Systemd/tdb

tdb - a trivial database system
tridge@linuxcare.com December 1999
==================================

This is a simple database API. It was inspired by the realisation that
in Samba we have several ad-hoc bits of code that essentially
implement small databases for sharing structures between parts of
Samba. As I was about to add another I realised that a generic
database module was called for to replace all the ad-hoc bits.

I based the interface on gdbm. I couldn't use gdbm as we need to be
able to have multiple writers to the databases at one time.

Compilation
-----------

add HAVE_MMAP=1 to use mmap instead of read/write
add TDB_DEBUG=1 for verbose debug info
add NOLOCK=1 to disable locking code

Testing
-------

Compile tdbtest.c and link with gdbm for testing. tdbtest will perform
identical operations via tdb and gdbm then make sure the result is the
same

Also included is tdbtool, which allows simple database manipulation
on the commandline.

tdbtest and tdbtool are not built as part of Samba, but are included
for completeness.

Interface
---------

The interface is very similar to gdbm except for the following:

- different open interface. The tdb_open call is more similar to a
  traditional open()
- no tdbm_reorganise() function
- no tdbm_sync() function. No operations are cached in the library anyway
- added a tdb_traverse() function for traversing the whole database

A general rule for using tdb is that the caller frees any returned
TDB_DATA structures. Just call free(p.dptr) to free a TDB_DATA
return value called p. This is the same as gdbm.

here is a full list of tdb functions with brief descriptions.


----------------------------------------------------------------------
TDB_CONTEXT *tdb_open(char *name, int hash_size, int tdb_flags,
		      int open_flags, mode_t mode)

   open the database, creating it if necessary 

   The open_flags and mode are passed straight to the open call on the database
   file. A flags value of O_WRONLY is invalid

   The hash size is advisory, use zero for a default value. 

   return is NULL on error

   possible tdb_flags are:
    TDB_CLEAR_IF_FIRST - clear database if we are the only one with it open
    TDB_INTERNAL - don't use a file, instaed store the data in
                   memory. The filename is ignored in this case.
    TDB_NOLOCK - don't do any locking
    TDB_NOMMAP - don't use mmap

----------------------------------------------------------------------
char *tdb_error(TDB_CONTEXT *tdb);

     return a error string for the last tdb error

----------------------------------------------------------------------
int tdb_close(TDB_CONTEXT *tdb);

   close a database

----------------------------------------------------------------------
int tdb_update(TDB_CONTEXT *tdb, TDB_DATA key, TDB_DATA dbuf);

   update an entry in place - this only works if the new data size
   is <= the old data size and the key exists.
   on failure return -1

----------------------------------------------------------------------
TDB_DATA tdb_fetch(TDB_CONTEXT *tdb, TDB_DATA key);

   fetch an entry in the database given a key 
   if the return value has a null dptr then a error occurred

   caller must free the resulting data

----------------------------------------------------------------------
int tdb_exists(TDB_CONTEXT *tdb, TDB_DATA key);

   check if an entry in the database exists 

   note that 1 is returned if the key is found and 0 is returned if not found
   this doesn't match the conventions in the rest of this module, but is
   compatible with gdbm

----------------------------------------------------------------------
int tdb_traverse(TDB_CONTEXT *tdb, int (*fn)(TDB_CONTEXT *tdb,
                 TDB_DATA key, TDB_DATA dbuf, void *state), void *state);

   traverse the entire database - calling fn(tdb, key, data, state) on each 
   element.

   return -1 on error or the record count traversed

   if fn is NULL then it is not called

   a non-zero return value from fn() indicates that the traversal should stop

----------------------------------------------------------------------
TDB_DATA tdb_firstkey(TDB_CONTEXT *tdb);

   find the first entry in the database and return its key

   the caller must free the returned data

----------------------------------------------------------------------
TDB_DATA tdb_nextkey(TDB_CONTEXT *tdb, TDB_DATA key);

   find the next entry in the database, returning its key

   the caller must free the returned data

----------------------------------------------------------------------
int tdb_delete(TDB_CONTEXT *tdb, TDB_DATA key);

   delete an entry in the database given a key

----------------------------------------------------------------------
int tdb_store(TDB_CONTEXT *tdb, TDB_DATA key, TDB_DATA dbuf, int flag);

   store an element in the database, replacing any existing element
   with the same key 

   If flag==TDB_INSERT then don't overwrite an existing entry
   If flag==TDB_MODIFY then don't create a new entry

   return 0 on success, -1 on failure

----------------------------------------------------------------------
int tdb_writelock(TDB_CONTEXT *tdb);

   lock the database. If we already have it locked then don't do anything

----------------------------------------------------------------------
int tdb_writeunlock(TDB_CONTEXT *tdb);
   unlock the database

----------------------------------------------------------------------
int tdb_lockchain(TDB_CONTEXT *tdb, TDB_DATA key);

   lock one hash chain. This is meant to be used to reduce locking
   contention - it cannot guarantee how many records will be locked

----------------------------------------------------------------------
int tdb_unlockchain(TDB_CONTEXT *tdb, TDB_DATA key);

   unlock one hash chain