path: root/lib/ntdb/doc/TDB_porting.txt

Interface differences between TDB and NTDB.

- ntdb shares 'struct TDB_DATA' with tdb, but TDB defines the TDB_DATA
  typedef, whereas ntdb defines NTDB_DATA (ie. both are compatible).
  If you include both ntdb.h and tdb.h, #include tdb.h first,
  otherwise you'll get a compile error when tdb.h re-defined struct
  TDB_DATA.

  Example:
	#include <tdb.h>
	#include <ntdb.h>

- ntdb functions return NTDB_SUCCESS (ie 0) on success, and a negative
  error on failure, whereas tdb functions returned 0 on success, and
  -1 on failure.  tdb then used tdb_error() to determine the error;
  this API is nasty if we ever want to support threads, so is not supported.

  Example:
	#include <tdb.h>
	#include <ntdb.h>

	void tdb_example(struct tdb_context *tdb, TDB_DATA key, TDB_DATA d)
	{
		if (tdb_store(tdb, key, d) == -1) {
			printf("store failed: %s\n", tdb_errorstr(tdb));
		}
	}

	void ntdb_example(struct ntdb_context *ntdb, NTDB_DATA key, NTDB_DATA d)
	{
		enum NTDB_ERROR e;

		e = ntdb_store(ntdb, key, d);
		if (e) {
			printf("store failed: %s\n", ntdb_errorstr(e));
		}
	}

- ntdb's ntdb_fetch() returns an error, tdb's returned the data directly
  (or tdb_null, and you were supposed to check tdb_error() to find out why).

  Example:
	#include <tdb.h>
	#include <ntdb.h>

	void tdb_example(struct tdb_context *tdb, TDB_DATA key)
	{
		TDB_DATA data;

		data = tdb_fetch(tdb, key);
		if (!data.dptr) {
			printf("fetch failed: %s\n", tdb_errorstr(tdb));
		}
	}

	void ntdb_example(struct ntdb_context *ntdb, NTDB_DATA key)
	{
		NTDB_DATA data;
		enum NTDB_ERROR e;

		e = ntdb_fetch(ntdb, key, &data);
		if (e) {
			printf("fetch failed: %s\n", ntdb_errorstr(e));
		}
	}

- ntdb's ntdb_nextkey() frees the old key's dptr, in tdb you needed to do
  this manually.

  Example:
	#include <tdb.h>
	#include <ntdb.h>

	void tdb_example(struct tdb_context *tdb)
	{
		TDB_DATA key, next, data;

		for (key = tdb_firstkey(tdb); key.dptr; key = next) {
			printf("Got key!\n");
			next = tdb_nextkey(tdb, key);
			free(key.dptr);
		}
	}


	void ntdb_example(struct ntdb_context *ntdb)
	{
		NTDB_DATA k, data;
		enum NTDB_ERROR e;

		for (e = ntdb_firstkey(ntdb,&k); !e; e = ntdb_nextkey(ntdb,&k))
			printf("Got key!\n");
	}

- Unlike tdb_open/tdb_open_ex, ntdb_open does not allow NULL names,
  even for NTDB_INTERNAL dbs, and thus ntdb_name() never returns NULL.

  Example:
	#include <tdb.h>
	#include <ntdb.h>

	struct tdb_context *tdb_example(void)
	{
		return tdb_open(NULL, 0, TDB_INTERNAL, O_RDWR, 0);
	}

	struct ntdb_context *ntdb_example(void)
	{
		return ntdb_open("example", NTDB_INTERNAL, O_RDWR, 0);
	}

- ntdb uses a linked list of attribute structures to implement logging and
  alternate hashes.  tdb used tdb_open_ex, which was not extensible.

  Example:
	#include <tdb.h>
	#include <ntdb.h>

	/* Custom hash function */
	static unsigned int my_tdb_hash_func(TDB_DATA *key)
	{
		return key->dsize;
	}

	struct tdb_context *tdb_example(void)
	{
		return tdb_open_ex("example.tdb", 0, TDB_DEFAULT,
		                   O_CREAT|O_RDWR, 0600, NULL, my_hash_func);
	}

	/* Custom hash function */
	static unsigned int my_ntdb_hash_func(const void *key, size_t len,
					      uint32_t seed, void *data)
	{
		return len;
	}

	struct ntdb_context *ntdb_example(void)
	{
		union ntdb_attribute hash;

		hash.base.attr = NTDB_ATTRIBUTE_HASH;
		hash.base.next = NULL;
		hash.hash.fn = my_ntdb_hash_func;
		return ntdb_open("example.ntdb", NTDB_DEFAULT,
		                   O_CREAT|O_RDWR, 0600, &hash);
	}

- tdb's tdb_open/tdb_open_ex took an explicit hash size, defaulting to
  131.  ntdb's uses an attribute for this, defaulting to 8192.

