# Chapter 4 Introduction to {golem}

The {golem} (Guyader et al. 2020) package is a framework for building production-grade Shiny Application. Lot of the patterns and methodologies described in this book are linked to {golem} and packages from the golemverse. Of course, all the advice developed in this book will still be valid even if you do not plan to use {golem}.

We have quickly introduced {golem} in the last chapter, and we will come back to this package many times in the following chapters. So, let’s start with an introduction to this package. Note that the version used at the time of writing this book is 0.2.1.9010.

## 4.1 What is {golem}?

{golem} is a toolkit for simplifying the creation, development and deployment of a Shiny application. It’s focused on building applications that will be sent to production, but of course starting with {golem} from the very beginning is also possible, even recommended: it is easier to start with {golem} than to refactor your codebase to fit into the framework.

The stable release can be found on CRAN and installed with:

install.packages("golem")

{golem} development version can be found on GitHub and installed with:

remotes::install_github("Thinkr-open/golem")

The version of the package on CRAN at the time of writing this book is:

library(dplyr, warn.conflicts = FALSE)
tools::CRAN_package_db() %>%
filter(Package == "golem") %>%
select(Version)
  Version
1   0.2.1

While the current development version is:

x <- tempfile()
url <- paste0(
"https://raw.githubusercontent.com/",
"ThinkR-open/golem/dev/DESCRIPTION"
)
desc::desc_get_version(x)
[1] '0.2.1.9021'

The motivation behind {golem} is that building a Proof of Concept application is easy, but things change when the application becomes larger and more complex, and especially when you need to send that app to production. Until recently there has not been any real framework for building and deploying production-grade Shiny Apps. This is where {golem} comes into play: offering Shiny developers a toolkit for making a stable, easy-to-maintain, and robust production web application with R. {golem} has been developed to abstract away the most common engineering tasks (for example, module creation, addition and linking of external CSS or JavaScript file, etc.), so you can focus on what matters: building the application. Once your application is ready to be deployed, {golem} guides you through testing and brings tools for deploying to common platforms.

Some things to keep in mind before using {golem}:

• A {golem} application is contained inside a package. Knowing how to build a package is heavily recommended. The good news is also that everything you know about package development can be applied to {golem}.

• A {golem} app works better if you are working with shiny modules. Knowing how modules work is also recommended but not necessary.

## 4.2 Understanding {golem} app structure

A {golem} application is an R package. Having an R package architecture is perfectly suited for production-ready programs, as we developed in the previous chapter.

Let’s focus on the architecture of the default {golem} app, and present what part each file plays and how you can use (or not use) each of them.

You can create a {golem} project, here called golex, with RStudio “New project” creation or with command line:

golem::create_golem("golex")

The project will start with this specific architecture:

fs::dir_tree("golex")
golex
├── DESCRIPTION
├── NAMESPACE
├── R
│   ├── app_config.R
│   ├── app_server.R
│   ├── app_ui.R
│   ├── mod_my_first_module.R
│   └── run_app.R
├── dev
│   ├── 01_start.R
│   ├── 02_dev.R
│   ├── 03_deploy.R
│   └── run_dev.R
├── inst
│   ├── app
│   │   └── www
│   │       ├── custom.css
│   │       ├── favicon.ico
│   │       ├── plop.js
│   │       └── script.js
│   └── golem-config.yml
└── man
└── run_app.Rd

If you are familiar with building R packages, this structure will look familiar to you. And for a good reason: a {golem} app IS a package.

### 4.2.1DESCRIPTION & NAMESPACE

The DESCRIPTION and NAMESPACE are standard package files (i.e. they are not {golem}-specific). In DESCRIPTION, you will add a series of metadata about your package, for example who wrote the package, what is the package version, what is its goal, who to complain to if things go wrong, and also information about external dependencies, the license, the encoding and so forth.

This DESCRIPTION file will be filled automatically by the first function you will run in dev/01_start.R, and by other functions from the dev/ scripts. In other words, most of the time you will not interact with it directly, but through wrappers from {golem} and {usethis} (Wickham and Bryan 2020b) which are listed in the dev scripts.

