DBZ

Section: C Library Functions (3)
Updated: 6 Sep 1997
 

NAME

dbzinit, dbzfresh, dbzagain, dbzclose - database routines
dbzexists, dbzfetch, dbzstore - database routines
dbzsync, dbzsize, dbzgetoptions, dbzsetoptions, dbzdebug - database routines  

SYNOPSIS

#include <dbz.h>

BOOL dbzinit(const char *base)

BOOL dbzclose(void)

BOOL dbzfresh(const char *base, const long size)

BOOL dbzagain(const char *base, const char *oldbase)

BOOL dbzexists(const HASH key)

OFFSET_T dbzfetch(const HASH key)
BOOL dbzfetch(const HASH key, void *ivalue)

BOOL dbzstore(const HASH key, const OFFSET_T offset)
BOOL dbzstore(const HASH key, void *ivalue)

BOOL dbzsync(void)

long dbzsize(const long nentries)

void dbzgetoptions(dbzoptions *opt)

void dbzsetoptions(const dbzoptions opt)

BOOL dbzdebug(const BOOL newvalue)
 

DESCRIPTION

These functions provide an indexing system for rapid random access to a text file (the base file).

Dbz stores offsets into the base text file for rapid retrieval. All retrievals are keyed on a hash value that is generated by the HashMessageID() function.

Dbzinit opens a database, an index into the base file base, consisting of files base.dir , base.index , and base.hash which must already exist. (If the database is new, they should be zero-length files.) Subsequent accesses go to that database until dbzclose is called to close the database.

Dbzfetch searches the database for the specified key, returning the corresponding value if any, if <--enable-tagged-hash at configure> is specified. If <--enable-tagged-hash at configure> is not specified, it returns TRUE and content of ivalue is set. Dbzstore stores the key - value pair in the database, if <--enable-tagged-hash at configure> is specified. If <--enable-tagged-hash at configure> is not specified, it stores the content of ivalue. Dbzstore will fail unless the database files are writable. Dbzexists will verify whether or not the given hash exists or not. Dbz is optimized for this operation and it may be significantly faster than dbzfetch().

Dbzfresh is a variant of dbzinit for creating a new database with more control over details.

Dbzfresh's size parameter specifies the size of the first hash table within the database, in key-value pairs. Performance will be best if the number of key-value pairs stored in the database does not exceed about 2/3 of size. (The dbzsize function, given the expected number of key-value pairs, will suggest a database size that meets these criteria.) Assuming that an fseek offset is 4 bytes, the .index file will be 4 * size bytes. The .hash file will be DBZ_INTERNAL_HASH_SIZE * size bytes (the .dir file is tiny and roughly constant in size) until the number of key-value pairs exceeds about 80% of size. (Nothing awful will happen if the database grows beyond 100% of size, but accesses will slow down quite a bit and the .index and .hash files will grow somewhat.)

Dbz stores up to DBZ_INTERNAL_HASH_SIZE bytes of the message-id's hash in the .hash file to confirm a hit. This eliminates the need to read the base file to handle collisions. This replaces the tagmask feature in previous dbz releases.

A size of ``0'' given to dbzfresh is synonymous with the local default; the normal default is suitable for tables of 5,000,000 key-value pairs. Calling dbzinit(name) with the empty name is equivalent to calling dbzfresh(name, 0).

When databases are regenerated periodically, as in news, it is simplest to pick the parameters for a new database based on the old one. This also permits some memory of past sizes of the old database, so that a new database size can be chosen to cover expected fluctuations. Dbzagain is a variant of dbzinit for creating a new database as a new generation of an old database. The database files for oldbase must exist. Dbzagain is equivalent to calling dbzfresh with a size equal to the result of applying dbzsize to the largest number of entries in the oldbase database and its previous 10 generations.

When many accesses are being done by the same program, dbz is massively faster if its first hash table is in memory. If the ``pag_incore'' flag is set to INCORE_MEM, an attempt is made to read the table in when the database is opened, and dbzclose writes it out to disk again (if it was read successfully and has been modified). Dbzsetoptions can be used to set the pag_incore and exists_incore flag to new value which should be ``INCORE_NO'', ``INCORE_MEM'', or ``INCORE_MMAP'' for the .hash and .index files separately; this does not affect the status of a database that has already been opened. The default is ``INCORE_NO'' for the .index file and ``INCORE_MMAP'' for the .hash file. The attempt to read the table in may fail due to memory shortage; in this case dbz fails with an error. Stores to an in-memory database are not (in general) written out to the file until dbzclose or dbzsync, so if robustness in the presence of crashes or concurrent accesses is crucial, in-memory databases should probably be avoided or the writethrough option should be set to ``TRUE'';

If the nonblock option is ``TRUE'', then writes to the .hash and .index files will be done using non-blocking I/O. This can be significantly faster if your platform supports non-blocking I/O with files.

Dbzsync causes all buffers etc. to be flushed out to the files. It is typically used as a precaution against crashes or concurrent accesses when a dbz-using process will be running for a long time. It is a somewhat expensive operation, especially for an in-memory database.

If dbz has been compiled with debugging facilities available (which makes it bigger and a bit slower), dbzdebug alters the value (and returns the previous value) of an internal flag which (when 1; default is 0) causes verbose and cryptic debugging output on standard output.

Concurrent reading of databases is fairly safe, but there is no (inter)locking, so concurrent updating is not.

An open database occupies three stdio streams and two file descriptors; Memory consumption is negligible (except for stdio buffers) except for in-memory databases.  

SEE ALSO

dbm(3), history(5), libinn(3)  

DIAGNOSTICS

Functions returning BOOL values return ``TRUE'' for success, ``FALSE'' for failure. Functions returning OFFSET_T values return a value with -1 for failure. Dbzinit attempts to have errno set plausibly on return, but otherwise this is not guaranteed. An errno of EDOM from dbzinit indicates that the database did not appear to be in dbz format.

If DBZTEST is defined at compile-time then a main() function will be included. This will do performance tests and integrity test.  

HISTORY

The original dbz was written by Jon Zeeff (zeeff@b-tech.ann-arbor.mi.us). Later contributions by David Butler and Mark Moraes. Extensive reworking, including this documentation, by Henry Spencer (henry@zoo.toronto.edu) as part of the C News project. MD5 code borrowed from RSA. Extensive reworking to remove backwards compatibility and to add hashes into dbz files by Clayton O'Neill (coneill@oneill.net)  

BUGS

Unlike dbm, dbz will refuse to dbzstore with a key already in the database. The user is responsible for avoiding this.

The RFC822 case mapper implements only a first approximation to the hideously-complex RFC822 case rules.

Dbz no longer tries to be call-compatible with dbm in any way.


 

Index

NAME
SYNOPSIS
DESCRIPTION
SEE ALSO
DIAGNOSTICS
HISTORY
BUGS
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