  Example:
	#include <tdb.h>
	#include <ntdb.h>

	struct tdb_context *tdb_example(void)
	{
		return tdb_open("example.tdb", 10007, TDB_DEFAULT,
		                O_CREAT|O_RDWR, 0600);
	}

	struct ntdb_context *ntdb_example(void)
	{
		union ntdb_attribute hashsize;

		hashsize.base.attr = NTDB_ATTRIBUTE_HASHSIZE;
		hashsize.base.next = NULL;
		hashsize.hashsize.size = 16384;
		return ntdb_open("example.ntdb", NTDB_DEFAULT,
		                   O_CREAT|O_RDWR, 0600, &hashsize);
	}

- ntdb does locking on read-only databases (ie. O_RDONLY passed to ntdb_open).
  tdb did not: use the NTDB_NOLOCK flag if you want to suppress locking.

  Example:
	#include <tdb.h>
	#include <ntdb.h>

	struct tdb_context *tdb_example(void)
	{
		return tdb_open("example.tdb", 0, TDB_DEFAULT, O_RDONLY, 0);
	}

	struct ntdb_context *ntdb_example(void)
	{
		return ntdb_open("example.ntdb", NTDB_NOLOCK, O_RDONLY, NULL);
	}

- ntdb's log function is simpler than tdb's log function.  The string
  is already formatted, is not terminated by a '\n', and it takes an
  enum ntdb_log_level not a tdb_debug_level, and which has only three
  values: NTDB_LOG_ERROR, NTDB_LOG_USE_ERROR and NTDB_LOG_WARNING.

	#include <tdb.h>
	#include <ntdb.h>

	static void tdb_log(struct tdb_context *tdb,
	                    enum tdb_debug_level level, const char *fmt, ...)
	{
		va_list ap;
		const char *name;

		switch (level) {
		case TDB_DEBUG_FATAL:
			fprintf(stderr, "FATAL: ");
			break;
		case TDB_DEBUG_ERROR:
			fprintf(stderr, "ERROR: ");
			break;
		case TDB_DEBUG_WARNING:
			fprintf(stderr, "WARNING: ");
			break;
		case TDB_DEBUG_TRACE:
			/* Don't print out tracing. */
			return;
		}

		name = tdb_name(tdb);
		if (!name) {
			name = "unnamed";
		}

		fprintf(stderr, "tdb(%s):", name);

		va_start(ap, fmt);
		vfprintf(stderr, fmt, ap);
		va_end(ap);
	}

	struct tdb_context *tdb_example(void)
	{
		struct tdb_logging_context lctx;

		lctx.log_fn = tdb_log;
		return tdb_open_ex("example.tdb", 0, TDB_DEFAULT,
		                   O_CREAT|O_RDWR, 0600, &lctx, NULL);
	}

	static void ntdb_log(struct ntdb_context *ntdb,
			     enum ntdb_log_level level,
			     enum NTDB_ERROR ecode,
			     const char *message,
			     void *data)
	{
		switch (level) {
		case NTDB_LOG_ERROR:
			fprintf(stderr, "ERROR: ");
			break;
		case NTDB_LOG_USE_ERROR:
			/* We made a mistake, so abort. */
			abort();
			break;
		case NTDB_LOG_WARNING:
			fprintf(stderr, "WARNING: ");
			break;
		}

		fprintf(stderr, "ntdb(%s):%s:%s\n",
			ntdb_name(ntdb), ntdb_errorstr(ecode), message);
	}

	struct ntdb_context *ntdb_example(void)
	{
		union ntdb_attribute log;

		log.base.attr = NTDB_ATTRIBUTE_LOG;
		log.base.next = NULL;
		log.log.fn = ntdb_log;
		return ntdb_open("example.ntdb", NTDB_DEFAULT,
		                 O_CREAT|O_RDWR, 0600, &log);
	}

- ntdb provides ntdb_deq() for comparing two NTDB_DATA, and ntdb_mkdata() for
  creating an NTDB_DATA.

