Database access class
#include <db2/db_cxx.h> static int Db::open(const char *fname, DBTYPE type, u_int32_t flags, int mode, DbEnv *dbenv, DbInfo *dbinfo, Db **dbpp); DBTYPE Db::get_type(void) const; int Db::close(u_int32_t flags); int Db::cursor(DbTxn *txnid, Dbc **cursorp); int Db::del(Dbt *key, DbTxn *txnid); int Db::fd(int *fdp); int Db::get(DbTxn *txnid, Dbt *key, Dbt *data, u_int32_t flags); int Db::put(DbTxn *txnid, Dbt *key, Dbt *data, u_int32_t flags); int Db::stat(void *sp, void *(*db_malloc)(size_t), u_int32_t flags); int Db::sync(u_int32_t flags);
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 Db class, which is the center of access activity.
The currently supported file formats are btree, hashed and recno. The btree format is a representation of a sorted, balanced tree structure. The hashed format is an extensible, dynamic hashing scheme. The recno format supports fixed or variable length records (optionally retrieved from a flat text file).
Storage and retrieval for the Db access methods are based on key/data pairs, using the Dbt class. See Dbt(3) for specific information on the structure and capabilities of a Dbt.
The Db::open method opens the database represented by file for both reading and writing. Files never intended to be shared or preserved on disk may be created by setting the file parameter to NULL.
The Db::open method copies a pointer to a Db object into the memory location referenced by dbpp. The methods of this object allow you to perform various database actions, as described below. The Db::open method throws a DbException(3) or returns the value of errno on failure and 0 on success.
Note, while most of the access methods use file as the name of an underlying file on disk, this is not guaranteed. Also, calling Db::open is a reasonably expensive operation. (This is based on a model where the DBMS keeps a set of files open for a long time rather than opening and closing them on each query.)
The type argument is of type DBTYPE (as defined in the <db2/db_cxx.h> include file) and must be set to one of DB_BTREE, DB_HASH, DB_RECNO or DB_UNKNOWN. If type is DB_UNKNOWN, the database must already exist and Db::open will then determine if it is of type DB_BTREE, DB_HASH or DB_RECNO.
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:
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.
Do not map this file (see DbMpool(3) for further information).
Open the database for reading only. Any attempt to write the database using the access methods will fail regardless of the actual permissions of any underlying files.
Cause the Db handle returned by the Db::open method to be useable by multiple threads within a single address space, i.e., to be ``free-threaded''.
``Truncate'' the database if it exists, i.e., behave as if the database were just created, discarding any previous contents.
All files created by the access methods 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.
See DbEnv(3) for a description of the dbenv argument, and DbInfo(3) for a description of the dbinfo argument.
The Db object returned by Db::open describes a database type, and includes a set of functions to perform various actions, as described below. The methods for Db are as follows:
The type of the underlying access method (and file format). Returns one of DB_BTREE, DB_HASH or DB_RECNO. This value may be used to determine the type of the database after a return from Db::open with the type argument set to DB_UNKNOWN.
A method to flush any cached information to disk, close any open cursors (see Dbc(3)), free any allocated resources, and close any underlying files. Since key/data pairs are cached in memory, failing to sync the file with the close or sync method may result in inconsistent or lost information.
The flags parameter must be set to 0 or the following value:
Do not flush cached information to disk.
The DB_NOSYNC flag is a dangerous option. It should only be set if the application is doing logging (with transactions) so that the database is recoverable after a system or application crash, or if the database is always generated from scratch after any system or application crash.
It is important to understand that flushing cached information to disk only minimizes the window of opportunity for corrupted data. While unlikely, it is possible for database corruption to happen if a system or application crash occurs while writing data to the database. To ensure that database corruption never occurs, applications must either: use transactions and logging with automatic recovery, use logging and application-specific recovery, or edit a copy of the database, and, once all applications using the database have successfully called close, replace the original database with the updated copy.
When multiple threads are using the Db handle concurrently, only a single thread may call the Db handle close method.
The close method throws a DbException(3) or returns the value of errno on failure and 0 on success.
A method to create a cursor and copy a pointer to it into the memory referenced by cursorp.
A cursor is an object used to provide sequential access through a database.
