1 This is a implementation of the netatalk CNID database support that
2 attempts to put all functionality into a separate daemon called cnid_dbd.
3 There is one such daemon per netatalk volume. The underlying database
4 structure is based on Berkeley DB and the database format is the same
5 as in the cdb CNID backend, so this can be used as a drop-in replacement.
9 - No locking issues or leftover locks due to crashed afpd daemons any
10 more. Since there is only one thread of control accessing the
11 database, no locking is needed and changes appear atomic.
13 - Berkeley DB transactions are difficult to get right with several
14 processes attempting to access the CNID database simultanously. This
15 is much easier with a single process and the database can be made nearly
16 crashproof this way (at a performance cost).
18 - No problems with user permissions and access to underlying database
19 files, the cnid_dbd process runs under a configurable user
20 ID that normally also owns the underlying database
21 and can be contacted by whatever afpd daemon accesses a volume.
23 - If an afpd process crashes, the CNID database is unaffected. If the
24 process was making changes to the database at the time of the crash,
25 those changes will be rolled back entirely (transactions).
26 If the process was not using the database at the time of the crash,
27 no corrective action is necessary. In any case, database consistency
32 - Performance in an environment of processes sharing the database
33 (files) is potentially better for two reasons:
36 ii) r/o access to database pages is possible by more than one
37 process at once, r/w access is possible for nonoverlapping regions.
39 The current implementation of cnid_dbd uses unix domain sockets as
40 the IPC mechanism. While this is not the fastest possible method, it
41 is very portable and the cnid_dbd IPC mechanisms can be extended to
42 use faster IPC (like mmap) on architectures where it is
43 supported. As a ballpark figure, 20000 requests/replies to the cnid_dbd
44 daemon take about 0.6 seconds on a Pentium III 733 Mhz running Linux
45 Kernel 2.4.18 using unix domain sockets. The requests are "empty"
46 (no database lookups/changes), so this is just the IPC
49 I have not measured the effects of the advantages of simultanous
53 Installation and configuration
55 cnid_dbd is part of the CNID framework whereby various CNID backends
56 can be selected for afpd as a runtime option for a given volume.
57 By default only last and dbd backend are built and dbd is the default.
59 There are two executeables that will be built in etc/cnid_dbd and
60 installed into the systems binaries directories of netatalk
61 (e.g. /usr/local/netatalk/sbin or whatever you specify with --sbindir
62 to configure): cnid_metad and cnid_dbd. cnid_metad should run all the
63 time with root permissions. It will be notified when an instance of
64 afpd starts up and will in turn make sure that a cnid_dbd daemon is
65 started for the volume that afpd wishes to access. The daemon runs as
66 long as necessary (see the idle_timeout option below) and services any
67 other instances of afpd that access the volume. You can safely kill it
68 with SIGTERM, it will be restarted automatically by cnid_metad as soon
69 as the volume is accessed again.
71 cnid_dbd changes to the Berkeley DB directory on startup and sets
72 effective UID and GID to owner and group of that directory. Database and
73 supporting files should therefore be writeable by that user/group.
77 - The parameter file parsing of db_param is very simpleminded. It is
78 easy to cause buffer overruns and the like.
79 Also, there is no support for blanks (or weird characters) in
80 filenames for the usock_file parameter.
82 - There is no protection against a malicious user connecting to the
83 cnid_dbd socket and changing the database.
85 Please feel free to grep the source in etc/cnid_dbd and the file
86 libatalk/cnid/dbd/cnid_dbd.c for the string TODO, which indicates
87 comments that adress other, less important points.