISTER flag), and DB_RECOVER is also specified, recovery will be performed and the open will proceed normally. If recovery needs to be performed and DB_RECOVER is not specified, DB_RUNRECOVERY will be returned. If recovery does not need to be performed, the DB_RECOVER flag will be ignored. See Architecting Transactional Data Store applications for more information.

  • DB_SYSTEM_MEM

    Allocate region memory from system shared memory instead of from heap memory or memory backed by the filesystem.

    See Shared Memory Regions for more information.

  • DB_THREAD

    Cause the DB_ENV handle returned by DB_ENV->open() to be free-threaded; that is, concurrently usable by multiple threads in the address space. The DB_THREAD flag should be specified if the DB_ENV handle will be concurrently used by more than one thread in the process, or if any DB handles opened in the scope of the DB_ENV handle will be concurrently used by more than one thread in the process.

    If this flag is specified, then any database opened using this environment handle will also be free-threaded.

    Be aware that enabling this flag will serialize calls to DB when using the handle across threads. If concurrent scaling is important to your application we recommend opening separate handles for each thread (and not specifying this flag), rather than sharing handles between threads.

    This flag is required when using the Replication Manager.

    You can also specify this flag in the DB_CONFIG configuration file. The syntax is a single line with the string "set_open_flags", one or more whitespace characters, the string "DB_THREAD", optionally one or more whitespace characters and the string "on" or "off". If the optional string is omitted, the default is "on"; for example, "set_open_flags DB_THREAD" or "set_open_flags DB_THREAD on". Because the DB_CONFIG file is read when the database environment is opened, it will silently overrule configuration done before that time.

mode

On Windows systems, the mode parameter is ignored.

On UNIX systems or in IEEE/ANSI Std 1003.1 (POSIX) environments, files created by Berkeley DB are created with mode mode (as described in chmod(2)) and modified by the process' umask value at the time of creation (see umask(2)). Created files are owned by the process owner; the group ownership of created files is based on the system and directory defaults, and is not further specified by Berkeley DB. System shared memory segments created by Berkeley DB are created with mode mode, unmodified by the process' umask value. If mode is 0, Berkeley DB will use a default mode of readable and writable by both owner and group.

Errors

The DB_ENV->open() method may fail and return one of the following non-zero errors:

DB_RUNRECOVERY

Either the DB_REGISTER flag was specified, a failure occurred, and no recovery flag was specified, or the DB_FAILCHK flag was specified and recovery was deemed necessary.

DB_VERSION_MISMATCH

The version of the Berkeley DB library doesn't match the version that created the database environment.

EAGAIN

The shared memory region was locked and (repeatedly) unavailable.

EINVAL

If the DB_THREAD flag was specified and fast mutexes are not available for this architecture; The DB_HOME or TMPDIR environment variables were set, but empty; An incorrectly formatted NAME VALUE entry or line was found; or if an invalid flag value or parameter was specified.

ENOENT

The file or directory does not exist.

Class

DB_ENV

See Also

Database Environments and Related Methods