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$Cambridge: exim/exim-doc/doc-txt/dbm.discuss.txt,v 1.1 2004/10/07 15:04:35 ph10 Exp $
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DBM Libraries for use with Exim
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-------------------------------
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Exim uses direct-access (so-called "dbm") files for a number of different
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purposes. These are files arranged so that the data they contain is indexed and
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can quickly be looked up by quoting an appropriate key. They are used as
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1. Exim keeps its "hints" databases in dbm files.
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2. The configuration can specify that certain things (e.g. aliases) be looked
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3. The configuration can contain expansion strings that involve lookups in dbm
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4. The filter commands "mail" and "vacation" have a facility for replying only
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once to each incoming address. The record of which addresses have already
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received replies may be kept in a dbm file, depending on the configuration
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option once_file_size.
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The runtime configuration can be set up without specifying 2 or 3, but Exim
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always requires the availability of a dbm library, for 1 (and 4 if configured
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The original library that provided the dbm facility in Unix was called "dbm".
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This seems to have been superseded quite some time ago by a new version called
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"ndbm" which permits several dbm files to be open at once. Several operating
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systems, including those from Sun, contain ndbm as standard.
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A number of alternative libraries also exist, the most common of which seems to
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be Berkeley DB (just called DB hereinafter). Release 1.85 was around for
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some time, and various releases 2.x began to appear towards the end of 1997. In
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November 1999, version 3.0 was released, and the ending of support for 2.7.7,
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the last 2.x release, was announced for November 2000. (Support for 1.85 has
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already ceased.) There were further 3.x releases, but by the end of 2001, the
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current release was 4.0.14.
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There are major differences in implementation and interface between the DB 1.x
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and 2.x/3.x/4.x releases, and they are best considered as two independent dbm
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libraries. Changes to the API were made for 3.0 and again for 3.1.
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Another DBM library is the GNU library, gdbm, though this does not seem to be
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Yet another dbm library is tdb (Trivial Data Base) which has come out of the
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Samba project. The first releases seem to have been in mid-2000.
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Some older Linux releases contain gdbm as standard, while others contain no dbm
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library. More recent releases contain DB 1.85 or 2.x or later, and presumably
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will track the development of the DB library. Some BSD versions of Unix include
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DB 1.85 or later. All of the non-ndbm libraries except tdb contain
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compatibility interfaces so that programs written to call the ndbm functions
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should, in theory, work with them, but there are some potential pitfalls which
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have caught out Exim users in the past.
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Exim has been tested with ndbm, gdbm, DB 1.85, DB 2.x, DB 3.1, DB 4.0.14, and
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tdb 1.0.2, in various different modes in some cases, and is believed to work
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with all of them if it and they are properly configured.
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I have considered the possibility of calling different dbm libraries for
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different functions from a single Exim binary. However, because all bar one of
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the libraries provide ndbm compatibility interfaces (and therefore the same
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function names) it would require a lot of complicated, error-prone trickery to
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achieve this. Exim therefore uses a single library for all its dbm activities.
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However, Exim does also support cdb (Constant Data Base), an efficient file
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arrangement for indexed data that does not change incrementally (for example,
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alias files). This is independent of any dbm library and can be used alongside
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The configuration option EXIMDB_LOCK_TIMEOUT controls how long Exim waits to
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get a lock on a hints database. From version 1.80 onwards, Exim does not
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attempt to take out a lock on an actual database file (this caused problems in
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the past). Instead, it takes out an fcntl() lock on a separate file whose name
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ends in ".lockfile". This ensures that Exim has exclusive access to the
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database before even attempting to open it. Exim creates the lock file the
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first time it needs it. It should never be removed.
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The OS-specific configuration files that are used to build Exim specify the use
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of Berkeley DB on those systems where it is known to be standard. In the
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absence of any special configuration options, Exim uses the ndbm set of
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functions to control its dbm databases. This should work with any of the dbm
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libraries because those that are not ndbm have compatibility interfaces.
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However, there is one awful pitfall:
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Exim #includes a header file called ndbm.h which defines the functions and the
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interface data block; gdbm and DB 1.x provide their own versions of this header
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file, later DB versions do not. If it should happen that the wrong version of
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nbdm.h is seen by Exim, it may compile without error, but fail to operate
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correctly at runtime.
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This situation can easily arise when more than one dbm library is installed on
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a single host. For example, if you decide to use DB 1.x on a system where gdbm
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is the standard library, unless you are careful in setting up the include
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directories for Exim, it may see gdbm's ndbm.h file instead of DB's. The
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situation is even worse with later versions of DB, which do not provide an
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One way out of this for gdbm and any of the versions of DB is to configure Exim
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to call the DBM library in its native mode instead of via the ndbm
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compatibility interface, thus avoiding the use of ndbm.h. This is done by
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setting the USE_DB configuration option if you are using Berkeley DB, or
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USE_GDBM if you are using gdbm. This is the recommended approach.
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The ndbm library holds its data in two files, with extensions .dir and .pag.
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This makes atomic updating of, for example, alias files, difficult, because
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simple renaming cannot be used without some risk. However, if your system has
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ndbm installed, Exim should compile and run without any problems.
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The gdbm library, when called via the ndbm compatibility interface, makes two
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hard links to a single file, with extensions .dir and .pag. As mentioned above,
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gdbm provides its own version of the ndbm.h header, and you must ensure that
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this is seen by Exim rather than any other version. This is not likely to be a
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problem if gdbm is the only dbm library on your system.
