Add POST requests to {shiny} with {brochure}

[Disclaimer] The package presented in this blog post is still experimental at the time of writing these lines (2021-02-28), so it might face some API changes in the future.

Yesterday Jacqueline’s tweeted about her need to add POST requests to a {shiny} application.

I'm astounded that it doesn't seem like a Shiny app is able to accept POST requests. I have data I wanna pass to my server :/

— Dr. Jacqueline Nolis (@skyetetra) February 27, 2021

This is actually one of the things I had in mind when building {brochure}: more flexibility with the HTTP requests and how they are handled.

So here is a working example of adding a POST handler to a {shiny} app, using {brochure}.

library(shiny)
library(brochure)
# We'll start by defining a home page
home <- function(
  httr_code
){
  page(
    href = "/",
    # Simple UI, no server side
    ui = tagList(
      tags$h1("Hello world!"), 
      tags$p("Open a new R console and run:"),
      tags$pre(
        httr_code
      )
    )
  )
}

postpage <- function(){
  page(
    href = "/post",
    # We'll handle POST requests via a request handler
    req_handlers = list(
      function(req){
        # This is where the magic happens
        # Our req object contains a `REQUEST_METHOD` 
        # entry that contains the HTTP verb used 
        # to perform the request
        if( req$REQUEST_METHOD == "POST" ){
          print("In POST!")
          # Because we want the HTTP request to be 
          # completed here, we return an httpResponse object here. 
          # httpResponse() is exported since {shiny} 1.6.0, 
          # otherwise you'll have to ::: (shiny:::httpResponse)
          return(
            httpResponse(
              # 201 is the HTTP code you'll send back when 
              # you have created a resource on the server
              status = 201, 
              content = "ok"
            )
          )
        } 
        # Whenever we're not in a POST, we'll simply return 
        # req, which will the move to standard {shiny} handling, 
        # i.e. calling ui and server.
        return(req)
      }
    ), 
    ui = tagList(
      tags$p("Hello from /post!")
    )
  )
}
# For the sake of reproducibility:
options(shiny.port = 2811)

brochureApp(
  home(
    httr_code = "httr::POST('http://127.0.0.1:2811/post')"
  ),
  postpage()
)

If you open another terminal, you can run:

> httr::POST("http://127.0.0.1:2811/post")

Response [http://127.0.0.1:2811/post]
  Date: 2021-02-28 21:47
  Status: 201
  Content-Type: text/html; charset=UTF-8
  Size: 2 B

And your R console running the {shiny} app should print a message:

Listening on http://127.0.0.1:2811
[1] "In POST!"

Of course, what’s interesting with POST is that you can actually send a body along the http request. Using {brochure}, you can access it inside the request handler, via req$.bodyData. It’s an external pointer to a file, so we can read it with readLines.

str(req$.bodyData)
 'file' int 3
 - attr(*, "conn_id")=<externalptr> 

Let’s add this to our app:

postpage <- function(){
  page(
    href = "/post",
    req_handlers = list(
      function(req){
        if( req$REQUEST_METHOD == "POST" ){
          # reading the req$.bodyData. 
          body <- readLines(
            req$.bodyData,
            # The file will have no final EOL
            # and we don't want a message to be 
            # printed to the console
            warn = FALSE
          )
          # Here, you can define your own handling 
          # logic of the body. 
          # We'll simply print it to the console 
          # in this example.
          print(body)
          return(
            httpResponse(
              status = 201, 
              content = "Created"
            )
          )
        } 
        return(req)
      }
    ), 
    ui = tagList(
      tags$p("Hello from /post!")
    )
  )
}
brochureApp(
  home(
    httr_code = "httr::POST('http://127.0.0.1:2811/post', body = 'plop')"
  ),
  postpage()
)

Now inside another console, send:

> (resp <- httr::POST("http://127.0.0.1:2811/post", body = "plop"))
Response [http://127.0.0.1:2811/post]
  Date: 2021-02-28 21:57
  Status: 201
  Content-Type: text/html; charset=UTF-8
  Size: 7 B

The R console running the {shiny} app should print a message:

Listening on http://127.0.0.1:2811
[1] "plop"

Back to the other console, the content() of the {httr} resp will be the content defined in the httpResponse:

> httr::content(resp)
{html_document}
<html>
[1] <body><p>Created</p></body>

Hope this helps!