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Variants

ajohns edited this page Apr 9, 2019 · 10 revisions

Overview

Packages in rez can contain different variants. Think of these as different flavors of the same package version. Each variant has one or more package dependencies that differ to the other variants in the same package.

Use of variants is best illustrated with an example. Consider a maya plugin, my_maya_plugin. Let us assume that there are two active versions of maya currently in use at your studio - 2016.sp2 and 2017. If your plugin is compiled, you may need to build it separately for each maya version, even though the source is no different.

You would use variants to create a version of your plugin that will work with both maya versions. Consider the following package definition for our plugin:

name = "my_maya_plugin"

version = "1.0.0"

requires = [
    "openexr-2.2"
]

variants = [
    ["maya-2016.sp2"],
    ["maya-2017"]
]

When you build and install this package, two separate builds will occur - one using maya-2016.sp2, and the other maya-2017. When an environment is resolved that includes my_maya_plugin, the correct variant will be selected depending on the version of maya present. Only one variant of a package is ever used in a given configured environment.

Each variant entry is a list of dependencies, no different to the packages listed in the requires field. These dependencies are appended to the requires list for each variant. Thus the first variant requires openexr-2.2 and maya-2016.sp1, and the second variant requires openexr-2.2 and maya-2017.

Disk Structure

Package variants are stored within the package, under subdirectories that match the variant requirements. For example, continuing on with our my_maya_plugin package, the installation of that package would look like so:

/rez/packages/my_maya_plugin/1.0.0/maya-2016.sp2/<PAYLOAD>
                                  /maya-2017/<PAYLOAD>

The anatomy of a package with variants is illustrated in the following diagram:

The root of a package is the root directory of its current variant (the one the current environment is configured to use); the base of a package is the directory containing its variants. In a package that does not have variants, base and root are the same.

Hashed Variants

There are two problems with the variant subpath as illustrated above:

  • The variant install path can become long if there are many requirements;
  • If some variant requirements contain characters such as ! and <, they can cause escaping problems that affect build systems; and, depending on the platform, may not be a valid filesystem path.

You can avoid these issues by using hashed variants. This sets the variant subpath to a hash of its requirements, rather than the requirements themselves. The resulting subdirectly is somewhat unwieldy (example: 83e0c415db1b602f9d59cee028da6ac785e9bacc). However, another feature - variant shortlinks - deals with this. A shortlink is a symlink to each variant, created in a separate subdirectory (default _v).

Here is an example hashed variant path:

/rez/packages/my_maya_plugin/1.0.0/83e0c415db1b602f9d59cee028da6ac785e9bacc

Here is the matching shortlink, which is what will be used in a resolved environment:

/rez/packages/my_maya_plugin/1.0.0/_v/a

Hashed variants must be enabled explicitly for a package. To do this, simply set this in your package definition:

hashed_variants = True

Platform As Variant

It is not uncommon to see the platform, architecture and/or operating system packages in package variants (recall that rez represents these as packages). For example, you might see variants like this:

# in package.py
variants = [
    ["platform-linux", "arch-x86_64", "os-Ubuntu-12.04"],
    ["platform-linux", "arch-x86_64", "os-Ubuntu-16.04"]
]

This indicates that the package has been built for multiple platforms. The correct variant will be selected for you, because you probably have the relevant implicit packages set to limit packages to the current platform.

Single Variants

You may often see packages with just one variant. There are two reasons for this:

  • Future proofing. Let's say you have a compiled package that links against python. It may currently support python-2.7, however it's conceivable that support for newer python versions may be added later. It is not possible to add new variants to a package that does not have any; so by adding the python-2.7 variant now, you can add variants later without needing to move to a newer version.
  • Installation path. People often expect to see platform, architecture and/or operating system information in the installation path of a piece of software (and may also expect the same of python version, or other core packages). By putting these dependencies into a variant, we ensure that they appear in the installation path of the package.

