X-Git-Url: https://arthur.barton.de/cgi-bin/gitweb.cgi?a=blobdiff_plain;f=doc%2FDEVELOPER;h=2cc708eb9cd3e21a655b24bd2c2f1e519bca3771;hb=e00467826a7ea8dd706d0728191b13894d25618d;hp=297398f7430aafa7351776001615b4ef3486a565;hpb=fbd6b007f0121c117e86d02efa0a88e1e5ff6676;p=netatalk.git

diff --git a/doc/DEVELOPER b/doc/DEVELOPER
index 297398f7..2cc708eb 100644
--- a/doc/DEVELOPER
+++ b/doc/DEVELOPER
@@ -218,3 +218,85 @@ It has been slightly modified:
   as sepereta args instead of one string of the form "section:parameter" in the original
   library
 
+CNID Database Daemons
+=====================
+
+The CNID database daemons cnid_metad and cnid_dbd are a implementation of
+the netatalk CNID database support that attempts to put all functionality
+into separate daemons.
+There is one cnid_dbd 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
+
+There are two executeables that will be built in etc/cnid_dbd and
+installed into the systems binaries directories of netatalk
+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 cnid_dbd daemon runs as
+long as necessary 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.