If the file is being accessed under transaction protection, the txnid parameter is a transaction ID returned from txn_begin, otherwise, NULL. If transaction protection is enabled, cursors must be opened and closed within the context of a transaction, and the txnid parameter specifies the transaction context in which the cursor may be used. See Dbc(3) for more information.
The cursor method throws a DbException(3) or returns the value of errno on failure and 0 on success.
A method to remove key/data pairs from the database. The key/data pair associated with the specified key is discarded from the database. In the presence of duplicate key values, all records associated with the designated key will be discarded.
If the file is being accessed under transaction protection, the txnid parameter is a transaction ID returned from txn_begin, otherwise, NULL.
The flags parameter is currently unused, and must be set to 0.
The del method throws a DbException(3) or returns the value of errno on failure, 0 on success, and DB_NOTFOUND if the specified key did not exist in the file.
A method that copies a file descriptor representative of the underlying database into the memory referenced by fdp. A file descriptor referencing the same file will be returned to all processes that call Db::open with the same file argument. This file descriptor may be safely used as an argument to the fcntl(2) and flock(2) locking functions. The file descriptor is not necessarily associated with any of the underlying files used by the access method.
The fd method only supports a coarse-grained form of locking. Applications should use the lock manager where possible.
The fd method throws a DbException(3) or returns the value of errno on failure and 0 on success.
A method that is an interface for keyed retrieval from the database. The address and length of the data associated with the specified key are returned in the object referenced by data.
In the presence of duplicate key values, get will return the first data item for the designated key. Duplicates are sorted by insert order except where this order has been overridden by cursor operations. Retrieval of duplicates requires the use of cursor operations. See Dbc(3) for details.
If the file is being accessed under transaction protection, the txnid parameter is a transaction ID returned from txn_begin, otherwise, NULL.
The flags parameter must be set to 0 or the following value:
Retrieve a specific numbered record from a database. Upon return, both the key and data items will have been filled in, not just the data item as is done for all other uses of the get method.
For DB_GET_RECNO to be specified, the underlying database must be of type btree, and it must have been created with the DB_RECNUM flag (see Db::open(3)). In this case, the data field of the key must be a pointer to a memory location of type db_recno_t, as described in Dbt(3).
If the database is a recno database and the requested key exists, but was never explicitly created by the application or was later deleted, the get method returns DB_KEYEMPTY. Otherwise, if the requested key isn't in the database, the get method returns DB_NOTFOUND. Otherwise, the get method throws a DbException(3) or returns the value of errno on failure and 0 on success.
A method to store key/data pairs in the database. If the database supports duplicates, the put method adds the new data value at the end of the duplicate set.
If the file is being accessed under transaction protection, the txnid parameter is a transaction ID returned from txn_begin, otherwise, NULL.
The flags value is specified by or'ing together one or more of the following values:
Append the key/data pair to the end of the database. For DB_APPEND to be specified, the underlying database must be of type recno. The record number allocated to the record is returned in the specified key.
Enter the new key/data pair only if the key does not already appear in the database.
The default behavior of the put method is to enter the new key/data pair, replacing any previously existing key if duplicates are disallowed, or to add a duplicate entry if duplicates are allowed. Even if the designated database allows duplicates, a call to put with the DB_NOOVERWRITE flag set will fail if the key already exists in the database.
The put method throws a DbException(3) or returns the value of errno on failure, 0 on success, and DB_KEYEXIST if the DB_NOOVERWRITE flag was set and the key already exists in the file.
A method to flush any cached information to disk. If the database is in memory only, the sync method has no effect and will always succeed.
The flags parameter is currently unused, and must be set to 0.
See the close method description above for a discussion of Db and cached data.
The sync method throws a DbException(3) or returns the value of errno on failure and 0 on success.
void *(*db_malloc)(size_t), u_int32_t flags);
A method to create a statistical structure and copy a pointer to it into user-specified memory locations. Specifically, if sp is non-NULL, a pointer to the statistics for the database are copied into the memory location it references.
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.
In the presence of multiple threads or processes accessing an active database, the returned information may be out-of-date.
This method may access all of the pages in the database, and therefore may incur a severe performance penalty and have obvious negative effects on the underlying buffer pool.
The flags parameter must be set to 0 or the following value:
In the case of a btree or recno database, fill in the bt_nrecs field, but do not collect any other information. This flag makes it reasonable for applications to request a record count from a database without incurring a performance penalty.
The stat method throws a DbException(3) or returns the value of errno on failure and 0 on success.
