DbInfo
#include <db_cxx.h>
DbInfo::DbInfo();
DbInfo::~DbInfo();
DbInfo::DbInfo(const DbInfo &);
DbInfo::DbInfo &operator = (const DbInfo &);
int DbInfo::get_lorder() const;
void DbInfo::set_lorder(int);
size_t DbInfo::get_cachesize() const;
void DbInfo::set_cachesize(size_t);
size_t DbInfo::get_pagesize() const;
void DbInfo::set_pagesize(size_t);
typedef void *(*db_malloc_fcn)(size_t);
DbInfo::db_malloc_fcn DbInfo::get_malloc() const;
void DbInfo::set_malloc(db_malloc_fcn);
int DbInfo::get_bt_maxkey() const;
void DbInfo::set_bt_maxkey(int);
int DbInfo::get_bt_minkey() const;
void DbInfo::set_bt_minkey(int);
typedef int (*bt_compare_fcn)(const DBT *, const DBT *);
bt_compare_fcn DbInfo::get_bt_compare() const;
void DbInfo::set_bt_compare(bt_compare_fcn);
typedef size_t (*bt_prefix_fcn)(const DBT *, const DBT *);
bt_prefix_fcn DbInfo::get_bt_prefix() const;
void DbInfo::set_bt_prefix(bt_prefix_fcn);
unsigned int DbInfo::get_h_ffactor() const;
void DbInfo::set_h_ffactor(unsigned int);
unsigned int DbInfo::get_h_nelem() const;
void DbInfo::set_h_nelem(unsigned int);
typedef u_int32_t (*h_hash_fcn)(const void *, u_int32_t);
h_hash_fcn DbInfo::get_h_hash() const;
void DbInfo::set_h_hash(h_hash_fcn);
int DbInfo::get_re_pad() const;
void DbInfo::set_re_pad(int);
int DbInfo::get_re_delim() const;
void DbInfo::set_re_delim(int);
u_int32_t DbInfo::get_re_len() const;
void DbInfo::set_re_len(u_int32_t);
char DbInfo::*get_re_source() const;
void DbInfo::set_re_source(char *);
u_int32_t DbInfo::get_flags() const;
void DbInfo::set_flags(u_int32_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 DbInfo class. A DbInfo
object is used in conjunction with the Db::open method
(see Db(3)) to specify particular configuration options
for the open. The DbInfo class provides simple access to
an underlying data structure, whose elements can be
examined or changed using the set_ or get_ methods. The
remainder of the manual page refers to these accesses
using the underlying name, e.g., cachesize instead of
get_cachesize and set_cachesize. The default constructor
sets all elements of the underlying structure to zero.
Some of the fields are specific to a type of file format
(one of btree, hashed and recno) and are thus named with
an underscore separated string, ``bt'', ``h'' and ``re'',
respectively. For example, the method set_bt_maxkey sets
the underlying bt_maxkey field, and this field is only
used when opening a btree file.
The fields that are common to all access methods are
listed here; those specific to an individual access method
are described below. No reference to the DbInfo object is
maintained by Db, so it is possible to discard it as soon
as the Db::open call returns.
If possible, defaults appropriate for the system are used
for the DbInfo fields if dbinfo is NULL or any fields of
the DbInfo object are not explicitly set. The following
DbInfo fields may be initialized before calling Db::open:
size_t cachesize;
A suggested maximum size of the memory pool cache, in
bytes. If cachesize is not explicitly set, an
appropriate default is used. If the mp_info field is
also specified, this field is ignored.
Note, the minimum number of pages in the cache should
be no less than 10, and the access methods will fail
if an insufficiently large cache is specified. In
addition, for applications that exhibit strong
locality in their data access patterns, increasing
the size of the cache can significantly improve
application performance.
int lorder;
The byte order for integers in the stored database
metadata. The number should represent the order as
an integer, for example, big endian order is the
number 4,321, and little endian order is the number
1,234. If lorder is not explicitly set, the host
order of the machine where the Db library was
compiled is used.
The value of lorder is ignored except when databases
are being created. If a database already exists, the
byte order it uses is determined when the file is
read.
The access methods provide no guarantees about the
byte ordering of the application data stored in the
database, and applications are responsible for
maintaining any necessary ordering.
size_t pagesize;
The size of the pages used to hold items in the
database, in bytes. The minimum page size is 512
bytes and the maximum page size is 64K bytes. If
pagesize is not explicitly set, a page size is
selected based on the underlying filesystem I/O block
size. The selected size has a lower limit of 512
bytes and an upper limit of 16K bytes.
void *(*malloc)(size_t);
The flag DB_DBT_MALLOC, when specified in the Dbt
object, will cause the Db library to allocate memory
which then becomes the responsibility of the calling
application. See Dbt(3) for more information.
On systems where separate heaps are maintained for
applications and libraries (notably Windows NT),
specifying the DB_DBT_MALLOC flag will fail because
the Db library will allocate memory from a different
heap than the application will use to free it. To
avoid this problem, the malloc function should be set
to point to the application's allocation routine. If
malloc is not explicitly set, it will be used to
allocate the memory returned when the DB_DBT_MALLOC
flag is set. The malloc method must match the
calling conventions of the malloc(3) library routine.
