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+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, instead 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_chainlock(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_chainunlock(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.