Berkeley DB Reference Guide:
Logging Subsystem


Berkeley DB and logging

The Logging subsystem is the logging facility used by Berkeley DB. It is largely Berkeley DB-specific, although it is potentially useful outside of the Berkeley DB package for applications wanting write-ahead logging support. Applications wanting to use the log for purposes other than logging file modifications based on a set of open file descriptors will almost certainly need to make source code modifications to the Berkeley DB code base.

A log can be shared by any number of threads of control. The DB_ENV->open interface is used to open a log. When the log is no longer in use, it should be closed using the DB_ENV->close interface.

Individual log entries are identified by log sequence numbers. Log sequence numbers are stored in an opaque object, a DB_LSN.

The DB_ENV->log_cursor method is used to allocate a log cursor. Log cursors have two methods: DB_LOGC->get method to retrieve log records from the log, and DB_LOGC->close method to destroy the cursor.

There are additional interfaces for integrating the log subsystem with a transaction processing system:

Flushes the log up to a particular log sequence number.

Allows applications to compare any two log sequence numbers.

Maps a log sequence number to the specific log file that contains it.

Returns various sets of log filenames. These interfaces are used for database administration; for example, to determine if log files may safely be removed from the system.

The display db_stat utility uses the DB_ENV->log_stat interface to display statistics about the log.

The log meta-information (but not the log files themselves) may be removed using the DB_ENV->remove interface.

Logging Subsystem and Related MethodsDescription
DB_ENV->log_archiveList log and database files
DB_ENV->log_cursorCreate a log cursor handle
DB_ENV->log_fileMap Log Sequence Numbers to log files
DB_ENV->log_flushFlush log records
DB_ENV->log_putWrite a log record
DB_ENV->set_lg_bsizeSet log buffer size
DB_ENV->set_lg_dirSet the environment logging directory
DB_ENV->set_lg_maxSet log file size
DB_ENV->set_lg_regionmaxSet logging region size
log_compareCompare two Log Sequence Numbers
DB_ENV->log_statReturn log subsystem statistics
DB_LOGC->closeClose a log cursor
DB_LOGC->getRetrieve a log record


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