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Installing Dependencies
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=======================
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One of the key features for ease of use when utilizing in ensemble is
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the ability to have it fetch and deploy formulas from remote
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repositories with dependency resolution. Ie. invoking::
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ensemble deploy wordpress
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Will lookup a formula named wordpress in the configured remote formula
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repositories, and should pick up any required interfaces of wordpress
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and satisfy those dependencies within the environment if they don't
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This last part is key, as it embodies ensemble moniker as a network
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enabled apt, where dependency resolution of installed packages is
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satisifed by services available within the local network/environment.
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Ensemble allows for multiple formula repositories to be utilized when
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searching for formulas, unlike apt where multiple repositories are
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overlayed into a single global namespace, ensemble provides for
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each repository as an independent namespace. ie. Once a package is
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installed from a repository, it will not be upgraded automatically
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by a newer version in a different repository. Updates will only
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be detected against its origin repository, unless explicitly modified
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Dependencies are not looked up directly by formula name, but by
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interface names. In the example above wordpress requires something
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providing the mysql interface. Ensemble will first attempt to satisfy
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this dependency by examining the services already deployed within the
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environment to determine if any of them provide the required
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If the interface dependency cannot be satisified from the local
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environment, the repository of the formula which specified the
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dependency will be utilized. So in the case of deploying a wordpress
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formula, the repository that contains the wordpress formula would be
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searched next for a formula that provides the required dependency.
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If the required dependencies are not found in this repository,
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ensemble fallbacks to its repository lookup algorithm, searching each
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repository in turn for the dependency. The repository lookup algorithm
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can be configured by the user in their environment configuration. It
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defaults to searching any command line specified formula directories
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or repositories, and then the central ubuntu formula repositories.
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docs/source/specifications/auto-dependency.rst