Variant Selection

As mentioned, rez will automatically select the correct variant of a package depending on the environment being resolved. For example, consider:

]$ rez-env my_maya_plugin maya-2017 -- echo '$REZ_MY_MAYA_PLUGIN_ROOT'
/rez/packages/my_maya_plugin/1.0.0/maya-2017
]$ rez-env my_maya_plugin maya-2016 -- echo '$REZ_MY_MAYA_PLUGIN_ROOT'
/rez/packages/my_maya_plugin/1.0.0/maya-2016.sp2

You can see how the correct variant (ie the one that does not conflict with other packages in the request) has been selected. But what if both variants are valid for the given request? Consider:

]$ rez-env my_maya_plugin -- echo '$REZ_MY_MAYA_PLUGIN_ROOT'
/rez/packages/my_maya_plugin/1.0.0/maya-2017

Here maya was not in the request. Either variant of my_maya_plugin would have satisfied the request, since we have not specified which version of maya we actually want.

By default, rez will prefer the variant with the higher-versioned packages, which is why the maya-2017 variant was selected in this example. If there are multiple packages in the variant, priority is given to those that were in the request list, if any; following that, priority is given to packages listed earlier in the variant. For example, consider:

name = "foo"

variants = [
    ["python-2.6", "maya-2017"],
    ["python-2.7", "maya-2016"]
]

If I run "rez-env foo", which variant will I get? The answer is not clear. In this case it will be the second variant, since python is given priority (it is the first listed package in the variant), and the second variant has the higher version of python. However, if I ran "rez-env foo maya", I would get the first variant - because priority is now given to maya, because it's listed in my request, and the first variant has the higher version of maya.

The rez setting variant_select_mode affects this selection behavior. The default mode just described is version_priority, but there is another mode - intersection_priority. In this mode, variants are preferred that have the most number of packages present in the request; version priority is secondary.

Mutual Exclusivity

In all the examples we've seen so far, a package's variants have been mutually exclusive. For example, you cannot have both python-2.6 and python-2.7 in the same environment, so when we request "foo python-2.6" we can be sure of which variant we will get.

Variants, however, do not need to be mutually exclusive. In fact, you may use variants in order to provide support for different DCCs for your package. Consider a package with the following variants:

name = "geocache"

variants = [
    ["maya-2016"],
    ["houdini-14"]
]

Which variant will I get if I run "rez-env geocache"? Behavior in this case is undefined - rez gives no guarantees as to which variant will be selected. We cannot meaningfully compare version numbers across packages, so maya will not have preference simply because 2016 > 14. However, version_priority mode does give priority to packages listed in the request. So if we ran "rez-env geocache maya", we will get the first variant... probably.

Probably?

The operative word here is preference. Because the variants are not mutually exclusive, we can't make guarantees. A resolve is still deterministic - you aren't going to get differing results when requesting the same environment - but predicting which variant you'll get can be tricky.

Consider the following request:

]$ rez-env geocache maya animtools-1.4

We would expect to get the maya-2016 variant of geocache. However, what if animtools requires maya-2017? This makes the first geocache variant impossible to select, since a conflict would occur, and so the houdini variant of geocache will be selected - quite possibly not what you expected.

I plan on adding a new package request syntax, that is able to explicitly select package variants. This will avoid the ambiguity in cases like the one described here.

Why Use Variants?

Variants are a powerful mechanism in rez. They avoid the need to maintain separate branches of a package in order to support varying dependencies. You may have had problems in the past where a common library depends on, say, boost, and is used in various DCCs (maya, nuke etc), and depended on by many other packages in your pipeline. When a DCC moves to a new version of boost (or python, or OIIO, etc) you now have to branch this library, which potentially affects many other packages. The problem gets worse if you have multiple dependencies with varying versions. Variants solve that problem - you simply add another boost variant to your library, and other dependent packages are not affected. Rez will correctly select the package variant that does not conflict with the resolved environment.

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