dagger/docs/core-concepts/1221-action.md

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Dagger Actions

Actions are the basic building block of the Dagger platform. An action encapsulates an arbitrarily complex automation into a simple software component that can be safely shared, and repeatably executed by any Dagger engine.

Actions can be executed directly with dagger do, or integrated as a component of a more complex action.

There are two types of actions: core actions and composite actions.

Core Actions

Core Actions are primitives implemented by the Dagger Engine itself. They can be combined into higher-level composite actions. Their definitions can be imported in the dagger.io/dagger/core package.

To learn more about core actions, see the core action reference.

Composite Actions

Composite Actions are actions made of other actions. Dagger supports arbitrary nesting of actions, so a composite action can be assembled from any combination of core and composite actions.

One consequence of arbitrary nesting is that Dagger doesn't need to distinguish between "pipelines" and "steps": everything is an action. Some actions are just more complex and powerful than others. This is a defining feature of Dagger.

Lifecycle of an Action

A composite action's lifecycle has 4 stages:

1. Definition
2. Integration
3. Discovery
4. Execution

Definition

A new action is defined in a declarative template called a CUE definition. This definition describes the action's inputs, outputs, sub-actions, and the wiring between them.

Here is an example of a simple action definition:

package hello

import (
    "dagger.io/dagger"
    "dagger.io/dagger/core"
)

// Write a greeting to a file, and add it to a directory
#AddHello: {
    // The input directory
    dir: dagger.#FS

    // The name of the person to greet
    name: string | *"world"

    write: core.#WriteFile & {
        input: dir
        path: "hello-\(name).txt"
        contents: "hello, \(name)!"
    }

    // The directory with greeting message added
    result: write.output
}

Note that this action includes one sub-action: core.#WriteFile. An action can incorporate any number of sub-actions.

Also note the free-form structure: an action definition is not structured by a rigid schema. It is simply a CUE struct with fields of various types.

• "inputs" are simply fields which are not complete, and therefore can receive an external value at integration. For example, dir and name are inputs.
• "outputs" are simply fields which produce a value that can be referenced externally at integration. For example, result is an output.
• "sub-actions" are simply fields which contain another action definition. For example, write is a sub-action.

There are no constraints to an action's field names or types.

Integration

Action definitions cannot be executed directly: they must be integrated into a plan.

A plan is an execution context for actions. It specifies:

• What actions to present to the end user
• Dependencies between those tasks, if any
• Interactions between the tasks and the client system, if any

Actions are integrated into a plan by merging their CUE definition into the plan's CUE definition.

Here is an example of a plan:

package main

import (
    "dagger.io/dagger"
)

dagger.#Plan & {
    // Say hello by writing to a file
    actions: hello: #AddHello & {
        dir: client.filesystem.".".read.contents
    }
    client: filesystem: ".": {
        read: contents: dagger.#FS
        write: contents: actions.hello.result
    }
}

Note that #AddHello was integrated directly into the plan, whereas core.#WriteFile was integrated indirectly, by virtue of being a sub-action of #AddHello.

To learn more about the structure of a plan, see it all begins with a plan.

Discovery

Once integrated into a plan, actions can be discovered by end users, by using the familiar convention of usage messages:

$ dagger do --help
Execute a dagger action.

Available Actions:
 hello   Say hello by writing to a file

Usage:
  dagger do [OPTIONS] ACTION [SUBACTION...] [flags]

Flags:
  [...]

Execution

Once the end user has discovered the action that they need, they can execute it with dagger do. For example:

dagger do hello