path: root/doc/dbm.discuss.txt

DBM Libraries for use with Exim
-------------------------------

Background
----------

Exim uses direct-access (so-called "dbm") files for a number of different
purposes. These are files arranged so that the data they contain is indexed and
can quickly be looked up by quoting an appropriate key. They are used as
follows:

1. Exim keeps its "hints" databases in dbm files.

2. The configuration can specify that certain things (e.g. aliases) be looked
   up in dbm files.

3. The configuration can contain expansion strings that involve lookups in dbm
   files.

4. The filter commands "mail" and "vacation" have a facility for replying only
   once to each incoming address. The record of which addresses have already
   received replies may be kept in a dbm file, depending on the configuration
   option once_file_size.

The runtime configuration can be set up without specifying 2 or 3, but Exim
always requires the availability of a dbm library, for 1 (and 4 if configured
that way).


DBM Libraries
-------------

The original library that provided the dbm facility in Unix was called "dbm".
This seems to have been superseded quite some time ago by a new version called
"ndbm" which permits several dbm files to be open at once. Several operating
systems, including those from Sun, contain ndbm as standard.

A number of alternative libraries also exist, the most common of which seems to
be Berkeley DB (just called DB hereinafter). Release 1.85 was around for
some time, and various releases 2.x began to appear towards the end of 1997. In
November 1999, version 3.0 was released, and the ending of support for 2.7.7,
the last 2.x release, was announced for November 2000. (Support for 1.85 has
already ceased.) There were further 3.x releases, but by the end of 2001, the
current release was 4.0.14.  In 2022 it was 5.3.28 on Linux (the then-owner
has developed it further but Exim does not support anything after 5.x).

There are major differences in implementation and interface between the DB 1.x
and 2.x/3.x/4.x releases, and they are best considered as two independent dbm
libraries. Changes to the API were made for 3.0 and again for 3.1.

Another DBM library is the GNU library, gdbm, though this does not seem to be
very widespread.

Yet another dbm library is tdb (Trivial Data Base) which has come out of the
Samba project. The first releases seem to have been in mid-2000.

Some older Linux releases contain gdbm as standard, while others contain no dbm
library. More recent releases contain DB 1.85 or 2.x or later, and presumably
will track the development of the DB library. Some BSD versions of Unix include
DB 1.85 or later. All of the non-ndbm libraries except tdb contain
compatibility interfaces so that programs written to call the ndbm functions
should, in theory, work with them, but there are some potential pitfalls which
have caught out Exim users in the past.

Exim has been tested with ndbm, gdbm, DB 1.85, DB 2.x, DB 3.1, DB 4.0.14, and
tdb 1.0.2, in various different modes in some cases, and is believed to work
with all of them if it and they are properly configured.

I have considered the possibility of calling different dbm libraries for
different functions from a single Exim binary. However, because all bar one of
the libraries provide ndbm compatibility interfaces (and therefore the same
function names) it would require a lot of complicated, error-prone trickery to
achieve this. Exim therefore uses a single library for all its dbm activities.

However, Exim does also support cdb (Constant Data Base), an efficient file
arrangement for indexed data that does not change incrementally (for example,
alias files). This is independent of any dbm library and can be used alongside
any of them.


Locking
-------

The configuration option EXIMDB_LOCK_TIMEOUT controls how long Exim waits to
get a lock on a hints database. From version 1.80 onwards, Exim does not
attempt to take out a lock on an actual database file (this caused problems in
the past). Instead, it takes out an fcntl() lock on a separate file whose name
ends in ".lockfile". This ensures that Exim has exclusive access to the
database before even attempting to open it. Exim creates the lock file the
first time it needs it. It should never be removed.


Main Pitfall
------------

The OS-specific configuration files that are used to build Exim specify the use
of Berkeley DB on those systems where it is known to be standard. In the
absence of any special configuration options, Exim uses the ndbm set of
functions to control its dbm databases. This should work with any of the dbm
libraries because those that are not ndbm have compatibility interfaces.
However, there is one awful pitfall:

Exim #includes a header file called ndbm.h which defines the functions and the
interface data block; gdbm and DB 1.x provide their own versions of this header
file, later DB versions do not. If it should happen that the wrong version of
nbdm.h is seen by Exim, it may compile without error, but fail to operate
correctly at runtime.

This situation can easily arise when more than one dbm library is installed on
a single host. For example, if you decide to use DB 1.x on a system where gdbm
is the standard library, unless you are careful in setting up the include
directories for Exim, it may see gdbm's ndbm.h file instead of DB's. The
situation is even worse with later versions of DB, which do not provide an
ndbm.h file at all.

One way out of this for gdbm and any of the versions of DB is to configure Exim
to call the DBM library in its native mode instead of via the ndbm
compatibility interface, thus avoiding the use of ndbm.h. This is done by
setting the USE_DB configuration option if you are using Berkeley DB, or
USE_GDBM if you are using gdbm. This is the recommended approach.


NDBM
----

The ndbm library holds its data in two files, with extensions .dir and .pag.
This makes atomic updating of, for example, alias files, difficult, because
simple renaming cannot be used without some risk. However, if your system has
ndbm installed, Exim should compile and run without any problems.


GDBM
----

The gdbm library, when called via the ndbm compatibility interface, makes two
hard links to a single file, with extensions .dir and .pag. As mentioned above,
gdbm provides its own version of the ndbm.h header, and you must ensure that
this is seen by Exim rather than any other version. This is not likely to be a
problem if gdbm is the only dbm library on your system.

If gdbm is called via the native interface (by setting USE_GDBM in your
Local/Makefile), it uses a single file, with no extension on the name, and the
ndbm.h header is not required.

