tower

Dead simple middleware for R Shiny.

Summary

tower is a simple library for adding middleware to Shiny applications. It is inspired by the tower crate for Rust. It is designed to enable package authors and Shiny developers to extend Shiny a little bit more than what is usually possible.

You can use tower to add middlewares that forward, modify, or intercept requests in Shiny applications. This can be useful for adding logging, authentication, caching, or routing to your Shiny applications.

Installation

You can install the development version of tower from GitHub with:

# install.packages("remotes")
remotes::install_github("ixpantia/tower")

Example

We may want to add a new route to our Shiny application that adds a count to a counter every time a user visits the route. We can do this with tower by adding a middleware that intercepts the request and increments the counter.

library(shiny)
library(tower)

# Counter environment
global_counter <- new.env()
global_counter$count <- 0

# Middleware to increment the counter
increment_counter <- function(req) {
  global_counter$count <- global_counter$count + 1
  response_builder() |>
    add_body(paste("Counter is now", global_counter$count)) |>
    build_response()
}

# A very empty Shiny app (not necesarry for the demo)
ui <- fluidPage()
server <- function(input, output, session) {}

shinyApp(ui, server) |>
  create_tower() |>
  add_get_route("/counter", increment_counter) |>
  build_tower()

If you run the code above and visit the route /counter in your browser, you will see the counter increment every time you visit the route.

How it works

Basically, tower adds layers to a Shiny application. A layer is a function that takes a request and returns either a response or NULL. If a layer returns a response, the response is sent to the client and the request is not forwarded to the next layer. If a layer returns NULL, the request is forwarded to the next layer.