This is a implementation of the netatalk CNID database support that
attempts to put all functionality into a separate daemon called cnid_dbd.
There is one such daemon per netatalk volume. The underlying database
structure is based on Berkeley DB and the database format is the same
as in the cdb CNID backend, so this can be used as a drop-in replacement.

Advantages: 

- No locking issues or leftover locks due to crashed afpd daemons any
  more. Since there is only one thread of control accessing the
  database, no locking is needed and changes appear atomic.

- Berkeley DB transactions are difficult to get right with several
  processes attempting to access the CNID database simultanously. This 
  is much easier with a single process and the database can be made nearly 
  crashproof this way (at a performance cost).

- No problems with user permissions and access to underlying database
  files, the cnid_dbd process runs under a configurable user
  ID that normally also owns the underlying database
  and can be contacted by whatever afpd daemon accesses a volume.

- If an afpd process crashes, the CNID database is unaffected. If the
  process was making changes to the database at the time of the crash,
  those changes will be rolled back entirely (transactions).
  If the process was not using the database at the time of the crash,
  no corrective action is necessary. In any case, database consistency
  is assured.

Disadvantages:

- Performance in an environment of processes sharing the database
  (files) is potentially better for two reasons:

  i)  IPC overhead.
  ii) r/o access to database pages is possible by more than one
      process at once, r/w access is possible for nonoverlapping regions.

  The current implementation of cnid_dbd uses unix domain sockets as
  the IPC mechanism. While this is not the fastest possible method, it
  is very portable and the cnid_dbd IPC mechanisms can be extended to
  use faster IPC (like mmap) on architectures where it is
  supported. As a ballpark figure, 20000 requests/replies to the cnid_dbd
  daemon take about 0.6 seconds on a Pentium III 733 Mhz running Linux
  Kernel 2.4.18 using unix domain sockets. The requests are "empty"
  (no database lookups/changes), so this is just the IPC
  overhead.
  
  I have not measured the effects of the advantages of simultanous
  database access.


Installation and configuration

cnid_dbd is part of the CNID framework whereby various CNID backends
can be selected for afpd as a runtime option for a given volume.
By default only last and dbd backend are built and dbd is the default.

There are two executeables that will be built in etc/cnid_dbd and
installed into the systems binaries directories of netatalk
(e.g. /usr/local/netatalk/sbin or whatever you specify with --sbindir
to configure): cnid_metad and cnid_dbd. cnid_metad should run all the
time with root permissions. It will be notified when an instance of
afpd starts up and will in turn make sure that a cnid_dbd daemon is
started for the volume that afpd wishes to access. The daemon runs as
long as necessary (see the idle_timeout option below) and services any
other instances of afpd that access the volume. You can safely kill it
with SIGTERM, it will be restarted automatically by cnid_metad as soon
as the volume is accessed again.

cnid_dbd changes to the Berkeley DB directory on startup and sets
effective UID and GID to owner and group of that directory. Database and
supporting files should therefore be writeable by that user/group.

Current shortcomings:

- The parameter file parsing of db_param is very simpleminded. It is
easy to cause buffer overruns and the like.
Also, there is no support for blanks (or weird characters) in
filenames for the usock_file parameter.

- There is no protection against a malicious user connecting to the
cnid_dbd socket and changing the database.

Please feel free to grep the source in etc/cnid_dbd and the file
libatalk/cnid/dbd/cnid_dbd.c for the string TODO, which indicates
comments that adress other, less important points.


The Netatalk Team