DbLog
#include <db_cxx.h>
static int
DbLog::open(const char *dir,
u_int32_t flags, int mode, DbEnv *dbenv, DbLog **regionp);
int
DbLog::close();
int
DbLog::flush(const DbLsn *lsn);
int
DbLog::get(DbLsn *lsn, Dbt *data, u_int32_t flags);
static int
DbLog::compare(const DbLsn *lsn0, const DbLsn *lsn1);
int
DbLog::file(const DbLsn *lsn, char *namep, size_t len);
int
DbLog::put(DbLsn *lsn, const Dbt *data, u_int32_t flags);
static int
DbLog::unlink(const char *dir, int force, DbEnv *);
int
DbLog::archive(char **list[],
u_int32_t flags, void *(*db_malloc)(size_t));
int
DbLog::db_register(const Db *dbp,
const char *name, DBTYPE type, u_int32_t *fidp);
int
DbLog::db_unregister(u_int32_t fid);
int
DbLog::stat(DB_LOG_STAT **spp, void *(*db_malloc)(size_t));
DESCRIPTION
The DB library is a family of classes that provides a
modular programming interface to transactions and record-
oriented file access. The library includes support for
transactions, locking, logging and file page caching, as
well as various indexed access methods. Many of the
classes (e.g., the file page caching class) are useful
independent of the other DB classes, although some classes
are explicitly based on other classes (e.g., transactions
and logging). For a general description of the DB
package, see db_intro(3).
This manual page describes the specific details of the log
class.
This class provide a general-purpose logging facility
sufficient for transaction management. Logs can be shared
by multiple processes.
The DB transaction log is represented by a directory
containing a set of files. The log is a record-oriented,
append-only file, with records identified and accessed via
DbLsn's (database log sequence numbers).
DbLsn's are returned on each DbLog::put operation, and
only those DbLsn's returned by DbLog::put can later be
used to retrieve records from the log.
DbLog::open
The DbLog::open method copies a pointer, to the log
identified by the directory dir, into the memory location
referenced by regionp.
If the dbenv argument to DbLog::open was initialized using
DbEnv::appinit, dir is interpreted as described by
DbEnv(3).
Otherwise, if dir is not NULL, it is interpreted relative
to the current working directory of the process. If dir
is NULL, the following environment variables are checked
in order: ``TMPDIR'', ``TEMP'', and ``TMP''. If one of
them is set, log files are created relative to the
directory it specifies. If none of them are set, the
first possible one of the following directories is used:
/var/tmp, /usr/tmp, /temp, /tmp, C:/temp and C:/tmp.
All files associated with the log are created in this
directory. This directory must already exist when
DbLog::open is called. If the log already exists, the
process must have permission to read and write the
existing files. If the log does not already exist, it is
optionally created and initialized.
If the log region is being created and log files are
already present, the log files are ``recovered'' and
subsequent log writes are appended to the end of the log.
The log is stored in one or more files in the specified
directory. Each file is named using the format
log.NNNNN
where ``NNNNN'' is the sequence number of the file within
the log.
The flags and mode arguments specify how files will be
opened and/or created when they don't already exist. The
flags value is specified by or'ing together one or more of
the following values:
DB_CREATE
Create any underlying files, as necessary. If the
files do not already exist and the DB_CREATE flag is
not specified, the call will fail.
DB_THREAD
Cause the DbLog handle returned by the DbLog::open
method to be useable by multiple threads within a
single address space, i.e., to be ``free-threaded''.
All files created by the log subsystem 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)). The group ownership of created files is based
on the system and directory defaults, and is not further
specified by DB.
The logging subsystem is configured based on which set
methods have been used. It is expected that applications
will use a single DbEnv object as the argument to all of
the subsystems in the DB package. The fields of the DbEnv
object used by DbLog::open are described below. As
references to the DbEnv object may be maintained by
DbLog::open, it is necessary that the DbEnv object and
memory it references be valid until the object is
destroyed. Any of the DbEnv fields that are not
explicitly set will default to appropriate values.