The NAMESPACE file is the file you will NEVER edit by hand! The NAMESPACE file defines how to interact with the rest of the package: what functions to import and from which package and what functions to export, i.e. what functions are available to the user when you do library(golex). This file will be built when running the documenting process in your R package: {roxygen2} will scan all your .R files, and build the man/ + the NAMESPACE, by scanning the roxygen tags there.

Explaining how these files are to be filled and how to document your functions is out of the scope of this book, as they are deeply linked to how you would do that for any other package. If you want to learn more about these, here are some resources you can refer to:

### 4.2.2 R/

The R/ folder is the standard folder where you will store all your app functions. When you start your project with {golem}, this folder is pre-populated with three .R files: app_server.R, app_ui.R and run_app.R.

During the process of building your application, all the core functionalities of your app will be stored in the R/ directory. Note that these files are the “core” features of your application itself, and that other .R files also exists. For example, when you will need to deploy your application on RStudio platforms, {golem} will create an app.R at the root of your directory17 . This file should not go into the R/ folder, as it is not in the core of the package mechanic. The dev/ folder also contains .R scripts, and they are inside this folder as they should not live inside the R/ folder: they are utilitarian files for development, not core functionalities of your application.

Inside these .R files, you will find the content of your modules (the one added with golem::add_modules()) and the utilitarian / business logic functions, built with golem::add_utils() and golem::add_fct(). If you want to add a standard file (that is to say out of {golem} nomenclature), you can also call usethis::use_r("name"), which will create a R/name.R file.

If you have built a “classic” Shiny App, i.e before {golem} was available, or if you started with the standard RStudio template, you might have a series of source() and library() calls all over the place. This is not to be done with a {golem}-based application: you are leveraging the package infrastructure so that everything inside the R/ folder is made available internally, without having to source() scripts.

Note also that this folder must not contain any sub-folders.

#### 4.2.2.1 app_server.R

#' The application server-side
#'
#' @param input,output,session Internal parameters for {shiny}.
#'     DO NOT REMOVE.
#' @import shiny
#' @noRd
app_server <- function( input, output, session ) {
# List the first level callModules here

}

The app_server.R file contains the function for the server logic. If you are familiar with the classic ‘ui.R / server.R’ methodology, this function can be seen as a replacement for the content of the function you have in your server.R.

Building a complex Shiny application commonly implies using Shiny modules. If so, you will be adding there a series of callModule(), the ones you will get on the very bottom of the file created with golem::add_module().

You will also find global elements from your server-logic: top-level reactiveValues(), connections to databases, setting options and so forth.

#### 4.2.2.2 app_ui.R

#' The application User-Interface
#'
#' @param request Internal parameter for {shiny}.
#'     DO NOT REMOVE.
#' @import shiny
#' @noRd
app_ui <- function(request) {
tagList(
# Leave this function for adding external resources
# List the first level UI elements here
fluidPage(
h1("golex")
)
)
}

This piece of the app_ui.R is designed to receive the counterpart of what you put in your server. Everything here is to be put after the  # List the first level UI elements here  line. Just as with their server counterparts, the UI side of these elements are the one from the bottom of the file you are creating with golem::add_module().

By default, {golem} uses a fluidPage(), which is {shiny} (Chang et al. 2020) most commonly used template. If ever you want to use navBarPage(), this is where you will define this: replace one with the other, and you will be good to go. You can also define any other template page, for example with an htmlTemplate(). Keep in mind that removing the fluidPage() here implies that there is no available CSS/JS template to be used anymore, and you will need to be adding your own there.

#' Add external Resources to the Application
#'
#' This function is internally used to add external
#' resources inside the Shiny application.
#'
#' @import shiny
#' @importFrom golem add_resource_path activate_js favicon bundle_resources
#' @noRd

