For the distribution of data the most commonly used format is the file.
The tools to work with files exist on any operating system and are
integrated in most applications.  There are two ways to transport a file
from one computer to another. One is to use removable media (floppy,
tape, CD-ROM, etc) and the other is via a network (FTP, NFS, email, etc).
The TCP/IP protocol suite has become the most popular protocol to connect
computers via a network. Many applications have been build on top of
TCP/IP, the most common one for transporting files is FTP.

In the DWD there is a great demand for distributing its products from
the headquarter to all its regional centres and customers. In the beginning
scripts where used to distribute files via FTP. However, this concept
proved to be not very efficient and flexible. Thus the DWD decided to
develop its own file distributing system AFD (Automatic File Distributor)
which has the following features:

 - The AFD can handle the protocols FTP and SMTP directly which effectively
   means that it has its own FTP and SMTP-client. This proved to be
   a very important feature since it made the control and monitoring
   of file distributing much more flexible. Portability was also
   increased with this feature since there is no need to take care of any
   implementation dependent features of the client that comes with the
   implementation.

 - A single configuration file with which the administrator can control
   the distribution of files. The configuration file holds the information
   about which directories the AFD has to monitor and how files are to be
   distributed when files appear in this directory. These files can then
   be distributed to more then one recipient with different options.

 - Files can be retrieved from a remote host via FTP and FTPS.

 - It can handle parallel transfers. If a big file is currently being
   transfered it is still possible to send a smaller more important file
   in parallel.

 - Net capacity is always limited. To overcome this limit files can be
   send with a priority.

 - The AFD is not necessary to receive files. The receiver only needs an
   FTP-daemon.

 - When errors occur the AFD will retry until the file has been
   successfully transmitted or (if configured) deleted if the file has
   reached a certain age.

 - Extensive log files are kept so that each file that has entered the AFD
   can be traced. It is also possible to monitor the files that each
   recipient receives.

 - There is a very compact and efficient graphical user interface (X11)
   with which the administrator can monitor and control each recipient.

 - The code has been written entirely in C and ported to Linux, FreeBSD,
   SGI, FTX, SunOS, HPUX, AIX, MacOS and SCO.

There are now more then 100 installations of the AFD in the DWD which
distribute three million files with 750 GBytes of data daily.