The following fields in the DbEnv object may be
initialized, using the appropriate set method, before
calling DbLog::open:
void *(*db_errcall)(char *db_errpfx, char *buffer);
FILE *db_errfile;
const char *db_errpfx;
class ostream *db_error_stream;
int db_verbose;
The error fields of the DbEnv behave as described for
DbEnv(3).
u_int32_t lg_max;
The maximum size of a single file in the log.
Because DbLsn file offsets are unsigned 4-byte
values, lg_max may not be larger than the maximum
unsigned 4-byte value.
If lg_max is 0, a default value is used.
See the section "LOG FILE LIMITS" below, for further
information.
The DbLog::open method throws a DbException(3) or returns
the value of errno on failure and 0 on success.
DbLog::close
The DbLog::close method closes the log associated with the
DbLog object.
In addition, if the dir argument to DbLog::open was NULL
and dbenv was not initialized using DbEnv::appinit, all
files created for this shared region will be removed, as
if DbLog::unlink were called.
When multiple threads are using the DbLog handle
concurrently, only a single thread may call the
DbLog::close method.
The DbLog::close method throws a DbException(3) or returns
the value of errno on failure and 0 on success.
DbLog::flush
The DbLog::flush method guarantees that all log records
whose DbLsns are less than or equal to the lsn parameter
have been written to disk. If lsn is NULL, all records in
the log are flushed.
The DbLog::flush method throws a DbException(3) or returns
the value of errno on failure and 0 on success.
DbLog::get
The DbLog::get method implements a cursor inside of the
log, retrieving records from the log according to the lsn
and flags parameters.
The data field of the data object is set to the record
retrieved and the size field indicates the number of bytes
in the record. See Dbt(3) for a description of other
fields in the data object. When multiple threads are
using the returned DbLog handle concurrently, either the
DB_DBT_MALLOC or DB_DBT_USERMEM flags must be specified
for any Dbt used for data retrieval.
The flags parameter must be set to exactly one of the
following values:
DB_CHECKPOINT
The last record written with the DB_CHECKPOINT flag
specified to the DbLog::put method is returned in the
data argument. The lsn argument is overwritten with
the DbLsn of the record returned. If no record has
been previously written with the DB_CHECKPOINT flag
specified, the first record in the log is returned.
If the log is empty the DbLog::get method will return
DB_NOTFOUND.
DB_FIRST
The first record from any of the log files found in
the log directory is returned in the data argument.
The lsn argument is overwritten with the DbLsn of the
record returned.
If the log is empty the DbLog::get method will return
DB_NOTFOUND.
DB_LAST
The last record in the log is returned in the data
argument. The lsn argument is overwritten with the
DbLsn of the record returned.
If the log is empty, the DbLog::get method will
return DB_NOTFOUND.
DB_NEXT
The current log position is advanced to the next
record in the log and that record is returned in the
data argument. The lsn argument is overwritten with
the DbLsn of the record returned.
If the pointer has not been initialized via DB_FIRST,
DB_LAST, DB_SET, DB_NEXT, or DB_PREV, DbLog::get will
return the first record in the log. If the last log
record has already been returned or the log is empty,
the DbLog::get method will return DB_NOTFOUND.
If the log was opened with the DB_THREAD flag set,
calls to DbLog::get with the DB_NEXT flag set will
return EINVAL.
DB_PREV
The current log position is moved to the previous
record in the log and that record is returned in the
data argument. The lsn argument is overwritten with
the DbLsn of the record returned.
If the pointer has not been initialized via DB_FIRST,
DB_LAST, DB_SET, DB_NEXT, or DB_PREV, DbLog::get will
return the last record in the log. If the first log
record has already been returned or the log is empty,
the DbLog::get method will return DB_NOTFOUND.
If the log was opened with the DB_THREAD flag set,
calls to DbLog::get with the DB_PREV flag set will
return EINVAL.
DB_CURRENT
Return the log record currently referenced by the
log.
If the log pointer has not been initialized via
DB_FIRST, DB_LAST, DB_SET, DB_NEXT, or DB_PREV, or if
the log was opened with the DB_THREAD flag set,
DbLog::get will return EINVAL.
