[skip changelog] Add Arduino CLI integration options documentation (#733)

Arduino CLI provides three interfaces for integration:
- command line
- gRPC
- Go library

This is an article written by Massimiliano Pippi, which I have merely converted to Markdown and added some links to.

Author: per1234

Diff for: docs/img/CLI_Go_library_interface_screenshot.png

@@ -0,0 +1,138 @@
+# The three pillars of the Arduino CLI
+
+The Arduino CLI is an open source Command Line Application written in [Golang]
+that can be used from a terminal to compile, verify and upload sketches to
+Arduino boards and that’s capable of managing all the software and tools needed
+in the process. But don’t get fooled by its name: Arduino CLI can do much more
+than the average console application, as shown by the [Arduino Pro IDE] and
+[Arduino Create], which rely on it for similar purposes but each one in a
+completely different way from the other. In this article we introduce the three
+pillars of the Arduino CLI, explaining how we designed the software so that it
+can be effectively leveraged under different scenarios.
+
+## The first pillar: command line interface
+
+### Console applications for humans
+
+As you might expect, the first way to use the Arduino CLI is from a terminal and
+by a human, and user experience plays a key role here. The UX is under a
+continuous improvement process as we want the tool to be powerful without being
+too complicated. We heavily rely on sub-commands to provide a rich set of
+different operations logically grouped together, so that users can easily
+explore the interface while getting very specific contextual help.
+
+![contextual help screenshot][]
+
+### Console applications for robots
+
+Humans are not the only type of customers we want to support and the Arduino CLI
+was also designed to be used programmatically - think about automation pipelines
+or a [CI][continuous integration]/[CD][continuous deployment] system.
+There are some niceties to observe when you write software that’s supposed to be
+easy to run when unattended and one in particular is the ability to run without
+a configuration file. This is possible because every configuration option you
+find in the arduino-cli.yaml configuration file can be provided either through a
+command line flag or by setting an environment variable. To give an example, the
+following commands are all equivalent and will proceed fetching the unstable
+package index that can be used to work with experimental versions of cores:
+
+![configuration methods screenshot][]
+
+One note about the example above: passing a value through a command line flag
+always takes precedence over reading an environment variable, which in turn
+always takes precedence over reading the same value from the configuration file
+(if you have one).
+Consistent with the previous paragraph, when it comes to providing output the
+Arduino CLI aims to be user friendly but also slightly verbose, something that
+doesn’t play well with robots. This is why we added an option to provide output
+that’s easy to parse, for example the following figure shows what getting the
+software version in [JSON] format looks like.
+
+![JSON output screenshot][]
+
+Even if not related to software design, one last feature that’s worth mentioning
+is the availability of a one-line [installation script] that can be used to make
+the latest version of the Arduino CLI available on most systems with an HTTP
+client like curl or wget and a shell like bash.
+
+For more information on Arduino CLI's command line interface, see the
+[command reference].
+
+## The second pillar: gRPC interface
+
+[gRPC] is a high performance [RPC] framework that can efficiently connect client
+and server applications. The Arduino CLI can act as a gRPC server (we call it
+[daemon mode]), exposing a set of procedures that implement the very same set of
+features of the command line interface and waiting for clients to connect and
+use them. To give an idea, the following is some [Golang] code capable of
+retrieving the version number of a remote running Arduino CLI server instance:
+
+![gRPC interface screenshot][]
+
+gRPC is language agnostic: even if the example is written in Golang, the
+programming language used for the client can be Python, JavaScript or any of the
+many [supported ones][gRPC supported languages], leading to a variety of
+possible scenarios. The new [Arduino Pro IDE] is a good example of how to
+leverage the daemon mode of the Arduino CLI with a clean separation of concerns:
+the Pro IDE knows nothing about how to download a core, compile a sketch or talk
+to an Arduino board and it demands all these features of an Arduino CLI instance.
+Conversely, the Arduino CLI doesn’t even know that the client that’s connected
+is the Pro IDE, and neither does it care.
+
+For more information on Arduino CLI's gRPC interface, see the
+[gRPC interface reference].
+
+## The third pillar: embedding
+
+Arduino CLI is written in [Golang] and the code is organized in a way that makes
+it easy to use it as a library by including the modules you need in another
+Golang application at compile time. Both the first and second pillars rely on a
+common Golang API, a set of functions that abstract all the functionalities
+offered by the Arduino CLI, so that when we provide a fix or a new feature, they
+are automatically available to both the command line and gRPC interfaces.
+The source modules implementing this API can be imported in other Golang
+programs to embed a full fledged Arduino CLI. For example, this is how some
+backend services powering [Arduino Create] can compile sketches and manage
+libraries. Just to give you a taste of what it means to embed the Arduino CLI,
+here is how to search for a core using the API:
+
+![Go library interface screenshot][]
+
+Embedding the Arduino CLI is limited to Golang applications and requires a deep
+knowledge of its internals; for the average use case the gRPC interface might be
+a better alternative, nevertheless this remains a valid option that we use and
+provide support for.
+
+## Conclusions
+
+You can start playing with the Arduino CLI right away. The code is open source
+and [the repo][Arduino CLI repository] contains
+[example code showing how to implement a gRPC client][gRPC client example]. If
+you’re curious about how we designed the low level API, have a look at the
+[commands package] and don’t hesitate to leave feedback on the [issue tracker]
+if you’ve got a use case that doesn’t fit one of the three pillars.
+
+
+[Golang]: https://golang.org/
+[Arduino Pro IDE]: https://www.arduino.cc/pro/arduino-pro-ide
+[Arduino Create]: https://create.arduino.cc
+[continuous integration]: https://en.wikipedia.org/wiki/Continuous_integration
+[continuous deployment]: https://en.wikipedia.org/wiki/Continuous_deployment
+[JSON]: https://www.json.org
+[installation script]: installation.md#use-the-install-script
+[command reference]: ../commands/arduino-cli
+[gRPC]: https://grpc.io/
+[RPC]: https://en.wikipedia.org/wiki/Remote_procedure_call
+[daemon mode]: ../commands/arduino-cli_daemon
+[gRPC interface reference]: ../rpc/commands
+[gRPC supported languages]: https://grpc.io/docs/languages/
+[Arduino CLI repository]: https://github.com/arduino/arduino-cli
+[gRPC client example]: https://github.com/arduino/arduino-cli/blob/master/client_example
+[commands package]: https://github.com/arduino/arduino-cli/tree/master/commands
+[issue tracker]: https://github.com/arduino/arduino-cli/issues
+
+[contextual help screenshot]: img/CLI_contextual_help_screenshot.png
+[configuration methods screenshot]: img/CLI_configuration_methods_screenshot.png
+[JSON output screenshot]: img/CLI_JSON_output_screenshot.png
+[gRPC interface screenshot]: img/CLI_gRPC_interface_screenshot.png
+[Go library interface screenshot]: img/CLI_Go_library_interface_screenshot.png

Diff for: docs/img/CLI_JSON_output_screenshot.png

@@ -99,6 +99,7 @@ nav:
     - monitor: rpc/monitor.md
     - settings: rpc/settings.md
     - debug: rpc/debug.md
+  - Integration options: integration-options.md
   - sketch-build-process.md
   - sketch-specification.md
   - library-specification.md