BTREE
The btree data structure is a sorted, balanced tree
structure storing associated key/data pairs. Searches,
insertions, and deletions in the btree will all complete
in O (lg base N) where base is the average number of keys
per page. Often, inserting ordered data into btrees
results in pages that are half-full. This implementation
has been modified to make ordered (or inverse ordered)
insertion the best case, resulting in nearly perfect page
space utilization.
Space freed by deleting key/data pairs from the database
is never reclaimed from the filesystem, although it is
reused where possible. This means that the btree storage
structure is grow-only. If sufficiently many keys are
deleted from a tree that shrinking the underlying database
file is desirable, this can be accomplished by creating a
new tree from a scan of the existing one.
The following additional fields and flags may be
initialized in the DbInfo object before calling Db::open,
when using the btree access method:
int (*bt_compare)(const Dbt *, const Dbt *);
The bt_compare function is the key comparison method.
It must return an integer less than, equal to, or
greater than zero if the first key argument is
considered to be respectively less than, equal to, or
greater than the second key argument. The same
comparison method must be used on a given tree every
time it is opened. If bt_compare is NULL, the keys
are compared lexically, with shorter keys collating
before longer keys.
int bt_minkey;
The minimum number of keys that will be stored on any
single page. This value is used to determine which
keys will be stored on overflow pages, i.e. if a key
or data item is larger than the pagesize divided by
the bt_minkey value, it will be stored on overflow
pages instead of in the page itself. The bt_minkey
value specified must be at least 2; if bt_minkey is
not explicitly set, a value of 2 is used.
size_t (*bt_prefix)(const Dbt *, const Dbt *);
The bt_prefix function is the prefix comparison
method. If specified, this method must return the
number of bytes of the second key argument that are
necessary to determine that it is greater than the
first key argument. If the keys are equal, the key
length should be returned.
This is used to compress the keys stored on the btree
internal pages. The usefulness of this is data
dependent, but in some data sets can produce
significantly reduced tree sizes and search times.
If bt_prefix is not explicitly set, and no comparison
method is specified, a default lexical comparison
method is used. If bt_prefix is NULL and a
comparison method is specified, no prefix comparison
is done.
unsigned long flags;
The following additional flags may be specified:
DB_DUP
Permit duplicate keys in the tree, i.e.
insertion when the key of the key/data pair
being inserted already exists in the tree will
be successful. The ordering of duplicates in
the tree is determined by the order of
insertion, unless the ordering is otherwise
specified by use of a cursor (see Dbc(3) for
more information.) It is an error to specify
both DB_DUP and DB_RECNUM.
DB_RECNUM
Support retrieval from btrees using record
numbers. For more information, see the
DB_GET_RECNO flag to the db->get method (below),
and the cursor Dbc::get method (in Dbc(3)).
Logical record numbers in btrees are mutable in
the face of record insertion or deletion. See
the DB_RENUMBER flag in the RECNO section below
for further discussion.
Maintaining record counts within a btree
introduces a serious point of contention, namely
the page locations where the record counts are
stored. In addition, the entire tree must be
locked during both insertions and deletions,
effectively single-threading the tree for those
operations. Specifying DB_RECNUM can result in
serious performance degradation for some
applications and data sets.
It is an error to specify both DB_DUP and
DB_RECNUM.
HASH
The hash data structure is an extensible, dynamic hashing
scheme. Backward compatible interfaces to the functions
described in dbm(3), ndbm(3) and hsearch(3) are provided,
however these interfaces are not compatible with previous
file formats.
The following additional fields and flags may be
initialized in the DbInfo object before calling Db::open,
when using the hash access method:
unsigned int h_ffactor;
The desired density within the hash table. It is an
approximation of the number of keys allowed to
accumulate in any one bucket, determining when the
hash table grows or shrinks. The default value is 0,
indicating that the fill factor will be selected
dynamically as pages are filled.
u_int32_t (*h_hash)(const void *, u_int32_t);
The h_hash field is a user defined hash method; if
h_hash is NULL, a default hash method is used. Since
no hash method performs equally well on all possible
data, the user may find that the built-in hash method
performs poorly with a particular data set. User
specified hash functions must take a pointer to a
byte string and a length as arguments and return a
u_int32_t value.
If a hash method is specified, hash_open will attempt
to determine if the hash method specified is the same
as the one with which the database was created, and
will fail if it detects that it is not.
unsigned int h_nelem;
An estimate of the final size of the hash table. If
not set or set too low, hash tables will expand
gracefully as keys are entered, although a slight
performance degradation may be noticed. The default
value is 1.
unsigned long flags;
The following additional flags may be specified by
or'ing together one or more of the following values:
DB_DUP
Permit duplicate keys in the tree, i.e.
insertion when the key of the key/data pair
being inserted already exists in the tree will
be successful. The ordering of duplicates in
the tree is determined by the order of
insertion, unless the ordering is otherwise
specified by use of a cursor (see Dbc(3) for
more information.)
