Revise Europa Docs - Core Concepts - It all starts with a plan

Thank you Tanguy for the "fresh pair of eyes" perspective.

Thank you Tom for the link suggestions.

Supersedes https://github.com/dagger/dagger/pull/1847

Signed-off-by: Gerhard Lazu <gerhard@lazu.co.uk>

docs/core-concepts/1202-plan.md

@@ -5,15 +5,19 @@ displayed_sidebar: europa
 
 # It all starts with a plan
 
-A CI/CD pipeline declared in Dagger starts with a plan, specifically `dagger.#Plan`
+## Plan structure
 
-This plan is the entrypoint for everything that runs within a pipeline. The simplest plan will usually:
+A config declared in Dagger starts with a plan, specifically `dagger.#Plan`
 
-- interact with the client filesystem to read (e.g. source code) or write files (e.g. build output)
+Within this plan we can:
-- read environment variables
-- declare a few actions, e.g. deps, test & build
 
-This is our **Getting Started** example app plan structure:
+- interact with the `client` filesystem
+  - read files, usually the current directory as `.`
+  - write files, usually the build output as `_build`
+- read `env` variables, such as `NETLIFY_TEAM` in our example
+- declare a few `actions`, e.g. `deps`, `test` & `build`
+
+This is our **Getting Started** todoapp plan structure:
 
 ```cue file=../tests/core-concepts/plan/structure.cue.fragment
 ```
@@ -28,46 +32,80 @@ When the above plan gets executed via `dagger do build`, it produces the followi
 [✔] actions.build.run.script                                 0.0s
 [✔] actions.build.run                                        0.0s
 [✔] actions.build.contents                                   0.0s
-[✔] client.filesystem.build.write                                 0.1s
+[✔] client.filesystem."./_build".write                       0.1s
 ```
 
-Since these actions have run before, they are cached and take less than 1 second to complete.
+Since these actions have run before, they are cached and take less than 2 seconds to complete.
 
-While the names used for the actions above - `deps`, `test` & `build` - are short & descriptive,
+While the names used for the actions above - `deps`, `test` & `build` - are short and descriptive,
 any other names would have worked. Put differently, action naming does not affect plan execution.
 
-In the example above, the `deps` action is an instance of the docker package build definition.
+Lastly, notice that even if the `deploy` action is defined, we did not run it.
+Similar to Makefile targets, we have the option of running specific actions.
 
-This is written as `deps: docker.#Build`
+We ran the `dagger do build` command, which only runs the `build` action (and all its dependent actions).
+This Dagger property enables us to keep the entire CI/CD config in a single file, while keeping the integration execution separate from the deployment one.
+Separating CI & CD concerns becomes essential as our pipelines grow in complexity, and we learn about operational and security constraints specific to our systems.
 
-Default definition configuration - `docker.#Build` in this case - can be modified via curly brackets, e.g.
+## Packages & imports
+
+In order to understand the correlation between actions, definitions and packages, let us focus on the following fragment from our **Getting Started** todoapp config:
 
 ```cue
-    deps: docker.#Build & {
+package todoapp
+
+import (
+  "dagger.io/dagger"
+  "universe.dagger.io/netlify"
+)
+
+dagger.#Plan & {
+  // ...
+  actions: {
+    // ...
+    deploy: netlify.#Deploy & {
       // ...
     }
+  // ...
+  }
+}
+```
+
+We start by declaring the package name, `package todoapp` above.
+
+Next, we import the packages that we use in our plan.
+
+The first import is needed for the `dagger.#Plan` definition to be available.
+
+The second import is for `netlify.#Deploy` to work.
+
+:::info
+Which other imports we are missing?
+Look at all the actions in the plan structure at the top of this page.
+Now check all the available packages in [universe.dagger.io](https://github.com/dagger/dagger/tree/v0.2.0/pkg/universe.dagger.io).
+:::
+
+We now understand that the `deploy` action is the deploy definition from the netlify package, written as `deploy: netlify.#Deploy`
+
+Each definition has default values that can be modified via curly brackets. This is what that looks like in practice for our deploy action:
+
+```cue
+// ...
+deploy: netlify.#Deploy & {
+  contents: build.contents.output
+  site:     client.env.APP_NAME
+  token:    client.env.NETLIFY_TOKEN
+  team:     client.env.NETLIFY_TEAM
+}
+// ...
 ```
 
 We can build complex pipelines efficiently by referencing any definition, from any package in our actions.
-This is one of the fundamental concepts that makes Dagger a powerful language for CI/CD.
+This is one of the fundamental concepts that makes Dagger a powerful language for building CI/CD pipelines.
-Before we can use a package in a plan, we need to declare it at the top of the pipeline configuration, like this:
 
-```cue
+If you want to learn more packages in the context of CUE, the config language used by Dagger configs, check out the [Packages](1215-what-is-cue.md#packages) section on the **What is CUE?** page.
-import (
-  "universe.dagger.io/docker"
-)
-```
-
-Since we are using the plan definition from the dagger package - `dagger.#Plan` - we also need to declare it at the top of the pipeline configuration:
-
-```cue
-import (
-  "dagger.io/dagger"
-  "universe.dagger.io/docker"
-)
-```
 
 :::tip
 Now that we understand the basics of a Dagger plan, we are ready to learn more about how to interact with the client environment.
-This will enable us to configure plans just-in-time, save build artefacts, and perform other interactions with the environment within which Dagger runs.
+We can read the env (including secrets), run commands, use local sockets, etc.
 :::

docs/tests/core-concepts/plan/structure.cue.fragment

@@ -1,3 +1,8 @@
+// ...
+// A plan has pre-requisited that we cover below.
+// For now we focus on the dagger.#Plan structure.
+// ...
+
 dagger.#Plan & {
   client: {
     filesystem: {
@@ -22,5 +27,8 @@ dagger.#Plan & {
         // ...
       }
     }
+    deploy: netlify.#Deploy & {
+      // ...
+    }
   }
 }

pkg/universe.dagger.io/examples/todoapp/todoapp.cue

@@ -1,4 +1,4 @@
-package netlify
+package todoapp
 
 import (
 	"dagger.io/dagger"
@@ -23,12 +23,12 @@ dagger.#Plan & {
 				contents: dagger.#FS
 				exclude: [
 					"README.md",
-					"build",
+					"_build",
-					"netlify.cue",
+					"todoapp.cue",
 					"node_modules",
 				]
 			}
-			build: write: contents: actions.build.contents.output
+			"./_build": write: contents: actions.build.contents.output
 		}
 		env: {
 			APP_NAME:      string
@@ -50,8 +50,6 @@ dagger.#Plan & {
 					contents: client.filesystem.".".read.contents
 					dest:     "/src"
 				},
-				// bash.#Run is a superset of docker.#Run
-				// install yarn dependencies
 				bash.#Run & {
 					workdir: "/src"
 					mounts: {