On May 13, 2010, at 9:16 AM, Oleg Goldshmidt wrote:
"Installed at the same time" and "operational at the same time" are
not the same thing. This is one of the things that "alternatives"
helps to achieve - you switch between MTAs with a single command
(assuming they are all properly configured, etc.). I suppose you also
need to stop and start the corresponding services.
Not with MTA's. They have various files with the same names, files in
the same locations, etc. that are incompatible. While everyone likes
to put them in /etc/mail (with some linked by the same file name in /
etc), you would need to have an /etc/mail/sendmail, /etc/mail/qmail, /
etc/mail/postfix, and so on tree, a /var/mail (or /var/spool/mail)
sendmail or postfix or whatever tree and, and a whole bunch of
executables switched with alternatives, among them sendmail,
newaliases, and so on.
You would also have to switch startup files, and parameter files and
so on.
That's why an MTA package is set up as if it were the only one in the
house. It's just too complicated to maintain.
Something like Java or GCC is simple beacuse it was designed with
multiple versions being installed with one single front end module. So
you can have cc, gcc, gcc-3, and gcc-4 executables all pointing to
different executables which when they were compiled used a specific
object and library tree, while the non release specific stuff (such as
standard includes, kernerl includes, etc) are in the places you expect
them, and they are compatible across all the releases.
There are also some specific problems if you want both alternatives
active at the same time if the alternatives are packaged not to be
that way. For example, mpg123 and mpg321. Both install a binary with a
different name and the last alternative installed links /usr/bin/
mpg123 and /usr/bin/mpg321 to its binary. Fine for the average dumb
user, but once you find out they are not really compatible and need
both, you have to junk the alternative system and make sure that those
packages are never updated by the system.
I guess if you were willing to persue the old "it's a bug, not a
feature" debate with the developers, you could get mpg321 to be a 100%
replacement for mpg123, but then all the people who expect mpg321 to
act the way it does would be upset.
It comes down to the point that you have to decide if you want to let
them (whomever "they" are) maintain your system for you, or do it
yourself and how far away from reality as they see it you want to
stray. For example, when I found that sendmail would not do what I
needed without being manually compiled and installed, but the UBUNTU
postfix would, although it took a lot of effort, I switched to postfix.
It was stressfull, but unless the packagers truely go "off the deep
end" as I see it, I just have to install my config files, which use a
lot of different values for options than theirs. I can just use their
packages and if they get updated automaticly, the package manager is
smart enough not to replace my config files.
Geoff.
--
geoffrey mendelson N3OWJ/4X1GM
Jerusalem Israel geoffreymendel...@gmail.com
New word I coined 12/13/09, "Sub-Wikipedia" adj, describing knowledge
or understanding, as in he has a sub-wikipedia understanding of the
situation. i.e possessing less facts or information than can be found
in the Wikipedia.
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