doc/doc-txt/dbm.discuss.txt

   1 $Cambridge: exim/doc/doc-txt/dbm.discuss.txt,v 1.1 2004/10/07 15:04:35 ph10 Exp $
   2
   3 DBM Libraries for use with Exim
   4 -------------------------------
   5
   6 Background
   7 ----------
   8
   9 Exim uses direct-access (so-called "dbm") files for a number of different
  10 purposes. These are files arranged so that the data they contain is indexed and
  11 can quickly be looked up by quoting an appropriate key. They are used as
  12 follows:
  13
  14 1. Exim keeps its "hints" databases in dbm files.
  15
  16 2. The configuration can specify that certain things (e.g. aliases) be looked
  17    up in dbm files.
  18
  19 3. The configuration can contain expansion strings that involve lookups in dbm
  20    files.
  21
  22 4. The filter commands "mail" and "vacation" have a facility for replying only
  23    once to each incoming address. The record of which addresses have already
  24    received replies may be kept in a dbm file, depending on the configuration
  25    option once_file_size.
  26
  27 The runtime configuration can be set up without specifying 2 or 3, but Exim
  28 always requires the availability of a dbm library, for 1 (and 4 if configured
  29 that way).
  30
  31
  32 DBM Libraries
  33 -------------
  34
  35 The original library that provided the dbm facility in Unix was called "dbm".
  36 This seems to have been superseded quite some time ago by a new version called
  37 "ndbm" which permits several dbm files to be open at once. Several operating
  38 systems, including those from Sun, contain ndbm as standard.
  39
  40 A number of alternative libraries also exist, the most common of which seems to
  41 be Berkeley DB (just called DB hereinafter). Release 1.85 was around for
  42 some time, and various releases 2.x began to appear towards the end of 1997. In
  43 November 1999, version 3.0 was released, and the ending of support for 2.7.7,
  44 the last 2.x release, was announced for November 2000. (Support for 1.85 has
  45 already ceased.) There were further 3.x releases, but by the end of 2001, the
  46 current release was 4.0.14.
  47
  48 There are major differences in implementation and interface between the DB 1.x
  49 and 2.x/3.x/4.x releases, and they are best considered as two independent dbm
  50 libraries. Changes to the API were made for 3.0 and again for 3.1.
  51
  52 Another DBM library is the GNU library, gdbm, though this does not seem to be
  53 very widespread.
  54
  55 Yet another dbm library is tdb (Trivial Data Base) which has come out of the
  56 Samba project. The first releases seem to have been in mid-2000.
  57
  58 Some older Linux releases contain gdbm as standard, while others contain no dbm
  59 library. More recent releases contain DB 1.85 or 2.x or later, and presumably
  60 will track the development of the DB library. Some BSD versions of Unix include
  61 DB 1.85 or later. All of the non-ndbm libraries except tdb contain
  62 compatibility interfaces so that programs written to call the ndbm functions
  63 should, in theory, work with them, but there are some potential pitfalls which
  64 have caught out Exim users in the past.
  65
  66 Exim has been tested with ndbm, gdbm, DB 1.85, DB 2.x, DB 3.1, DB 4.0.14, and
  67 tdb 1.0.2, in various different modes in some cases, and is believed to work
  68 with all of them if it and they are properly configured.
  69
  70 I have considered the possibility of calling different dbm libraries for
  71 different functions from a single Exim binary. However, because all bar one of
  72 the libraries provide ndbm compatibility interfaces (and therefore the same
  73 function names) it would require a lot of complicated, error-prone trickery to
  74 achieve this. Exim therefore uses a single library for all its dbm activities.
  75
  76 However, Exim does also support cdb (Constant Data Base), an efficient file
  77 arrangement for indexed data that does not change incrementally (for example,
  78 alias files). This is independent of any dbm library and can be used alongside
  79 any of them.
  80
  81
  82 Locking
  83 -------
  84
  85 The configuration option EXIMDB_LOCK_TIMEOUT controls how long Exim waits to
  86 get a lock on a hints database. From version 1.80 onwards, Exim does not
  87 attempt to take out a lock on an actual database file (this caused problems in
  88 the past). Instead, it takes out an fcntl() lock on a separate file whose name
  89 ends in ".lockfile". This ensures that Exim has exclusive access to the
  90 database before even attempting to open it. Exim creates the lock file the
  91 first time it needs it. It should never be removed.