The gdbm library does its own locking of the single file that it uses. From
version 1.80 onwards, Exim locks on an entirely separate file before accessing
a hints database, so gdbm's locking should always succeed.


Berkeley DB 1.8x
----------------

1.85 was the most widespread DB 1.x release; there is also a 1.86 bug-fix
release, but the belief is that the bugs it fixes will not affect Exim.
However, maintenance for 1.x releases has been phased out.

This dbm library can be called by Exim in one of two ways: via the ndbm
compatibility interface, or via its own native interface. There are two
advantages to doing the latter: (1) you don't run the risk of Exim's seeing the
"wrong" version of the ndbm.h header, as described above, and (2) the
performance is better. It is therefore recommended that you set USE_DB=yes in an
appropriate Local/Makefile-xxx file. (If you are compiling for just one OS, it
can go in Local/Makefile itself.)

When called via the compatibility interface, DB 1.x creates a single file with
a .db extension. When called via its native interface, no extension is added to
the file name handed to it.

DB 1.x does not do any locking of its own.


Berkeley DB 2.x
---------------

DB 2.x was released in 1997. It is a major re-implementation and its native
interface is incompatible with DB 1.x, though a compatibility interface was
introduced in DB 2.1.0, and there is also an ndbm.h compatibility interface.

Like 1.x, it can be called from Exim via the ndbm compatibility interface or
via its native interface, and once again setting USE_DB in order to get the
native interface is recommended. If USE_DB is *not* set, then you will have to
provide a suitable version of ndbm.h, because one does not come with the DB 2.x
distribution. A suitable version is:

  /*************************************************
  *              ndbm.h header for DB 2.x          *
  *************************************************/

  /* This header should replace any other version of ndbm.h when Berkeley DB
  version 2.x is in use via the ndbm compatibility interface. Otherwise, any
  extant version of ndbm.h may cause programs to misbehave. There doesn't seem
  to be a version of ndbm.h supplied with DB 2.x, so I made this for myself.

  Philip Hazel 12/Jun/97
  */

  #define DB_DBM_HSEARCH
  #include <db.h>

  /* End */

When called via the compatibility interface, DB 2.x creates a single file with
a .db extension. When called via its native interface, no extension is added to
the file name handed to it.

DB 2.x does not do any automatic locking of its own; it does have a set of
functions for various forms of locking, but Exim does not use them.


Berkeley DB 3.x
---------------

DB 3.0 was released in November 1999 and 3.1 in June 2000. The 3.x series is a
development of the 2.x series and the above comments apply. Exim can
automatically distinguish between the different versions, so it copes with the
changes to the API without needing any special configuration.

When Exim creates a DBM file using DB 3.x (e.g. when creating one of its hints
databases), it specified the "hash" format. However, when it opens a DB 3 file
for reading only, it specifies "unknown". This means that it can read DB 3
files in other formats that are created by other programs.


Berkeley DB 4.x
---------------

The 4.x series is a development of the 2.x and 3.x series, and the above
comments apply.


tdb
---

tdb 1.0.2 was released in September 2000. Its origin is the database functions
that are used by the Samba project.



Testing Exim's dbm handling
---------------------------

Because there have been problems with dbm file locking in the past, I built
some testing code for Exim's dbm functions. This is very much a lash-up, but it
is documented here so that anybody who wants to check that their configuration
is locking properly can do so. Now that Exim does the locking on an entirely
separate file, locking problems are much less likely, but this code still
exists, just in case. Proceed as follows:

. Build Exim in the normal way. This ensures that all the makesfiles etc. get
  set up.

. From within the build directory, obey "make test_dbfn". This makes a binary
  file called test_dbfn. If you are experimenting with different configurations
  you *must* do "make makefile" after changing anything, before obeying "make
  test_dbfn" again, because the make target for test_dbfn isn't integrated
  with the making of the makefile.

. Identify a scratch directory where you have write access. Create a sub-
  directory called "db" in the scratch directory.

. Type the command "test_dbfn <scratch-directory>". This will output some
  general information such as

    Exim's db functions tester: interface type is db (v2)
    DBM library: Berkeley DB: Sleepycat Software: DB 2.1.0: (6/13/97)
    USE_DB is defined

  It then says

    Test the functions
    >

. At this point you can type commands to open a dbm file and read and write
  data in it. First type the command "open <name>", e.g. "open junk". The
  response should look like this

    opened DB file <scratch-directory>/db/junk: flags=102
    Locked
    opened 0
    >

  The tester will have created a dbm file within the db directory of the
  scratch directory. It will also have created a file with the extension
  ".lockfile" in the same directory. Unlike Exim itself, it will not create
  the db directory for itself if it does not exist.

. To test the locking, don't type anything more for the moment. You now need to
  set up another process running the same test_dbfn command, e.g. from a
  different logon to the same host. This time, when you attempt to open the
  file it should fail after a minute with a timeout error because it is
  already in use.

. If the second process doesn't produce any error message, but gets back to the
  > prompt, then the locking is not working properly.

. You can check that the second process gets the lock when the first process
  releases it by exiting from the first process with ^D, q, or quit; or by
  typing the command "close".

. There are some other commands available that are not related to locking:

     write <key> <data>
     e.g.
     write abcde the quick brown fox

  writes a record to the database,

     read <key>
     delete <key>

  read and delete a record, respectively, and

     scan

  scans the entire database. Note that the database is purely for testing the
  dbm functions. It is *not* one of Exim's regular databases, and you should
  not try running this testing program on any of Exim's real database
  files.

Philip Hazel
Last update: June 2002