In the case of a btree or recno database, the statistics are stored in a structure of type DB_BTREE_STAT (typedef'd in <db2/db2/db_cxx.h>). The following fields will be filled in:
Magic number that identifies the file as a btree file.
The version of the btree file type.
Permanent database flags, including DB_DUP, DB_FIXEDLEN, DB_RECNUM and DB_RENUMBER.
The bt_minkey value specified to Db::open(3), if any.
The re_len value specified to Db::open(3), if any.
The re_pad value specified to Db::open(3), if any.
Underlying tree page size.
Number of levels in the tree.
Number of data items in the tree (since there may be multiple data items per key, this number may not be the same as the number of keys).
Number of tree internal pages.
Number of tree leaf pages.
Number of tree duplicate pages.
Number of tree overflow pages.
Number of pages on the free list.
Number of pages made available for reuse because they were emptied.
Number of bytes free in tree internal pages.
Number of bytes free in tree leaf pages.
Number of bytes free in tree duplicate pages.
Number of bytes free in tree overflow pages.
Number of bytes saved by prefix compression.
Total number of tree page splits (includes fast and root splits).
Number of root page splits.
Number of fast splits. When sorted keys are added to the database, the Db btree implementation will split left or right to increase the page-fill factor. This number is a measure of how often it was possible to make such a split.
Number of keys added.
Number of keys deleted.
Number of keys retrieved. (Note, this value will not reflect any keys retrieved when the database was open for read-only access, as there is no permanent location to store the information in this case.)
Number of hits in tree fast-insert code. When sorted keys are added to the database, the Db btree implementation will check the last page where an insert occurred before doing a full lookup. This number is a measure of how often the lookup was successful.
Number of misses in tree fast-insert code. See the description of bt_cache_hit; this number is a measure of how often the lookup failed.
The following environment variables affect the execution of Db::open:
DB_HOME
If the dbenv argument to Db::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 Db::open, as described in db_appinit(3). Specifically, Db::open is affected by the configuration string value of DB_DATA_DIR.
Applications that create short-lived databases that are discarded or recreated when the system fails and are unconcerned with concurrent access and loss of data due to catastrophic failure, may wish to use the Db::open functionality without other parts of the Db library. Such applications will only be concerned with the Db access methods. The Db access methods will use the memory pool subsystem, but the application is unlikely to be aware of this. See the file examples_cxx/AccessExample.cpp in the Db source distribution for a C++ language code example of how such an application might use the Db library.
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 Db::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: Db::sync(3), DbLock::get(3), DbLock::put(3), DbLockTab::id(3), DbLockTab::vec(3), DbLog::db_register(3), DbLog::put(3), DbMpool::close(3), DbMpool::db_register(3), DbMpool::open(3), DbMpoolFile::close(3), DbMpoolFile::get(3), DbMpoolFile::open(3), DbMpoolFile::put(3), DbMpoolFile::set(3), DbMpoolFile::sync(3), calloc(3), close(2), fcntl(2), fflush(3), malloc(3), memcpy(3), memmove(3), memset(3), mmap(2), munmap(2), open(2), read(2), realloc(3), sigfillset(3), sigprocmask(2), stat(2), strcpy(3), strdup(3), strerror(3), strlen(3), time(3), and unlink(2).
In addition, the Db::open method may fail and throw a DbException(3) or return errno for the following conditions:
A lock was unavailable.
An invalid flag value or parameter was specified (e.g., unknown database type, page size, hash method, recno pad byte, byte order) or a flag value or parameter that is incompatible with the current file specification.
The DB_THREAD flag was specified and spinlocks are not implemented for this architecture.
There is a mismatch between the version number of file and the software.
A re_source file was specified with either the DB_THREAD flag or a non-NULL tx_info field in the DbEnv argument to Db::open.
A non-existent re_source file was specified.
Database corruption was detected. All subsequent database calls (other than Db::close) will return EPERM.
The Db::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: Db::sync(3), DbLock::get(3), DbLock::put(3), DbLockTab::vec(3), DbLog::db_register(3), DbLog::put(3), DbMpool::close(3), DbMpoolFile::close(3), DbMpoolFile::get(3), DbMpoolFile::put(3), DbMpoolFile::set(3), DbMpoolFile::sync(3), calloc(3), close(2), fflush(3), malloc(3), memcpy(3), memmove(3), memset(3), munmap(2), realloc(3), and strerror(3).
