dbz

(Database routines for InterNetNews)

SYNOPSIS

    #include <inn/dbz.h>

    #define DBZMAXKEY              ...
    #define DBZ_INTERNAL_HASH_SIZE ...

    typedef enum
    {
        DBZSTORE_OK,
        DBZSTORE_EXISTS,
        DBZSTORE_ERROR
    } DBZSTORE_RESULT;

    typedef enum
    {
        INCORE_NO,
        INCORE_MEM,
        INCORE_MMAP
    } dbz_incore_val;

    typedef struct {
        bool writethrough;
        dbz_incore_val pag_incore;
        dbz_incore_val exists_incore;
        bool nonblock;
    } dbzoptions;

    typedef struct {
        char hash[DBZ_INTERNAL_HASH_SIZE];
    } __attribute__((__packed__)) erec;

    extern bool dbzinit(const char *name);
    extern bool dbzclose(void);

    extern bool dbzfresh(const char *name, off_t size);
    extern bool dbzagain(const char *name, const char *oldname);
    extern bool dbzexists(const HASH key);
    extern bool dbzfetch(const HASH key, off_t *value);
    extern DBZSTORE_RESULT dbzstore(const HASH key, off_t data);
    extern bool dbzsync(void);
    extern long dbzsize(off_t contents);
    extern void dbzsetoptions(const dbzoptions options);
    extern void dbzgetoptions(dbzoptions *options);

DESCRIPTION

These functions provide an indexing system for rapid random access to a text file, hereafter named the base file.

dbz stores offsets into the base file for rapid retrieval. All retrievals are keyed on a hash value that is generated by the HashMessageID function in libinn(3).

dbzinit opens a database, an index into the base file name, consisting of files name.dir, name.index, and name.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. When tagged hash format is used (if --enable-tagged-hash was given at configure time), a name.pag file is used instead of .index and .hash.

dbzfetch searches the database for the specified key, assigning the offset of the base file for the corresponding key to value, if any.

dbzstore stores the key-data pair in the database. It will return DBZSTORE_EXISTS for duplicates (already existing entries), and DBZSTORE_OK for success. It will fail with DBZSTORE_ERROR if the database files are not writable or not opened, or if any other error occurs.

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. The size parameter specifies the size of the first hash table within the database, in number of 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 (by default, 4 bytes if tagged hash format is used, 6 otherwise) 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.

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. That default value is used by dbzinit.

When databases are regenerated periodically, as it is the case for the history file, 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 oldname 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 oldname 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 flags to different values which should be INCORE_NO (read from disk), INCORE_MEM (read from memory) or INCORE_MMAP (read from a mmap'ed file) 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 (telling to systematically write to the filesystem in addition to updating the in-memory database).

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. It is only applicable if you're not mmap'ing the database.

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.

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 in-memory databases (and stdio buffers).

DIAGNOSTICS

Functions returning bool values return true for success, false 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.

BUGS

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

The RFC5322 case mapper implements only a first approximation to the hideously-complex RFC5322 case rules.

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

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>. Rewritten into POD by Julien Elie.

SEE ALSO

dbm(3), history(5), libinn(3).

Last spun 2022-02-26 from POD modified 2022-02-09