Implementation details#

This document provides an overview on how the conda-libmamba-solver integrations are implemented, both within the conda_libmamba_solver package itself, and as a conda plugin.

Repository structure#

  • .devcontainer: Configuration for DevContainer development workflows.

  • .github/workflows/: CI pipelines to run unit and upstream tests, as well as linting and performance benchmarks. Some extra workflows might be added by the conda/infra settings.

  • conda_libmamba_solver/: The Python package. Check sections below for details.

  • recipe/: The conda-build recipe used for the PR build previews. It should be kept in sync with conda-forge and defaults.

  • dev/: Supporting configuration files to set up development environments.

  • docs/: Documentation sources.

  • tests/: pytest testing infrastructure.

  • pyproject.toml: Project metadata. See below for details.

Project metadata#

The pyproject.toml file stores the required packaging metadata, as well as some the configuration for some tools (black, pytest, etc.).

Some peculiarities:

  • hatchling is the chosen backend for the packaging.

  • The version is calculated from the git info with hatchling-vcs.

  • black uses a line length of 99 characters.

conda_libmamba_solver package#

The package is a flat namespace:

  • conda_libmamba_solver.__init__: Defines __version__ and the old-style plugin API.

  • conda_libmamba_solver.exceptions: Subclasses of conda.exceptions.

  • conda_libmamba_solver.index: Helper objects to deal with repodata fetching and loading, interfacing with libmamba helpers.

  • conda_libmamba_solver.mamba_utils: Utility functions to help set up the libmamba objects.

  • conda_libmamba_solver.models: Application-agnostic objects to assist in the metadata collection phases.

  • conda_libmamba_solver.plugin: The pluggy registration mechanism in conda.plugins.

  • conda_libmamba_solver.solver: The conda.core.solve.Solver subclass with all the libmamba-specific logic.

  • conda_libmamba_solver.state: Solver-agnostic objects to assist in the solver state specification and collection.

  • conda_libmamba_solver.utils: Other application-agnostic utility functions.


Refer to each module docstrings for further details!

Solver-agnostic parts#

The idea behind the module separation is to have better logic reusability and separation between the libmamba library and the preparation logic conda uses. The following paragraphs assume you have read the Deep Dive guides in the conda documentation, but as a refresher:

  • The Solver class will take a list of MatchSpec objects coming from diverse sources, like:

    • The packages the user requested in the command line or an environment file

    • The installed packages in the target environment

    • The explicitly requested packages in previous commands (the history)

    • … and some more coming from configured settings

  • All these MatchSpecs are then merged and sorted depending on certain preparation logic.

  • The final set of MatchSpec objects is used to query the so-called index of packages.

  • The index is a long list of PackageRecord objects, loaded from the repodata.json files obtained from the configured channels.

  • The job of the solver is to find the set of PackageRecord objects that better satisfies the optimization criteria for the given MatchSpec objects.

All the preparation steps before the actual SAT solver starts working are conda-specific, and solver-agnostic! In conda classic, this logic is spread across the different layers, but in conda-libmamba-solver we tried to synthesize it in a single module. This is the conda_libmamba_solver.state module, which contains the SolverInputState and SolverOutputState classes.

  • SolverInputState deals with the collection and management of the MatchSpec objects.

  • SolverOutputState will assist the Solver class maintain its state through the different solving attempts, and will finally export a list of PackageRecords. The early exit and post-solve logics are also expressed here.

Both SolverInputState and SolverOutputState classes are supported by the TrackedMap dictionary subclass, which logs its own changes for better debugging and developer experience while analyzing solver problems.

libmamba-specific parts#

conda_libmamba_solver interfaces with libmamba objects through three modules only:

  • .solver, which contains the conda.core.solve.Solver subclass. It relies heavily on conda_libmamba_solver.state in an effort to only contain the logic necessary to interface with libmamba.

  • .index, which deals with the repodata fetching and loading. Initially, it invoked the necessary libmamba objects to download and load the repodata JSON files. In later releases, downloading is done with conda objects, and we then pass the JSON files to the libmamba loaders.

  • .mamba_utils, which contains utility functions borrowed and adapted from mamba itself. Its main usage is the initialization of the libmamba.Context options from conda’s Context.

Integrations with conda#

With the plugin system#

Once co-installed with conda, conda_libmamba_solver registers itself via the conda.plugins.hookimpl-decorated function in conda.plugin, which yields a CondaSolver plugin instance.

After that, conda clients just need to get the configured solver via context.plugin_manager.get_cached_solver_backend().

Draft integrations (pre-plugin phase)#


This is just here as a historical trivia item. Please check the Plugin implementation section for current details!

The first experimental releases of conda_libmamba_solver used an ad-hoc mechanism based on try/except hooks.

On the conda/conda side, we had conda.core.solve._get_solver_class():

def _get_solver_class(key=None):
    key = key or conda.base.context.Context.experimental_solver
    if key == "classic":
        return conda.core.solve.Solver  # Classic
    if key.startswith("libmamba"):
            from conda_libmamba_solver import get_solver_class

            return get_solver_class(key)
        except ImportError as exc:
            raise CondaImportError(...)
    raise ValueError(...)

The key values were hard-coded in conda.base.constants. Not very extensible! This was only meant to be temporary as we iterated on the conda-libmamba-solver side. We had one more get_solver_class() function in conda_libmamba_solver so we could easily change the Solver object import path without changing conda itself.

The default value for the key was set by the Context object, which was populated by either:

  • The environment variable, CONDA_EXPERIMENTAL_SOLVER.

  • The command-line flag, --experimental-solver.

  • A configuration file (e.g. ~/.condarc).