The Db::cursor method may fail and throw a DbException(3) or return errno for any of the errors specified for the following DB and library functions: calloc(3).
In addition, the Db::cursor method may fail and throw a DbException(3) or return errno for the following conditions:
An invalid flag value or parameter was specified.
Database corruption was detected. All subsequent database calls (other than Db::close) will return EPERM.
The Db::del method may fail and throw a DbException(3) or return errno for any of the errors specified for the following DB and library functions: DbLock::get(3), DbLock::put(3), DbLockTab::id(3), DbLockTab::vec(3), DbLog::put(3), DbMpoolFile::get(3), DbMpoolFile::put(3), DbMpoolFile::set(3), calloc(3), fcntl(2), fflush(3), malloc(3), memcmp(3), memcpy(3), memmove(3), memset(3), realloc(3), and strerror(3).
In addition, the Db::del method may fail and throw a DbException(3) or return errno for the following conditions:
A lock was unavailable.
An invalid flag value or parameter was specified.
Database corruption was detected. All subsequent database calls (other than Db::close) will return EPERM.
In addition, the Db::fd method may fail and throw a DbException(3) or return errno for the following conditions:
The Db::fd method was called for an in-memory database, or no underlying file has yet been created.
Database corruption was detected. All subsequent database calls (other than Db::close) will return EPERM.
The Db::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: DbLock::get(3), DbLock::put(3), DbLockTab::id(3), DbLockTab::vec(3), DbLog::put(3), DbMpoolFile::get(3), DbMpoolFile::put(3), DbMpoolFile::set(3), Dbc::get(3), calloc(3), fcntl(2), fflush(3), malloc(3), memcmp(3), memcpy(3), memmove(3), memset(3), realloc(3), and strerror(3).
In addition, the Db::get method may fail and throw a DbException(3) or return errno for the following conditions:
A lock was unavailable.
An invalid flag value or parameter was specified.
The DB_THREAD flag was specified to the Db::open(3) method and neither the DB_DBT_MALLOC or DB_DBT_USERMEM flags were set in the Dbt.
A record number of 0 was specified.
Database corruption was detected. All subsequent database calls (other than Db::close) will return EPERM.
The Db::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: DbLock::get(3), DbLock::put(3), DbLockTab::id(3), DbLockTab::vec(3), DbLog::put(3), DbMpoolFile::get(3), DbMpoolFile::put(3), DbMpoolFile::set(3), calloc(3), fcntl(2), fflush(3), malloc(3), memcmp(3), memcpy(3), memmove(3), memset(3), realloc(3), and strerror(3).
In addition, the Db::put method may fail and throw a DbException(3) or return errno for the following conditions:
An attempt was made to modify a read-only database.
A lock was unavailable.
An invalid flag value or parameter was specified.
A record number of 0 was specified.
An attempt was made to add a record to a fixed-length database that was too large to fit.
An attempt was made to do a partial put.
Database corruption was detected. All subsequent database calls (other than Db::close) will return EPERM.
A btree exceeded the maximum btree depth (255).
The Db::stat method may fail and throw a DbException(3) or return errno for any of the errors specified for the following DB and library functions: DbLock::get(3), DbLock::put(3), DbLockTab::id(3), DbLockTab::vec(3), DbMpoolFile::get(3), DbMpoolFile::put(3), calloc(3), fcntl(2), fflush(3), malloc(3), memcpy(3), and memset(3).
The Db::sync method may fail and throw a DbException(3) or return errno for any of the errors specified for the following DB and library functions: Db::get(3), Db::sync(3), DbLock::get(3), DbLock::put(3), DbLockTab::id(3), DbLockTab::vec(3), DbLog::put(3), DbMpoolFile::get(3), DbMpoolFile::put(3), DbMpoolFile::set(3), DbMpoolFile::sync(3), calloc(3), close(2), fcntl(2), fflush(3), malloc(3), memcpy(3), memmove(3), memset(3), munmap(2), open(2), realloc(3), strerror(3), unlink(2), and write(2).
In addition, the Db::sync method may fail and throw a DbException(3) or return errno for the following conditions:
An invalid flag value or parameter was specified.
Database corruption was detected. All subsequent database calls (other than Db::close) will return EPERM.
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