  92
  93
  94 Main Pitfall
  95 ------------
  96
  97 The OS-specific configuration files that are used to build Exim specify the use
  98 of Berkeley DB on those systems where it is known to be standard. In the
  99 absence of any special configuration options, Exim uses the ndbm set of
 100 functions to control its dbm databases. This should work with any of the dbm
 101 libraries because those that are not ndbm have compatibility interfaces.
 102 However, there is one awful pitfall:
 103
 104 Exim #includes a header file called ndbm.h which defines the functions and the
 105 interface data block; gdbm and DB 1.x provide their own versions of this header
 106 file, later DB versions do not. If it should happen that the wrong version of
 107 nbdm.h is seen by Exim, it may compile without error, but fail to operate
 108 correctly at runtime.
 109
 110 This situation can easily arise when more than one dbm library is installed on
 111 a single host. For example, if you decide to use DB 1.x on a system where gdbm
 112 is the standard library, unless you are careful in setting up the include
 113 directories for Exim, it may see gdbm's ndbm.h file instead of DB's. The
 114 situation is even worse with later versions of DB, which do not provide an
 115 ndbm.h file at all.
 116
 117 One way out of this for gdbm and any of the versions of DB is to configure Exim
 118 to call the DBM library in its native mode instead of via the ndbm
 119 compatibility interface, thus avoiding the use of ndbm.h. This is done by
 120 setting the USE_DB configuration option if you are using Berkeley DB, or
 121 USE_GDBM if you are using gdbm. This is the recommended approach.
 122
 123
 124 NDBM
 125 ----
 126
 127 The ndbm library holds its data in two files, with extensions .dir and .pag.
 128 This makes atomic updating of, for example, alias files, difficult, because
 129 simple renaming cannot be used without some risk. However, if your system has
 130 ndbm installed, Exim should compile and run without any problems.
 131
 132
 133 GDBM
 134 ----
 135
 136 The gdbm library, when called via the ndbm compatibility interface, makes two
 137 hard links to a single file, with extensions .dir and .pag. As mentioned above,
 138 gdbm provides its own version of the ndbm.h header, and you must ensure that
 139 this is seen by Exim rather than any other version. This is not likely to be a
 140 problem if gdbm is the only dbm library on your system.
 141
 142 If gdbm is called via the native interface (by setting USE_GDBM in your
 143 Local/Makefile), it uses a single file, with no extension on the name, and the
 144 ndbm.h header is not required.
 145
 146 The gdbm library does its own locking of the single file that it uses. From
 147 version 1.80 onwards, Exim locks on an entirely separate file before accessing
 148 a hints database, so gdbm's locking should always succeed.
 149
 150
 151 Berkeley DB 1.8x
 152 ----------------
 153
 154 1.85 was the most widespread DB 1.x release; there is also a 1.86 bug-fix
 155 release, but the belief is that the bugs it fixes will not affect Exim.
 156 However, maintenance for 1.x releases has been phased out.
 157
 158 This dbm library can be called by Exim in one of two ways: via the ndbm
 159 compatibility interface, or via its own native interface. There are two
 160 advantages to doing the latter: (1) you don't run the risk of Exim's seeing the
 161 "wrong" version of the ndbm.h header, as described above, and (2) the
 162 performace is better. It is therefore recommended that you set USE_DB=yes in an
 163 appropriate Local/Makefile-xxx file. (If you are compiling for just one OS, it
 164 can go in Local/Makefile itself.)
 165
 166 When called via the compatibility interface, DB 1.x creates a single file with
 167 a .db extension. When called via its native interface, no extension is added to
 168 the file name handed to it.
 169
 170 DB 1.x does not do any locking of its own.
 171
 172
 173 Berkeley DB 2.x
 174 ---------------
 175
 176 DB 2.x was released in 1997. It is a major re-implementation and its native
 177 interface is incompatible with DB 1.x, though a compatibility interface was
 178 introduced in DB 2.1.0, and there is also an ndbm.h compatibility interface.
 179
 180 Like 1.x, it can be called from Exim via the ndbm compatibility interface or
 181 via its native interface, and once again setting USE_DB in order to get the
 182 native interface is recommended. If USE_DB is *not* set, then you will have to
 183 provide a suitable version of ndbm.h, because one does not come with the DB 2.x
 184 distribution. A suitable version is:
 185
 186   /*************************************************
 187   *              ndbm.h header for DB 2.x          *
 188   *************************************************/
 189
 190   /* This header should replace any other version of ndbm.h when Berkeley DB
 191   version 2.x is in use via the ndbm compatibility interface. Otherwise, any
 192   extant version of ndbm.h may cause programs to misbehave. There doesn't seem
 193   to be a version of ndbm.h supplied with DB 2.x, so I made this for myself.