DB_SET
Retrieve the record specified by the lsn argument.
If the specified DbLsn is invalid (e.g., does not
appear in the log) DbLog::get will return EINVAL.
Otherwise, the DbLog::get method throws a DbException(3)
or returns the value of errno on failure and 0 on success.
DbLog::compare
The DbLog::compare method allows the caller to compare two
DbLsn's. DbLog::compare returns 0 if the two DbLsn's are
equal, 1 if lsn0 is greater than lsn1, and -1 if lsn0 is
less than lsn1.
DbLog::file
The DbLog::file method maps DbLsn's to file names. The
DbLog::file method copies the name of the file containing
the record named by lsn into the memory location
referenced by namep. (This mapping of DbLsn to file is
needed for database administration. For example, a
transaction manager typically records the earliest DbLsn
needed for restart, and the database administrator may
want to archive log files to tape when they contain only
DbLsn's before the earliest one needed for restart.)
The len argument is the length of the namep buffer in
bytes. If namep is too short to hold the file name,
DbLog::file will return ENOMEM. Note, as described above,
log file names are quite short, on the order of 10
characters.
The DbLog::file method throws a DbException(3) or returns
the value of errno on failure and 0 on success.
DbLog::put
The DbLog::put method appends records to the log. The
DbLsn of the put record is returned in the lsn parameter.
The flags parameter may be set to one of the following
values:
DB_CHECKPOINT
The log should write a checkpoint record, recording
any information necessary to make the log structures
recoverable after a crash.
DB_CURLSN
The DB_LSN of the next record to be put is returned
in the lsn parameter.
DB_FLUSH
The log is forced to disk after this record is
written, guaranteeing that all records with DbLsns
less than or equal to the one being put are on disk
before this method returns (this function is most
often used for a transaction commit, see Db::Txn in
Db (3)).
The caller is responsible for providing any necessary
structure to data. (For example, in a write-ahead logging
protocol, the application must understand what part of
data is an operation code, what part is redo information,
and what part is undo information. In addition, most
transaction managers will store in data the DbLsn of the
previous log record for the same transaction, to support
chaining back through the transaction's log records during
undo.)
The DbLog::put method throws a DbException(3) or returns
the value of errno on failure and 0 on success.
DbLog::unlink
The DbLog::unlink method destroys the log region
identified by the directory dir, removing all files used
to implement the log region. (The directory dir is not
removed.) If there are processes that have called
DbLog::open without calling DbLog::close (i.e., there are
processes currently using the log region), DbLog::unlink
will fail without further action, unless the force flag is
set, in which case DbLog::unlink will attempt to remove
the log region files regardless of any processes still
using the log region.
The result of attempting to forcibly destroy the region
when a process has the region open is unspecified.
Processes using a shared memory region maintain an open
file descriptor for it. On UNIX systems, the region
removal should succeed and processes that have already
joined the region should continue to run in the region
without change, however processes attempting to join the
log region will either fail or attempt to create a new
region. On other systems, e.g., WNT, where the unlink(2)
system call will fail if any process has an open file
descriptor for the file, the region removal will fail.
In the case of catastrophic or system failure, database
recovery must be performed (see db_recover(1) or the
DB_RECOVER and DB_RECOVER_FATAL flags to
DbEnv::appinit(3)). Alternatively, if recovery is not
required because no database state is maintained across
failures, it is possible to clean up a log region by
removing all of the files in the directory specified to
the DbLog::open method, as log region files are never
created in any directory other than the one specified to
DbLog::open. Note, however, that this has the potential
to remove files created by the other DB subsystems in this
database environment.
The DbLog::unlink method throws a DbException(3) or
returns the value of errno on failure and 0 on success.
DbLog::archive
The DbLog::archive method creates a NULL-terminated array
of log or database file names and copies a pointer to them
into the user-specified memory location list.
By default, DbLog::archive returns the names of all of the
log files that are no longer in use (e.g., no longer
involved in active transactions), and that may be archived
for catastrophic recovery and then removed from the
system. If there were no file names to return, list will
be set to NULL.