'www', app_sys('app/www')
)

tags$head( favicon(), bundle_resources( path = app_sys('app/www'), app_title = 'golex' ) # Add here other external resources # for example, you can add shinyalert::useShinyalert() ) } The second part of this file contains the golem_add_external_resources() function, which is used to add, well, external resources. You may have noticed that this function is to be found above in the file, in the app_ui() function. This function is used for linking to external files inside your applications: notably the files you will create with golem::add_css_file() and friends. In golem_add_external_resources(), you can also define custom resourcesPath. The first line (the one with addResourcePath()) is the one allowing the inst/app/www folder to be mounted and be available at www with your app when you launch it. That link makes it possible for {golem} to bundle the CSS and JavaScript files automatically. The other part of this function, starting with tags$head, creates a <head> tag for your application. This <head> tag is a pretty standard tag, which is used in HTML to define a series of metadata about your app. We encourage you to add any new external file (e.g pictures) in this inst/app/www folder, so that you can later use it in the UI with the common www prefix. Another common pattern would be:

• Adding images in inst/app/img
• Calling addResourcePath( 'img', system.file('app/img', package = 'golex') )
• Adding elements to your UI with tags\$img(src = "img/name.png").

#### 4.2.2.3 run_app.R

#' Run the Shiny Application
#'
#' @param ... A series of options to be used inside the app.
#'
#' @export
#' @importFrom shiny shinyApp
#' @importFrom golem with_golem_options
run_app <- function(
...
) {
with_golem_options(
app = shinyApp(
ui = app_ui,
server = app_server
),
golem_opts = list(...)
)
}

The run_app() function is the one that you will use to launch the app18 .

The body of this function is wrapped inside with_golem_options(), which allows you to pass arguments to the run_app() function, which can be called later on with golem::get_golem_options(). Some examples of passing arguments include run_app(prod = FALSE) to run a verbose development version or run_app(user = "admin) to bypass authentication during development tests.

### 4.2.3golem-config

#### 4.2.3.1 app_config.R

Inside the R/ folder is the app_config.R file.

#' Access files in the current app
#'
#' @param ... Character vector specifying directory and or file to
#'     point to inside the current package.
#'
#' @noRd
app_sys <- function(...){
system.file(..., package = "golex")
}

#' Read App Config
#'
#' @param value Value to retrieve from the config file.
#' @param config R_CONFIG_ACTIVE value.
#' @param use_parent Logical, scan the parent directory for config file.
#'
#' @noRd
get_golem_config <- function(
value,
config = Sys.getenv("R_CONFIG_ACTIVE", "default"),
use_parent = TRUE
){
config::get(
value = value,
config = config,
# Modify this if your config file is somewhere else:
file = app_sys("golem-config.yml"),
use_parent = use_parent
)
}

This file is designed to handle two things:

• app_sys() is a wrapper around system.file(package = "golex"), and allows you to quickly reference to the files inside the inst/ folder. For example, app_sys("x.txt") points to inst/x.txt file inside your package.

• get_golem_config() helps you manipulate the config file located at inst/golem-config.yml.

#### 4.2.3.2 Manipulating golem-config.yml

Here is what the default config file looks like:

default:
golem_name: golex
golem_version: 0.0.0.9000
app_prod: no
production:
app_prod: yes
dev:
golem_wd: !expr here::here()

It is based on the {config} (Allaire 2018) format, and allows you to define contexts, with values associated with these specific contexts. For example, in the default example:

• default.golem_name, default.golem_version, default.app_prod are usable across the whole life of your golem app: while developing, and also when in production.
• production.app_prod might be used for adding elements that are to be used once the app is in production.
• dev.golem_wd is in a dev config because the only moment you might reliably use this config is while developing your app. Use the app_sys() function if you want to rely on the package path once the app is deployed.

These options are globally set with:

set_golem_options()

The functions reading the options in this config file are:

get_golem_name()
get_golem_wd()
get_golem_version()

You can set these with:

set_golem_name("this")
set_golem_wd(".")
set_golem_version("0.0.1")

If you are already familiar with the {config} package, you can use this file just as any config file.

{golem} comes with an amend_golem_config() function to add elements to it.

amend_golem_config(
key = "where",
value = "indev"
)
amend_golem_config(
key = "where",
value = "inprod",
config = "production"
)

In R/app_config.R, you will find a get_golem_config() function that allows you to retrieve config from this config file:

get_golem_config(
"where"
)
get_golem_config(
"where",
config = "production"
)

You can also use an environment variable (default {config} behavior):

Sys.setenv("R_CONFIG_ACTIVE" = "production")
get_golem_config("where")

The good news is that if you don’t want/need to use {config}, you can safely ignore this file, just leave it where it is: it is used internally by the {golem} functions.