 194
 195   Philip Hazel 12/Jun/97
 196   */
 197
 198   #define DB_DBM_HSEARCH
 199   #include <db.h>
 200
 201   /* End */
 202
 203 When called via the compatibility interface, DB 2.x creates a single file with
 204 a .db extension. When called via its native interface, no extension is added to
 205 the file name handed to it.
 206
 207 DB 2.x does not do any automatic locking of its own; it does have a set of
 208 functions for various forms of locking, but Exim does not use them.
 209
 210
 211 Berkeley DB 3.x
 212 ---------------
 213
 214 DB 3.0 was released in November 1999 and 3.1 in June 2000. The 3.x series is a
 215 development of the 2.x series and the above comments apply. Exim can
 216 automatically distinguish between the different versions, so it copes with the
 217 changes to the API without needing any special configuration.
 218
 219 When Exim creates a DBM file using DB 3.x (e.g. when creating one of its hints
 220 databases), it specified the "hash" format. However, when it opens a DB 3 file
 221 for reading only, it specifies "unknown". This means that it can read DB 3
 222 files in other formats that are created by other programs.
 223
 224
 225 Berkeley DB 4.x
 226 ---------------
 227
 228 The 4.x series is a developement of the 2.x and 3.x series, and the above
 229 comments apply.
 230
 231
 232 tdb
 233 ---
 234
 235 tdb 1.0.2 was released in September 2000. Its origin is the database functions
 236 that are used by the Samba project.
 237
 238
 239
 240 Testing Exim's dbm handling
 241 ---------------------------
 242
 243 Because there have been problems with dbm file locking in the past, I built
 244 some testing code for Exim's dbm functions. This is very much a lash-up, but it
 245 is documented here so that anybody who wants to check that their configuration
 246 is locking properly can do so. Now that Exim does the locking on an entirely
 247 separate file, locking problems are much less likely, but this code still
 248 exists, just in case. Proceed as follows:
 249
 250 . Build Exim in the normal way. This ensures that all the makesfiles etc. get
 251   set up.
 252
 253 . From within the build directory, obey "make test_dbfn". This makes a binary
 254   file called test_dbfn. If you are experimenting with different configurations
 255   you *must* do "make makefile" after changing anything, before obeying "make
 256   test_dbfn" again, because the make target for test_dbfn isn't integrated
 257   with the making of the makefile.
 258
 259 . Identify a scratch directory where you have write access. Create a sub-
 260   directory called "db" in the scratch directory.
 261
 262 . Type the command "test_dbfn <scratch-directory>". This will output some
 263   general information such as
 264
 265     Exim's db functions tester: interface type is db (v2)
 266     DBM library: Berkeley DB: Sleepycat Software: DB 2.1.0: (6/13/97)
 267     USE_DB is defined
 268
 269   It then says
 270
 271     Test the functions
 272     >
 273
 274 . At this point you can type commands to open a dbm file and read and write
 275   data in it. First type the command "open <name>", e.g. "open junk". The
 276   response should look like this
 277
 278     opened DB file <scratch-directory>/db/junk: flags=102
 279     Locked
 280     opened 0
 281     >
 282
 283   The tester will have created a dbm file within the db directory of the
 284   scratch directory. It will also have created a file with the extension
 285   ".lockfile" in the same directory. Unlike Exim itself, it will not create
 286   the db directory for itself if it does not exist.
 287
 288 . To test the locking, don't type anything more for the moment. You now need to
 289   set up another process running the same test_dbfn command, e.g. from a
 290   different logon to the same host. This time, when you attempt to open the
 291   file it should fail after a minute with a timeout error because it is
 292   already in use.
 293
 294 . If the second process doesn't produce any error message, but gets back to the
 295   > prompt, then the locking is not working properly.
 296
 297 . You can check that the second process gets the lock when the first process
 298   releases it by exiting from the first process with ^D, q, or quit; or by
 299   typing the command "close".
 300
 301 . There are some other commands available that are not related to locking:
 302
 303      write <key> <data>
 304      e.g.
 305      write abcde the quick brown fox
 306
 307   writes a record to the database,
 308
 309      read <key>
 310      delete <key>
 311
 312   read and delete a record, respectively, and
 313
 314      scan
 315
 316   scans the entire database. Note that the database is purely for testing the
 317   dbm functions. It is *not* one of Exim's regular databases, and you should
 318   not try running this testing program on any of Exim's real database
 319   files.
 320
 321 Philip Hazel
 322 Last update: June 2002