Arrays of log file names are created in allocated memory.
If db_malloc is non-NULL, it is called to allocate the
memory, otherwise, the library function malloc(3) is used.
The function db_malloc must match the calling conventions
of the malloc(3) library routine. Regardless, the caller
is responsible for deallocating the returned memory. To
deallocate the returned memory, free each returned memory
pointer; pointers inside the memory do not need to be
individually freed.
The flags argument is specified by or'ing together one or
more of the following values:
DB_ARCH_ABS
All pathnames are returned as absolute pathnames,
instead of relative to the database home directory.
DB_ARCH_DATA
Return the database files that need to be archived in
order to recover the database from catastrophic
failure. If any of the database files have not been
accessed during the lifetime of the current log
files, DbLog::archive will not include them in this
list. It is also possible that some of the files
referenced in the log have since been deleted from
the system.
DB_ARCH_LOG
Return all the log file names regardless of whether
or not they are in use.
The DB_ARCH_DATA and DB_ARCH_LOG flags are mutually
exclusive.
The DbLog::archive method throws a DbException(3) or
returns the value of errno on failure and 0 on success.
The DbLog::archive method is based on the C log_archive
function, which is the underlying function used by the
db_archive(1) utility. See the source code for the
db_archive utility for an example of using log_archive in
a UNIX environment. See the db_archive(1) manual page for
more information on database archival procedures.
DbLog::db_register
The DbLog::db_register method registers a file name with
the log manager and copies a file identification number
into the memory location referenced by fidp. This file
identification number should be used in all subsequent log
messages that refer to operations on this file. The log
manager records all file name to file identification
number mappings at each checkpoint so that a recovery
process can identify the file to which a record in the log
refers.
The DbLog::db_register method is called when an access
method registers the open of a file. The dbp parameter
should be a pointer to the Db object which is being
returned by the access method.
The type parameter should be one of the Db types specified
in Db::open (seeDb(3)), e.g., DB_HASH.
The DbLog::db_register method throws a DbException(3) or
returns the value of errno on failure and 0 on success.
DbLog::db_unregister
The DbLog::db_unregister method disassociates the file
name to file identification number mapping for the file
identification number specified by the fid parameter. The
file identification number may then be reused.
The DbLog::db_unregister method throws a DbException(3) or
returns the value of errno on failure and 0 on success.
DbLog::stat
The DbLog::stat function creates a statistical structure
and copies a pointer to it into the user-specified memory
location.
Statistical structures are created in allocated memory.
If db_malloc is non-NULL, it is called to allocate the
memory, otherwise, the library function malloc(3) is used.
The function db_malloc must match the calling conventions
of the malloc(3) library routine. Regardless, the caller
is responsible for deallocating the returned memory. To
deallocate the returned memory, free each returned memory
pointer; pointers inside the memory do not need to be
individually freed. The log statistics are stored in a
structure of type DB_LOG_STAT (typedef'd in <db.h>). The
following DB_LOG_STAT fields will be filled in:
u_int32_t st_magic;
The magic number that identifies a file as a log
file.
u_int32_t st_version;
The version of the log file type.
u_int32_t st_refcnt;
The number of references to the region.
u_int32_t st_regsize;
The size of the region.
u_int32_t st_mode;
The mode of any created log files.
u_int32_t st_lg_max;
The maximum size of any individual file comprising
the log.
u_int32_t st_written;
The total number of bytes written to this log.
u_int32_t st_written_chkpt;
The number of bytes written since the last
checkpoint.
u_int32_t st_region_wait;
The number of times that a process was able to obtain
the region lock without waiting.
u_int32_t st_region_nowait;
The number of times that a process was forced to wait
before obtaining the region lock.
LOG FILE LIMITS
Log file sizes impose a time limit on the length of time a
database may be accessed under transaction protection,
before it needs to be dumped and reloaded (see db_dump(3)
and db_load(3)). Unfortunately, the limits are
potentially difficult to calculate.
The log file name consists of "log." followed by 5 digits,
resulting in a maximum of 99,999 log files. Consider an
application performing 600 transactions per second, for 15
hours a day, logged into 10Mb log files, where each
transaction is logging approximately 100 bytes of data.
