<a id="dependency-package"></a>

# Dependency package

The **`dependency`** package is a subpackage in [the Go worker package](https://documentation.ubuntu.com/juju/3.6/contributor/reference/worker-package.md#worker-package) that provides constructs
that help
manage shared resources and their lifetimes. In Juju, it provides constructs that enable [agents](https://documentation.ubuntu.com/juju/3.6/reference/agent.md#agent) to run
acyclic graphs of workers (i.e., workers and their dependencies, i.e., other workers).

> See more: [Go packages | `worker` > `dependency`](https://pkg.go.dev/github.com/juju/worker/v3@v3.3.0/dependency)

## List of most important `dependency` package constructs

This section lists all the most relevant `dependency` package constructs, following a top-down order (going from agent
to worker).

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## `NewEngine`

`dependency.NewEngine` is a function that is used in the definition of an agent to create and initialise an engine that
will maintain the dependency graph defined by any installed instances of the [Manifolds](#manifolds) type.

> See more: 
> [`NewEngine`](https://github.com/juju/worker/blob/HEAD/dependency/engine.go), [example usage of
> `NewEngine` in the model agent](https://github.com/juju/juju/blob/HEAD/cmd/jujud/agent/model.go)

## `Install`

`dependency.Install` is a function that is used in the definition of an agent to install instances of the [Manifolds](#manifolds) type.

> See more: 
> [`Install`](https://github.com/juju/worker/blob/HEAD/dependency/util.go),
> [example usage of
> `Install` in the model agent](https://github.com/juju/juju/blob/HEAD/cmd/jujud/agent/model.go)

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## `Manifolds`

`dependency.Manifolds` is a type that is used in the definition of an agent to define a collection of things of the  [Manifold](#manifold) type.

> See more: 
> [`Manifolds`](https://github.com/juju/worker/blob/HEAD/dependency/interface.go),
> [example usage of
> `Manifolds` in the model agent](https://github.com/juju/juju/blob/HEAD/cmd/jujud/agent/model.go)

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## `Manifold`

`dependency.Manifold` is a type that is used in the definition of a worker to declare its inputs, outputs, and start
function.

> See more: 
> [`Manifold`](https://github.com/juju/worker/blob/HEAD/dependency/interface.go),
> [example usage of
> `Manifold` in the firewaller worker](https://github.com/juju/juju/blob/HEAD/internal/worker/firewaller/manifold.go)