8.3: Metacello Features

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Metacello is consistent with the important features of Monticello and is based on the following ideas.

Declarative project descriptions. A Metacello project has named versions consisting of lists of Monticello package versions. Dependencies are explicitly expressed in terms of named versions of required projects. A required project is a reference to another Metacello project. Collectively, all of these descriptions are called the project metadata.

Project metadata are versioned. Metacello project metadata is represented as instance methods in a class. Managing such metadata as code brings a lot of power in comparison to XML, which is what most package management systems use. Metacello project metadata can themselves be stored as a Monticello package, and are thus subject to version control. As a result, concurrent updates to the project metadata can be managed easily: parallel versions of the metadata can be merged just like parallel versions of the code base itself.

Metacello has the following features:

Cross-platform: Metacello runs on all platforms that support Monticello, which currently means Pharo, Squeak and GLASS.

Conditional package loading: to enable projects to run on multiple platforms, Metacello supports conditional loading of platform-specific Monticello packages.

Configurations: Metacello manages configurations of projects. Large projects frequently have multiple variations with different sets of packages and projects required for each platform. Each unique configuration is labeled with a version string.

Metacello supports the definition of two kinds of entities (represented as methods): baselines and versions.

Baselines. A baseline defines the basic structure of a project. The baseline lists the packages and required projects that compose the project. The baseline defines the order in which packages are to be loaded and the repositories from which the packages are to be loaded.

Versions. A version identifies the exact version of each package and project that should be loaded. A version is based upon a baseline version. For each package in the baseline version, the Monticello file name (e.g., Metacello-Base-dkh.152) is specified. For each project in the baseline version, the Metacello version number is specified.

Example configurations diagram. — Figure \(\PageIndex{1}\): Configurations: groups of baselines and versions with dependencies.

ConfigurationOfProjectA, in Figure \(\PageIndex{1}\) contains two baselines (baseline 0.4 and 0.5) and four versions (version 0.4, 0.4.1, 0.5, and 0.6). Baseline 0.4 is composed of two packages (PackageA and PackageB). Version 0.4 is based on baseline 0.4 and specifies the version for each of the packages (PackageA-version.5 and PackageB-version.3). Version 0.4.1 is also based on baseline 0.4, but specifies a different version for PackageA (Package-version.7).

Baseline 0.5 is composed of 3 packages (PackageA, PackageB, and PackageC) and it depends on an external project (ProjectB). A new package (PackageC) and a project dependency (ProjectB) was added to the project so a new baseline version reflecting the new structure needed to be created. Version 0.5 is based on baseline 0.5 and specifies the versions of the packages (PackageA-version.6, PackageB-version.4 and PackageC-version.1) and version of the dependent project (ProjectB-version3).