[Code contributed by Scott Cotton and Joshua Marcus, IC Group, Inc.]

We've written code to add a mysql map type.  It utilizes the mysql
client library, which can be obtained from:

	http://www.mysql.com/downloads/
	http://sourceforge.net/projects/mysql/

In order to build postfix with mysql map support, you will need to add 
-DHAS_MYSQL and -I for the directory containing the mysql headers, and 
the mysqlclient library (and libm) to AUXLIBS, for example:

make -f Makefile.init makefiles \
	'CCARGS=-DHAS_MYSQL -I/usr/local/mysql/include' \
	'AUXLIBS=-L/usr/local/mysql/lib -lmysqlclient -lz -lm'

then, just run 'make'. This requires libz, the compression library.
Older mysql implementations build without libz.

Postfix installations which may benefit from using mysql map types
include sites that have a need for instantaneous updates of
forwarding, and sites that may benefit from having mail exchangers
reference a networked database, possibly working in conjunction with a
customer database of sorts.

Once postfix is built with mysql support, you can specify a map type
in main.cf like this:

alias_maps = mysql:/etc/postfix/mysql-aliases.cf

The file /etc/postfix/mysql-aliases.cf specifies lots of information
telling postfix how to reference the mysql database.  An example mysql 
map config file follows:

#
# mysql config file for alias lookups on postfix
# comments are ok.
#

# the user name and password to log into the mysql server
user = someone
password = some_password 

# the database name on the servers
dbname = customer_database

# the table name
table = mxaliases

#
select_field = forw_addr
where_field = alias

# you may specify additional_conditions here
additional_conditions = and status = 'paid'

# the above variables will result in a query of
# the form: 
# select forw_addr from mxaliases where alias = '$lookup' and status = 'paid'
# ($lookup is escaped so if it contains single quotes or other odd
# characters, it will not cause a parse error in the sql).
#
# the hosts that postfix will try to connect to
# and query from (in the order listed)
# specify unix: for unix-domain sockets, inet: for TCP connections (default)
hosts = host1.some.domain host2.some.domain unix:/file/name

# end mysql config file

Some notes:

This configuration interface setup allows for multiple mysql
databases: you can use one for a virtual table, one for an access
table, and one for an aliases table if you want.

Since sites that have a need for multiple mail exchangers may enjoy
the convenience of using a networked mailer database, but do not want 
to introduce a single point of failure to their system, we've included 
the ability to have postfix reference multiple hosts for access to a
single mysql map.  This will work if sites set up mirrored mysql
databases on two or more hosts.  Whenever queries fail with an error
at one host, the rest of the hosts will be tried in order.  Each host
that is in an error state will undergo a reconnection attempt every so 
often, and if no mysql server hosts are reachable, then mail will be
deferred until at least one of those hosts is reachable.

Performance of postfix with mysql has not been thoroughly tested,
however, we have found it to be stable.  Busy mail servers using mysql
maps will generate lots of concurrent mysql clients, so the mysql
server(s) should be run with this fact in mind.  Any further
performance information, in addition to any feedback is most welcome.