RECNO
The recno access method provides support for fixed and
variable length records, optionally backed by a flat text
(byte stream) file. Both fixed and variable length
records are accessed by their logical record number.
It is valid to create a record whose record number is more
than one greater than the last record currently in the
database. For example, the creation of record number 8,
when records 6 and 7 do not yet exist, is not an error.
However, any attempt to retrieve such records (e.g.,
records 6 and 7) will return DB_KEYEMPTY.
Deleting a record will not, by default, renumber records
following the deleted record (see DB_RENUMBER below for
more information). Any attempt to retrieve deleted
records will return DB_KEYEMPTY.
The following additional fields and flags may be
initialized in the DbInfo object before calling Db::open,
when using the recno access method:
int re_delim;
For variable length records, if the re_source file is
specified and the DB_DELIMITER flag is set, the
delimiting byte used to mark the end of a record in
the source file. If the re_source file is specified
and the DB_DELIMITER flag is not set, <newline>
characters (i.e. ``\n'', 0x0a) are interpreted as
end-of-record markers.
u_int32_t re_len;
The length of a fixed-length record.
int re_pad;
For fixed length records, if the DB_PAD flag is set,
the pad character for short records. If the DB_PAD
flag is not explicitly set, <space> characters (i.e.,
0x20) are used for padding.
char *re_source;
The purpose of the re_source field is to provide fast
access and modification to databases that are
normally stored as flat text files.
If the re_source field is explicitly set, it
specifies an underlying flat text database file that
is read to initialize a transient record number
index. In the case of variable length records, the
records are separated by the byte value re_delim.
For example, standard UNIX byte stream files can be
interpreted as a sequence of variable length records
separated by <newline> characters.
In addition, when cached data would normally be
written back to the underlying database file (e.g.,
the close or sync functions are called), the in-
memory copy of the database will be written back to
the re_source file.
By default, the backing source file is read lazily,
i.e., records are not read from the file until they
are requested by the application. If multiple
processes (not threads) are accessing a recno
database concurrently and either inserting or
deleting records, the backing source file must be
read in its entirety before more than a single
process accesses the database, and only that process
should specify the backing source file as part of the
Db::open call. See the DB_SNAPSHOT flag below for
more information.
Reading and writing the backing source file specified
by re_source cannot be transactionally protected
because it involves filesystem operations that are
not part of the Db transaction methodology. For this
reason, if a temporary database is used to hold the
records, i.e., a NULL was specified as the file
argument to Db::open, it is possible to lose the
contents of the re_source file, e.g., if the system
crashes at the right instant. If a file is used to
hold the database, i.e., a file name was specified as
the file argument to Db::open, normal database
recovery on that file can be used to prevent
information loss, although it is still possible that
the contents of re_source will be lost if the system
crashes.
The re_source file must already exist (but may be
zero-length) when Db::open is called.
For all of the above reasons, the re_source field is
generally used to specify databases that are read-
only for Db applications, and that are either
generated on the fly by software tools, or modified
using a different mechanism, e.g., a text editor.
unsigned long flags;
The following additional flags may be specified by
or'ing together one or more of the following values:
DB_DELIMITER
The re_delim field is set.
DB_FIXEDLEN
The records are fixed-length, not byte
delimited. The re_len value specifies the
length of the record, and the re_pad value is
used as the pad character.
Any records added to the database that are less
than re_len bytes long are automatically padded.
Any attempt to insert records into the database
that are greater than re_len bytes long will
cause the call to fail immediately and return an
error.
DB_PAD
The re_pad field is set.
DB_RENUMBER
Specifying the DB_RENUMBER flag causes the
logical record numbers to be mutable, and change
as records are added to and deleted from the
database. For example, the deletion of record
number 4 causes records numbered 5 and greater
to be renumbered downward by 1. If a cursor was
positioned to record number 4 before the
deletion, it will reference the new record
number 4, if any such record exists, after the
deletion. If a cursor was positioned after
record number 4 before the deletion, it will be
shifted downward 1 logical record, continuing to
reference the same record as it did before.
Using the Dbc::put or put interfaces to create
new records will cause the creation of multiple
records if the record number is more than one
greater than the largest record currently in the
database. For example, creating record 28, when
record 25 was previously the last record in the
database, will create records 26 and 27 as well
as 28. Attempts to retrieve records that were
created in this manner will result in an error
return of DB_KEYEMPTY.
If a created record is not at the end of the
database, all records following the new record
will be automatically renumbered upward by 1.
For example, the creation of a new record
numbered 8 causes records numbered 8 and greater
to be renumbered upward by 1. If a cursor was
positioned to record number 8 or greater before
the insertion, it will be shifted upward 1
logical record, continuing to reference the same
record as it did before.
For these reasons, concurrent access to a recno
database with the DB_RENUMBER flag specified may
be largely meaningless, although it is
supported.
DB_SNAPSHOT
This flag specifies that any specified re_source
file be read in its entirety when Db::open is
called. If this flag is not specified, the
re_source file may be read lazily.
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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