	#include <tdb.h>
	#include <ntdb.h>

	void tdb_example(struct tdb_context *tdb)
	{
		TDB_DATA data, key;

		key.dsize = strlen("hello");
		key.dptr = "hello";
		data = tdb_fetch(tdb, key);
		if (data.dsize == key.dsize
		    && !memcmp(data.dptr, key.dptr, key.dsize))
			printf("key is same as data\n");
		}
		free(data.dptr);
	}

	void ntdb_example(struct ntdb_context *ntdb)
	{
		NTDB_DATA data, key;

		key = ntdb_mkdata("hello", strlen("hello"));
		if (ntdb_fetch(ntdb, key, &data) == NTDB_SUCCESS) {
			if (ntdb_deq(key, data)) {
				printf("key is same as data\n");
			}
			free(data.dptr);
		}
	}

- Failure inside a transaction (such as a lock function failing) does
  not implicitly cancel the transaction; you still need to call
  ntdb_transaction_cancel().

	#include <tdb.h>
	#include <ntdb.h>

	void tdb_example(struct tdb_context *tdb, TDB_DATA key, TDB_DATA d)
	{
		if (tdb_transaction_start(tdb) == -1) {
			printf("transaction failed: %s\n", tdb_errorstr(tdb));
			return;
		}

		if (tdb_store(tdb, key, d) == -1) {
			printf("store failed: %s\n", tdb_errorstr(tdb));
			return;
		}
		if (tdb_transaction_commit(tdb) == -1) {
			printf("commit failed: %s\n", tdb_errorstr(tdb));
		}
	}

	void ntdb_example(struct ntdb_context *ntdb, NTDB_DATA key, NTDB_DATA d)
	{
		enum NTDB_ERROR e;

		e = ntdb_transaction_start(ntdb);
		if (e) {
			printf("transaction failed: %s\n", ntdb_errorstr(e));
			return;
		}

		e = ntdb_store(ntdb, key, d);
		if (e) {
			printf("store failed: %s\n", ntdb_errorstr(e));
			ntdb_transaction_cancel(ntdb);
		}

		e = ntdb_transaction_commit(ntdb);
		if (e) {
			printf("commit failed: %s\n", ntdb_errorstr(e));
		}
	}

- There is no NTDB_CLEAR_IF_FIRST flag; it has severe scalability and
  API problems.  If necessary, you can emulate this by using the open
  hook and placing a 1-byte lock at offset 4.  If your program forks
  and exits, you will need to place this lock again in the child before
  the parent exits.

  Example:

	#include <tdb.h>
	#include <ntdb.h>

	struct tdb_context *tdb_example(void)
	{
		return tdb_open("example.tdb", 0, TDB_CLEAR_IF_FIRST,
		                   O_CREAT|O_RDWR, 0600);
	}

	static enum NTDB_ERROR clear_if_first(int fd, void *unused)
	{
		/* We hold a lock offset 4 always, so we can tell if
		 * anyone else is. */
		struct flock fl;

		fl.l_type = F_WRLCK;
		fl.l_whence = SEEK_SET;
		fl.l_start = 4; /* ACTIVE_LOCK */
		fl.l_len = 1;

		if (fcntl(fd, F_SETLK, &fl) == 0) {
			/* We must be first ones to open it!  Clear it. */
			if (ftruncate(fd, 0) != 0) {
				return NTDB_ERR_IO;
			}
		}
		fl.l_type = F_RDLCK;
		if (fcntl(fd, F_SETLKW, &fl) != 0) {
			return NTDB_ERR_IO;
		}
		return NTDB_SUCCESS;
	}

	struct ntdb_context *ntdb_example(void)
	{
		union ntdb_attribute open_attr;

		open_attr.openhook.base.attr = NTDB_ATTRIBUTE_OPENHOOK;
		open_attr.openhook.base.next = NULL;
		open_attr.openhook.fn = clear_if_first;

		return ntdb_open("example.ntdb", NTDB_DEFAULT,
		                 O_CREAT|O_RDWR, 0600, &open_attr);
	}

- ntdb traversals are not reliable if the database is changed during
  the traversal, ie your traversal may not cover all elements, or may
  cover elements multiple times.  As a special exception, deleting the
  current record within ntdb_traverse() is reliable.

- There is no ntdb_traverse_read, since ntdb_traverse does not hold
  a lock across the entire traversal anyway.  If you want to make sure
  that your traversal function does not write to the database, you can
  set and clear the NTDB_RDONLY flag around the traversal.

- ntdb does not need tdb_reopen() or tdb_reopen_all().  If you call
  fork() after during certain operations the child should close the
  ntdb, or complete the operations before continuing to use the tdb:

	ntdb_transaction_start(): child must ntdb_transaction_cancel()
	ntdb_lockall(): child must call ntdb_unlockall()
	ntdb_lockall_read(): child must call ntdb_unlockall_read()
	ntdb_chainlock(): child must call ntdb_chainunlock()
	ntdb_parse() callback: child must return from ntdb_parse()

- ntdb will not open a non-ntdb file, even if O_CREAT is specified.  tdb
  will overwrite an unknown file in that case.