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If gdbm is called via the native interface (by setting USE_GDBM in your
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Local/Makefile), it uses a single file, with no extension on the name, and the
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ndbm.h header is not required.
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The gdbm library does its own locking of the single file that it uses. From
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version 1.80 onwards, Exim locks on an entirely separate file before accessing
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a hints database, so gdbm's locking should always succeed.
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1.85 was the most widespread DB 1.x release; there is also a 1.86 bug-fix
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release, but the belief is that the bugs it fixes will not affect Exim.
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However, maintenance for 1.x releases has been phased out.
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This dbm library can be called by Exim in one of two ways: via the ndbm
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compatibility interface, or via its own native interface. There are two
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advantages to doing the latter: (1) you don't run the risk of Exim's seeing the
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"wrong" version of the ndbm.h header, as described above, and (2) the
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performace is better. It is therefore recommended that you set USE_DB=yes in an
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appropriate Local/Makefile-xxx file. (If you are compiling for just one OS, it
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can go in Local/Makefile itself.)
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When called via the compatibility interface, DB 1.x creates a single file with
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a .db extension. When called via its native interface, no extension is added to
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the file name handed to it.
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DB 1.x does not do any locking of its own.
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DB 2.x was released in 1997. It is a major re-implementation and its native
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interface is incompatible with DB 1.x, though a compatibility interface was
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introduced in DB 2.1.0, and there is also an ndbm.h compatibility interface.
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Like 1.x, it can be called from Exim via the ndbm compatibility interface or
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via its native interface, and once again setting USE_DB in order to get the
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native interface is recommended. If USE_DB is *not* set, then you will have to
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provide a suitable version of ndbm.h, because one does not come with the DB 2.x
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distribution. A suitable version is:
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/*************************************************
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* ndbm.h header for DB 2.x *
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*************************************************/
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/* This header should replace any other version of ndbm.h when Berkeley DB
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version 2.x is in use via the ndbm compatibility interface. Otherwise, any
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extant version of ndbm.h may cause programs to misbehave. There doesn't seem
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to be a version of ndbm.h supplied with DB 2.x, so I made this for myself.
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Philip Hazel 12/Jun/97
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#define DB_DBM_HSEARCH
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When called via the compatibility interface, DB 2.x creates a single file with
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a .db extension. When called via its native interface, no extension is added to
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the file name handed to it.
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DB 2.x does not do any automatic locking of its own; it does have a set of
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functions for various forms of locking, but Exim does not use them.
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DB 3.0 was released in November 1999 and 3.1 in June 2000. The 3.x series is a
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development of the 2.x series and the above comments apply. Exim can
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automatically distinguish between the different versions, so it copes with the
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changes to the API without needing any special configuration.
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When Exim creates a DBM file using DB 3.x (e.g. when creating one of its hints
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databases), it specified the "hash" format. However, when it opens a DB 3 file
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for reading only, it specifies "unknown". This means that it can read DB 3
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files in other formats that are created by other programs.
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The 4.x series is a developement of the 2.x and 3.x series, and the above
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tdb 1.0.2 was released in September 2000. Its origin is the database functions
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that are used by the Samba project.
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Testing Exim's dbm handling
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---------------------------
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Because there have been problems with dbm file locking in the past, I built
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some testing code for Exim's dbm functions. This is very much a lash-up, but it
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is documented here so that anybody who wants to check that their configuration
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is locking properly can do so. Now that Exim does the locking on an entirely
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separate file, locking problems are much less likely, but this code still
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exists, just in case. Proceed as follows:
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. Build Exim in the normal way. This ensures that all the makesfiles etc. get
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. From within the build directory, obey "make test_dbfn". This makes a binary
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file called test_dbfn. If you are experimenting with different configurations
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you *must* do "make makefile" after changing anything, before obeying "make
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test_dbfn" again, because the make target for test_dbfn isn't integrated
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with the making of the makefile.
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. Identify a scratch directory where you have write access. Create a sub-
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directory called "db" in the scratch directory.
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. Type the command "test_dbfn <scratch-directory>". This will output some
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general information such as
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Exim's db functions tester: interface type is db (v2)
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DBM library: Berkeley DB: Sleepycat Software: DB 2.1.0: (6/13/97)
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. At this point you can type commands to open a dbm file and read and write
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data in it. First type the command "open <name>", e.g. "open junk". The
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response should look like this
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opened DB file <scratch-directory>/db/junk: flags=102
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The tester will have created a dbm file within the db directory of the
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scratch directory. It will also have created a file with the extension
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".lockfile" in the same directory. Unlike Exim itself, it will not create
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the db directory for itself if it does not exist.
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. To test the locking, don't type anything more for the moment. You now need to
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set up another process running the same test_dbfn command, e.g. from a
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different logon to the same host. This time, when you attempt to open the
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file it should fail after a minute with a timeout error because it is
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. If the second process doesn't produce any error message, but gets back to the
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> prompt, then the locking is not working properly.
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. You can check that the second process gets the lock when the first process
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releases it by exiting from the first process with ^D, q, or quit; or by
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typing the command "close".
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. There are some other commands available that are not related to locking:
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write abcde the quick brown fox
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writes a record to the database,
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read and delete a record, respectively, and
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scans the entire database. Note that the database is purely for testing the
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dbm functions. It is *not* one of Exim's regular databases, and you should
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not try running this testing program on any of Exim's real database
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Last update: June 2002
added
removed
doc/dbm.discuss.txt
