In some particular scenarios, it is necessary to add several repositories on a Debian system. In this example I'm talking about Proxmox, a virtualization solution based on Debian Wheezy, but this article applies to all additional apt repositories (and applies to Ubuntu and other Debian derivatives as well).
By default Proxmox installs its Enterprise repository in /etc/apt/sources.list.d/pve-enterprise.list:
$ cat /etc/apt/sources.list.d/pve-enterprise.list
deb https://enterprise.proxmox.com/debian wheezy pve-enterprise
However this repo is only accessible with a valid subscription. As an alternative, the community repository can be added and used:
$ cat /etc/apt/sources.list.d/pve--no-subscription.list
deb http://download.proxmox.com/debian wheezy pve-no-subscription
Some packages in the Proxmox repository are far newer than on the official Debian repository.
As an example, the glusterfs packages have a big version difference:
This causes apt-get install to fail because it doesn't know which version to install.
In such a situation it makes sense to set a preference on a repository over the others.
Such a preference can be set by setting different higher pin-priorities on the repositories in a preferences config file:
$ cat /etc/apt/preferences.d/proxmox
Pin: origin enterprise.proxmox.com
Pin: origin download.proxmox.com
Pin: origin security.debian.org
Pin: origin ftp.ch.debian.org
The content of the file is pretty much self-explaining. Each repository URL has its own Pin-Priority.
The higher the number, the higher is the priority.
It would also be possible to set the higher priority only for certain packages. In my example I have set the priorities for all packages (*).
Updated September 24, 2020
Using multiple repositories can create conflicts, too. A typical example is that the same package exists in both official Debian and another additional repository but the package dependencies of the additional repository don't match. Seen with kafkacat and a hiccup of the dependent package librdkafka1 and Confluent repositories:
# apt-get install librdkafka1
Reading package lists... Done
Building dependency tree
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:
The following packages have unmet dependencies:
librdkafka1 : Depends: libssl1.0.0 (>= 1.0.0) but it is not installable
E: Unable to correct problems, you have held broken packages.
Apt tried to install the librdkafka1 package from the Confluent repos - but the package was built against libssl1.0.0. Yet in the Debian repos, libssl1.1 is used.
In such a case it makes sense to prevent a certain package (librdkafka1 in this case) from a specific repository:
# cat /etc/apt/preferences.d/kafkacat
# do not install librdkafka1 (kafkacat needs it) from confluent repository
Pin: origin packages.confluent.io
The negative pin-priority (-1 here) prevents the listed packages (librdkafka1 here) to be installed from the origin (packages.confluent.io here).
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