The calculation:
(10 * 2^20 * 99999) /
(600 * 60 * 60 * 15 * 100) = 323.63
indicates that the system will run out of log file space
in roughly 324 days. If we increase the maximum size of
the files from 10Mb to 100Mb, the same calculation
indicates that the application will run out of log file
space in roughly 9 years.
There is no way to reset the log file name space in
Berkeley DB. If your application is reaching the end of
its log file name space, you should:
1. Archive your databases as if to prepare for
catastrophic failure (see db_archive(1) for more
information).
2. Dump and re-load all your databases (see db_dump(1)
and db_load(1) for more information).
3. Remove all of the log files from the database
environment (see db_archive(1) for more information).
4. Restart your applications.
ENVIRONMENT VARIABLES
The following environment variables affect the execution
of db_log:
DB_HOME
If the dbenv argument to DbLog::open was initialized
using db_appinit, the environment variable DB_HOME
may be used as the path of the database home for the
interpretation of the dir argument to DbLog::open, as
described in db_appinit(3). Specifically,
DbLog::open is affected by the configuration string
value of DB_LOG_DIR.
TMPDIR
If the dbenv argument to DbLog::open was NULL or not
initialized using db_appinit, the environment
variable TMPDIR may be used as the directory in which
to create the log, as described in the DbLog::open
section above.
ERRORS
Methods marked as returning errno will, by default, throw
an exception that encapsulates the error information. The
default error behavior can be changed, see DbException(3).
The DbLog::open method may fail and throw a DbException(3)
or return errno for any of the errors specified for the
following DB and library functions: DbLog::close(3),
DbLog::unlink(3), atoi(3), close(2), db_version(3),
fcntl(2), fflush(3), lseek(2), malloc(3), memcpy(3),
memset(3), mmap(2), munmap(2), open(2), opendir(3),
read(2), readdir(3), realloc(3), sigfillset(3),
sigprocmask(2), stat(2), strchr(3), strcpy(3), strdup(3),
strerror(3), strlen(3), strncmp(3), unlink(2), and
write(2).
In addition, the DbLog::open method may fail and throw a
DbException(3) or return errno for the following
conditions:
[EAGAIN]
The shared memory region was locked and (repeatedly)
unavailable.
[EINVAL]
An invalid flag value or parameter was specified.
The DB_THREAD flag was specified and spinlocks are
not implemented for this architecture.
The specified file size was too large.
The DbLog::close method may fail and throw a
DbException(3)
or return errno for any of the errors specified for the
following DB and library functions: close(2), fcntl(2),
fflush(3), munmap(2), and strerror(3).
The DbLog::flush method may fail and throw a
DbException(3)
or return errno for any of the errors specified for the
following DB and library functions: close(2), fcntl(2),
fflush(3), fsync(2), lseek(2), malloc(3), memcpy(3),
memset(3), open(2), sigfillset(3), sigprocmask(2),
stat(2), strcpy(3), strdup(3), strerror(3), strlen(3),
unlink(2), and write(2).
In addition, the DbLog::flush method may fail and throw a
DbException(3) or return errno for the following
conditions:
[EINVAL]
An invalid flag value or parameter was specified.
The DbLog::get method may fail and throw a DbException(3)
or return errno for any of the errors specified for the
following DB and library functions: atoi(3), close(2),
fcntl(2), fflush(3), lseek(2), malloc(3), memcpy(3),
memset(3), open(2), opendir(3), read(2), readdir(3),
realloc(3), sigfillset(3), sigprocmask(2), stat(2),
strchr(3), strcpy(3), strdup(3), strerror(3), strlen(3),
strncmp(3), and unlink(2).
In addition, the DbLog::get method may fail and throw a
DbException(3) or return errno for the following
conditions:
[EINVAL]
An invalid flag value or parameter was specified.
The DB_FIRST flag was specified and no log files were
found.