#### 4.2.3.3golem_config vs golem_options

There are two ways to configure golem apps:

• The golem_opts in the run_app() function
• The golem-config.yml file

The big difference between these two is that the golem options from run_app() are meant to be configured during runtime: you will be doing run_app(val = "this"), whereas the golem-config is meant to be used in the back-end, and will not be linked to the parameters passed to run_app() (even if this is technically possible, this is not the main objective).

It is also linked to the R_CONFIG_ACTIVE environment variable, just as any {config} file.

The idea is also that the golem-config.yml file is shareable across {golem} projects (golem_opts are application specific), and will be tracked by version control systems.

### 4.2.4inst/app/www/

The inst/app/www/ folder contains all files that are made available at application run time. Any web application has external files that allow it to run19 . For example, {shiny} and its fluidPage() function bundles a series of CSS and JavaScript files, notably the Bootstrap library, or jQuery. These external files enhance your app: CSS for the design part and JavaScript for the interactive part (more or less). On top of that, you can add your own files: your own design with CSS or your own JavaScript content (as we will see in the last chapters of this book). In order to work, you have to include a link to these files somewhere in the UI. This is what golem_add_external_resources() is made for: linking the external resources that you will build with the following functions.

• golem::add_css_file()
• golem::add_js_file()
• golem::add_js_handler()
• golem::use_external_css_file()
• golem::use_external_js_file()
• golem::use_favicon()

Be aware that these files are available under the www/ at application run time, i.e. that the www/ folder is available by your browser, not by R when it runs/generates your application. In other words, you can use the www prefix in the HTML generated in your UI, which is read by your browser, not from the R/server side. If you want to link to a file that is read during application generation, you will need to use the app_sys() function, with for example includeMarkdown( app_sys("app/www/plop.md") ).

### 4.2.5dev/

The dev/ folder is to be used as a notebook for your development process: you will find here a series of functions that can be used all along your project.

The content of these files are specific to {golem} here, but the concept of using a script to store all development steps is not restricted to a Shiny application: it could easily be done for any package, and this is something we recommend to do. The functions inside these files are the ones used to do some setup, like usethis::use_mit_license() or usethis::use_vignette("my-analysis"), add testing infrastructure like usethis::use_test("my-function") or devtools::check(). You will also find functions to populate the application like golem::add_module("my-module") or golem::add_js_file("my-script"). And finally, there are functions you will need once your application is ready: pkgdown::build_site(), rhub::check_for_cran() or golem::add_dockerfile().

We will come back to these files later in this book when we describe in more depth the {golem} workflow.

### 4.2.6man/

The man/ folder includes the package documentation. It is a common folder automatically filled when you document your app, notably when running the dev/run_dev.R script and the document_and_reload() function. As with the NAMESPACE and DESCRIPTION files, explaining this file is out of scope of this book (and to be honest, you will probably never have to interact with these files directly). To know more about documentation and how to build it, here are some external links:

### References

Allaire, JJ. 2018. Config: Manage Environment Specific Configuration Values. https://CRAN.R-project.org/package=config.

Chang, Winston, Joe Cheng, JJ Allaire, Yihui Xie, and Jonathan McPherson. 2020. Shiny: Web Application Framework for R. https://CRAN.R-project.org/package=shiny.

Guyader, Vincent, Colin Fay, Sébastien Rochette, and Cervan Girard. 2020. Golem: A Framework for Robust Shiny Applications. https://github.com/ThinkR-open/golem.

Wickham, Hadley, and Jennifer Bryan. 2020b. Usethis: Automate Package and Project Setup. https://CRAN.R-project.org/package=usethis.

1. {golem} will automatically add this file to the .Rbuildignore file.↩︎

2. Very technically speaking, it is the print() from the object outputed by run_app() that launches the app, but this is another story.↩︎

3. Some webpages do not need any external sources, as they do not have any design and are plain HTML, but generally speaking we will not call this format a web application.↩︎