On Wed, 19 Feb 2003, tim wrote:
> Am I to understand that the database lookups must only return one value?
> They can not return a list of values? The documentation seems to indicate
> that it's possible to return a list.
Maybe this is the tip you need:
Lookups can be used in two different situations, and what they return is
different in the two cases.
1. You can use a lookup in any expanded string. The syntax is
${lookup ..... }
In this case, whatever is looked up replaces the expansion item. It may
be one value or a list of values. Whether a single value or a list is
acceptable or not depends on where you are using the string expansion.
If it is for an option that expects just one value, then only one value
is allowed (for example).
2. You can make use of the lookup mechanism to test whether something
(typically a host name or IP address) is in a list. For example:
hosts = a : b : c
in an ACL tests whether the calling host's name matches "a", or "b", or
"c". Now, suppose you wanted to keep the list of names in a database, or
cdb file, or NIS map, or... By writing
hosts = pgsql;select ....
you are saying to Exim: "Run this lookup; if it succeeds, behave as if
the host is in the list; if it fails, the host is not in the list." You
are using the indexing mechanism of the database as a fast way of
checking a list. A simpler example is
hosts = lsearch;/some/file
where the file contains the list of hosts to be searched.
THE COMPLICATION
The complication happens when a list is first expanded before being
interpreted as a list. This happens in a lot of cases. You can therefore
write either of these:
hosts = cdb;/some/file
hosts = ${lookup{something}cdb{/some/file}}
but they have different meanings. The first means "see if the host name
is in the list in this file". The second means "run this lookup and use
the result of the lookup as a list of host items to check". In the
second case, the list could contain multiple values (colon separated),
and one of those values could even be "cdb;/some/file".
Flexibility does lead to complexity, I'm afraid.
--
Philip Hazel University of Cambridge Computing Service,
ph10@??? Cambridge, England. Phone: +44 1223 334714.
