Queue and Recno access method specific configuration
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## Queue and Recno access method specific configuration

There are a series of configuration tasks which you can perform when using the Queue and Recno access methods. They are described in the following sections.

### Managing record-based databases

When using fixed- or variable-length record-based databases, particularly with flat-text backing files, there are several items that the user can control. The Recno access method can be used to store either variable- or fixed-length data items. By default, the Recno access method stores variable-length data items. The Queue access method can only store fixed-length data items.

#### Record Delimiters

When using the Recno access method to store variable-length records, records read from any backing source file are separated by a specific byte value which marks the end of one record and the beginning of the next. This delimiting value is ignored except when reading records from a backing source file, that is, records may be stored into the database that include the delimiter byte. However, if such records are written out to the backing source file and the backing source file is subsequently read into a database, the records will be split where delimiting bytes were found.

For example, UNIX text files can usually be interpreted as a sequence of variable-length records separated by ASCII newline characters. This byte value (ASCII 0x0a) is the default delimiter. Applications may specify a different delimiting byte using the DB->set_re_delim() method. If no backing source file is being used, there is no reason to set the delimiting byte value.

#### Record Length

When using the Recno or Queue access methods to store fixed-length records, the record length must be specified. Since the Queue access method always uses fixed-length records, the user must always set the record length prior to creating the database. Setting the record length is what causes the Recno access method to store fixed-length, not variable-length, records.

The length of the records is specified by calling the DB->set_re_len() method. The default length of the records is 0 bytes. Any record read from a backing source file or otherwise stored in the database that is shorter than the declared length will automatically be padded as described for the DB->set_re_pad() method. Any record stored that is longer than the declared length results in an error. For further information on backing source files, see Flat-text backing files.

#### Record Padding Byte Value

When storing fixed-length records in a Queue or Recno database, a pad character may be specified by calling the DB->set_re_pad() method. Any record read from the backing source file or otherwise stored in the database that is shorter than the expected length will automatically be padded with this byte value. If fixed-length records are specified but no pad value is specified, a space character (0x20 in the ASCII character set) will be used. For further information on backing source files, see Flat-text backing files.

### Selecting a Queue extent size

In Queue databases, records are allocated sequentially and directly mapped to an offset within the file storage for the database. As records are deleted from the Queue, pages will become empty and will not be reused in normal queue operations. To facilitate the reclamation of disk space a Queue may be partitioned into extents. Each extent is kept in a separate physical file.

Extent files are automatically created as needed and marked for deletion when the head of the queue moves off the extent. The extent will not be deleted until all processes close the extent. In addition, Berkeley DB caches a small number of extents that have been recently used; this may delay when an extent will be deleted. The number of extents left open depends on queue activity.

The extent size specifies the number of pages that make up each extent. By default, if no extent size is specified, the Queue resides in a single file and disk space is not reclaimed. In choosing an extent size there is a tradeoff between the amount of disk space used and the overhead of creating and deleting files. If the extent size is too small, the system will pay a performance penalty, creating and deleting files frequently. In addition, if the active part of the queue spans many files, all those files will need to be open at the same time, consuming system and process file resources.

### Flat-text backing files

It is possible to back any Recno database (either fixed or variable length) with a flat-text source file. This provides fast read (and potentially write) access to databases that are normally created and stored as flat-text files. The backing source file may be specified by calling the DB->set_re_source() method.

The backing source file will be read to initialize the database. In the case of variable length records, the records are assumed to be separated as described for the DB->set_re_delim() method. For example, standard UNIX byte stream files can be interpreted as a sequence of variable length records separated by ASCII newline characters. This is the default.

When cached data would normally be written back to the underlying database file (for example, when the DB->close() or DB->sync() methods are called), the in-memory copy of the database will be written back to the backing source file.

The backing source file must already exist (but may be zero-length) when DB->open() is called. By default, the backing source file is read lazily, that is, records are not read from the backing source 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. This can be accomplished by calling the DB->set_flags() method with the DB_SNAPSHOT flag.

Reading and writing the backing source file cannot be transactionally protected because it involves filesystem operations that are not part of the Berkeley DB transaction methodology. For this reason, if a temporary database is used to hold the records (a NULL was specified as the file argument to DB->open()), it is possible to lose the contents of the backing source file if the system crashes at the right instant. If a permanent file is used to hold the database (a filename was specified as the file argument to DB->open()), normal database recovery on that file can be used to prevent information loss. It is still possible that the contents of the backing source file itself will be corrupted or lost if the system crashes.

For all of the above reasons, the backing source file is generally used to specify databases that are read-only for Berkeley DB applications, and that are either generated on the fly by software tools, or modified using a different mechanism such as a text editor.

### Logically renumbering records

Records stored in the Queue and Recno access methods are accessed by logical record number. In all cases in Btree databases, and optionally in Recno databases (see the DB->set_flags() method and the DB_RENUMBER flag for more information), record numbers are mutable. This means that the record numbers may change as records are added to and deleted from the database. The deletion of record number 4 causes any records numbered 5 and higher to be renumbered downward by 1; the addition of a new record after record number 4 causes any records numbered 5 and higher to be renumbered upward by 1. In all cases in Queue databases, and by default in Recno databases, record numbers are not mutable, and the addition or deletion of records to the database will not cause already-existing record numbers to change. For this reason, new records cannot be inserted between already-existing records in databases with immutable record numbers.

Cursors pointing into a Btree database or a Recno database with mutable record numbers maintain a reference to a specific record, rather than a record number, that is, the record they reference does not change as other records are added or deleted. For example, if a database contains three records with the record numbers 1, 2, and 3, and the data items "A", "B", and "C", respectively, the deletion of record number 2 ("B") will cause the record "C" to be renumbered downward to record number 2. A cursor positioned at record number 3 ("C") will be adjusted and continue to point to "C" after the deletion. Similarly, a cursor previously referring to the now deleted record number 2 will be positioned between the new record numbers 1 and 2, and an insertion using that cursor will appear between those records. In this manner records can be added and deleted to a database without disrupting the sequential traversal of the database by a cursor.

Only cursors created using a single DB handle can adjust each other's position in this way, however. If multiple DB handles have a renumbering Recno database open simultaneously (as when multiple processes share a single database environment), a record referred to by one cursor could change underfoot if a cursor created using another DB handle inserts or deletes records into the database. For this reason, applications using Recno databases with mutable record numbers will usually make all accesses to the database using a single DB handle and cursors created from that handle, or will otherwise single-thread access to the database, for example, by using the Berkeley DB Concurrent Data Store product.

In any Queue or Recno databases, creating new records will cause the creation of multiple records if the record number being created is more than one greater than the largest record currently in the database. For example, creating record number 28, when record 25 was previously the last record in the database, will implicitly create records 26 and 27 as well as 28. All first, last, next and previous cursor operations will automatically skip over these implicitly created records. So, if record number 5 is the only record the application has created, implicitly creating records 1 through 4, the DBC->get() method with the DB_FIRST flag will return record number 5, not record number 1. Attempts to explicitly retrieve implicitly created records by their record number will result in a special error return, DB_KEYEMPTY.

In any Berkeley DB database, attempting to retrieve a deleted record, using a cursor positioned on the record, results in a special error return, DB_KEYEMPTY. In addition, when using Queue databases or Recno databases with immutable record numbers, attempting to retrieve a deleted record by its record number will also result in the DB_KEYEMPTY return.

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