Anaconda Enterprise: Documentation

# Managing conda channels

It is possible for different channels to have the same package, so conda must handle these channel collisions. If you only use the defaults channel, or only use channels that only contain packages that do not exist in any of the other channels in your list, there will be no channel collisions. The way conda resolves these collisions only matters when you have multiple channels in your channel list that host the same package.

## Before conda 4.1.0

Before conda 4.1.0 was released on June 14th, 2016, when two channels hosted packages with the same name, conda would install the package with the highest version number. If there were two packages with the same version number, conda would install the one with the highest build number. Only if both the version numbers and build numbers were identical did your channel ordering make a difference. This had three problems.

1. Build numbers from different channels are not comparable. Channel A could do nightly builds while Channel B does weekly builds, so build 2 from Channel B could be newer than build 4 from Channel A.
2. Users could not specify a preferred channel. You might consider Channel B more reliable than Channel A and prefer to get packages from that channel even if the version is older than the package in Channel A. Before version 4.1.0 conda provided no way to choose that behavior. Only version and build numbers mattered.
3. Build number conflicts. This is an effect of the other two problems. If you were happily using package Alpha from Channel A and package Bravo from Channel B, and the provider from Channel B then added a version of Alpha to Channel B with a very high build number, your conda updates would start installing new versions of Alpha from Channel B whether you wanted that or not. This could cause both unintentional problems and a risk of deliberate attacks.

## After conda 4.1.0

By default conda now prefers packages from a higher priority channel over any version from a lower priority channel. Therefore you can now safely put channels at the bottom of your channel list to provide additional packages that are not in the default channels, and still be confident that these channels will not override the core package set.

By default conda collects all of the packages with the same name across all listed channels and sorts them from highest to lowest channel priority, then sorts packages that are tied from highest to lowest version number, then sorts packages that are still tied from highest to lowest build number, and then installs the first package on the sorted list that satisfies the installation specifications.

To make conda use the old method and install the newest version of a package in any listed channel, add `channel_priority: false` to your .condarc file. (You may instead run the equivalent command `conda config --set channel_priority false`.) Then conda will sort the package list from highest to lowest version number, then sort tied packages from highest to lowest channel priority, and then sort tied packages from highest to lowest build number. Because build numbers from different channels are not comparable, build number still comes after channel priority.

The command `conda config --add channels new_channel` adds the new channel to the top of the channel list, making it the highest priority. Conda now has an equivalent command `conda config --prepend channels new_channel`. Conda also now has a new command `conda config --append channels new_channel` that puts the new channel at the bottom of the channel list, making it the lowest priority.