The DbLog::file method may fail and throw a DbException(3)
or return errno for any of the errors specified for the
following DB and library functions: close(2), fcntl(2),
fflush(3), malloc(3), memcpy(3), memset(3), open(2),
sigfillset(3), sigprocmask(2), stat(2), strcpy(3),
strdup(3), strerror(3), strlen(3), and unlink(2).
In addition, the DbLog::file method may fail and throw a
DbException(3) or return errno for the following
conditions:
[ENOMEM]
The supplied buffer was too small to hold the log
file name.
The DbLog::put method may fail and throw a DbException(3)
or return errno for any of the errors specified for the
following DB and library functions: close(2), fcntl(2),
fflush(3), fsync(2), lseek(2), malloc(3), memcpy(3),
memset(3), open(2), sigfillset(3), sigprocmask(2),
stat(2), strcpy(3), strdup(3), strerror(3), strlen(3),
time(3), unlink(2), and write(2).
In addition, the DbLog::put method may fail and throw a
DbException(3) or return errno for the following
conditions:
[EINVAL]
An invalid flag value or parameter was specified.
The record to be logged is larger than the maximum
log record.
The DbLog::unlink method may fail and throw a
DbException(3)
or return errno for any of the errors specified for the
following DB and library functions: close(2), fcntl(2),
fflush(3), malloc(3), memcpy(3), memset(3), mmap(2),
munmap(2), open(2), sigfillset(3), sigprocmask(2),
stat(2), strcpy(3), strdup(3), strerror(3), strlen(3), and
unlink(2).
In addition, the DbLog::unlink method may fail and throw a
DbException(3) or return errno for the following
conditions:
[EBUSY]
The shared memory region was in use and the force
flag was not set.
The DbLog::archive method may fail and throw a
DbException(3)
or return errno for any of the errors specified for the
following DB and library functions: DbLog::compare(3),
DbLog::get(3), close(2), fcntl(2), fflush(3), getcwd(3),
malloc(3), memcpy(3), memset(3), open(2), qsort(3),
realloc(3), sigfillset(3), sigprocmask(2), stat(2),
strchr(3), strcmp(3), strcpy(3), strdup(3), strerror(3),
strlen(3), and unlink(2).
In addition, the DbLog::archive method may fail and throw
a DbException(3) or return errno for the following
conditions:
[EINVAL]
An invalid flag value or parameter was specified.
The log was corrupted.
The DbLog::db_register method may fail and throw a
DbException(3)
or return errno for any of the errors specified for the
following DB and library functions: close(2), fcntl(2),
fflush(3), fsync(2), lseek(2), malloc(3), memcmp(3),
memcpy(3), memset(3), open(2), realloc(3), sigfillset(3),
sigprocmask(2), stat(2), strcpy(3), strdup(3),
strerror(3), strlen(3), time(3), unlink(2), and write(2).
In addition, the DbLog::db_register method may fail and
throw a DbException(3) or return errno for the following
conditions:
[EINVAL]
An invalid flag value or parameter was specified.
The DbLog::db_unregister method may fail and throw a
DbException(3)
or return errno for any of the errors specified for the
following DB and library functions: close(2), fcntl(2),
fflush(3), fsync(2), lseek(2), malloc(3), memcpy(3),
memset(3), open(2), sigfillset(3), sigprocmask(2),
stat(2), strcpy(3), strdup(3), strerror(3), strlen(3),
time(3), unlink(2), and write(2).
In addition, the DbLog::db_unregister method may fail and
throw a DbException(3) or return errno for the following
conditions:
[EINVAL]
An invalid flag value or parameter was specified.
BUGS
The log files are not machine architecture independent.
Specifically, their metadata are not stored in a fixed
byte order.
SEE ALSO
db_archive(1), db_checkpoint(1), db_deadlock(1), db_dump(1),
db_load(1), db_recover(1), db_stat(1), db_intro(3),
db_internal(3), db_thread(3), Db(3), Dbc(3), DbEnv(3),
DbException(3), DbInfo(3), DbLock(3), DbLockTab(3), DbLog(3),
DbLsn(3), DbMpool(3), DbMpoolFile(3), Dbt(3), DbTxn(3),
DbTxnMgr(3)
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