Remove broken code in exim.h that tried to preserve EX_OK when it was
[exim.git] / 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 $
3 DBM Libraries for use with Exim
4 -------------------------------
6 Background
7 ----------
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:
14 1. Exim keeps its "hints" databases in dbm files.
16 2. The configuration can specify that certain things (e.g. aliases) be looked
17 up in dbm files.
19 3. The configuration can contain expansion strings that involve lookups in dbm
20 files.
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.
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).
32 DBM Libraries
33 -------------
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36 This seems to have been superseded quite some time ago by a new version called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.
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.
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.
52 Another DBM library is the GNU library, gdbm, though this does not seem to be
53 very widespread.
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.
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.
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.
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.
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.
82 Locking
83 -------
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.
94 Main Pitfall
95 ------------
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:
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.
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.
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.
124 NDBM
125 ----
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.
133 GDBM
134 ----
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.
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.
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.
151 Berkeley DB 1.8x
152 ----------------
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.
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.)
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.
170 DB 1.x does not do any locking of its own.
173 Berkeley DB 2.x
174 ---------------
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.
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:
186 /*************************************************
187 * ndbm.h header for DB 2.x *
188 *************************************************/
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.
195 Philip Hazel 12/Jun/97
196 */
198 #define DB_DBM_HSEARCH
199 #include <db.h>
201 /* End */
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.
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.
211 Berkeley DB 3.x
212 ---------------
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.
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.
225 Berkeley DB 4.x
226 ---------------
228 The 4.x series is a developement of the 2.x and 3.x series, and the above
229 comments apply.
232 tdb
233 ---
235 tdb 1.0.2 was released in September 2000. Its origin is the database functions
236 that are used by the Samba project.
240 Testing Exim's dbm handling
241 ---------------------------
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:
250 . Build Exim in the normal way. This ensures that all the makesfiles etc. get
251 set up.
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.
259 . Identify a scratch directory where you have write access. Create a sub-
260 directory called "db" in the scratch directory.
262 . Type the command "test_dbfn <scratch-directory>". This will output some
263 general information such as
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
269 It then says
271 Test the functions
272 >
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
278 opened DB file <scratch-directory>/db/junk: flags=102
279 Locked
280 opened 0
281 >
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.
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.
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.
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".
301 . There are some other commands available that are not related to locking:
303 write <key> <data>
304 e.g.
305 write abcde the quick brown fox
307 writes a record to the database,
309 read <key>
310 delete <key>
312 read and delete a record, respectively, and
314 scan
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.
321 Philip Hazel
322 Last update: June 2002