source: trunk/server/lib/tdb/docs/README

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 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
- added transactions support

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
    TDB_NOSYNC - don't synchronise transactions to disk
    TDB_SEQNUM - maintain a sequence number
    TDB_VOLATILE - activate the per-hashchain freelist, default 5
    TDB_ALLOW_NESTING - allow transactions to nest
    TDB_DISALLOW_NESTING - disallow transactions to nest

----------------------------------------------------------------------
TDB_CONTEXT *tdb_open_ex(char *name, int hash_size, int tdb_flags,
                         int open_flags, mode_t mode,
                         const struct tdb_logging_context *log_ctx,
                         tdb_hash_func hash_fn)

This is like tdb_open(), but allows you to pass an initial logging and
hash function. Be careful when passing a hash function - all users of
the database must use the same hash function or you will get data
corruption.


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

     return a error string for the last tdb error

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

   close a database

----------------------------------------------------------------------
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_parse_record(struct tdb_context *tdb, TDB_DATA key,
                     int (*parser)(TDB_DATA key, TDB_DATA data,
                                   void *private_data),
                     void *private_data);

   Hand a record to a parser function without allocating it.

   This function is meant as a fast tdb_fetch alternative for large records
   that are frequently read. The "key" and "data" arguments point directly
   into the tdb shared memory, they are not aligned at any boundary.

   WARNING: The parser is called while tdb holds a lock on the record. DO NOT
   call other tdb routines from within the parser. Also, for good performance
   you should make the parser fast to allow parallel operations.

   tdb_parse_record returns -1 if the record was not found.  If the record was
   found, the return value of "parser" is passed up to the caller.

----------------------------------------------------------------------
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. Traversal callbacks may not start transactions.

   WARNING: The data buffer given to the callback fn does NOT meet the
   alignment restrictions malloc gives you.

----------------------------------------------------------------------
int tdb_traverse_read(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, but marking the database read only during the
   traversal, so any write operations will fail. This allows tdb to
   use read locks, which increases the parallelism possible during the
   traversal.

   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. Traversal callbacks may not start transactions.

----------------------------------------------------------------------
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

----------------------------------------------------------------------
int tdb_transaction_start(TDB_CONTEXT *tdb)

   start a transaction. All operations after the transaction start can
   either be committed with tdb_transaction_commit() or cancelled with
   tdb_transaction_cancel(). 

   If you call tdb_transaction_start() again on the same tdb context
   while a transaction is in progress, then the same transaction
   buffer is re-used. The number of tdb_transaction_{commit,cancel}
   operations must match the number of successful
   tdb_transaction_start() calls.

   Note that transactions are by default disk synchronous, and use a
   recover area in the database to automatically recover the database
   on the next open if the system crashes during a transaction. You
   can disable the synchronous transaction recovery setup using the
   TDB_NOSYNC flag, which will greatly speed up operations at the risk
   of corrupting your database if the system crashes.

   Operations made within a transaction are not visible to other users
   of the database until a successful commit.

----------------------------------------------------------------------
int tdb_transaction_cancel(TDB_CONTEXT *tdb)

   cancel a current transaction, discarding all write and lock
   operations that have been made since the transaction started.


----------------------------------------------------------------------
int tdb_transaction_commit(TDB_CONTEXT *tdb)

   commit a current transaction, updating the database and releasing
   the transaction locks.

----------------------------------------------------------------------
int tdb_transaction_prepare_commit(TDB_CONTEXT *tdb)

   prepare to commit a current transaction, for two-phase commits.
   Once prepared for commit, the only allowed calls are
   tdb_transaction_commit() or tdb_transaction_cancel(). Preparing
   allocates disk space for the pending updates, so a subsequent
   commit should succeed (barring any hardware failures).

----------------------------------------------------------------------
int tdb_check(TDB_CONTEXT *tdb,
              int (*check)(TDB_DATA key, TDB_DATA data, void *private_data),
              void *private_data);)

   check the consistency of the database, calling back the check function
   (if non-NULL) with each record.  If some consistency check fails, or
   the supplied check function returns -1, tdb_check returns -1, otherwise
   0.  Note that logging function (if set) will be called with additional
   information on the corruption found.