httpuv provides low-level socket and protocol support for handling HTTP and WebSocket requests directly from within R. It uses a multithreaded architecture, where I/O is handled on one thread, and the R callbacks are handled on another.

It is primarily intended as a building block for other packages, rather than making it particularly easy to create complete web applications using httpuv alone. httpuv is built on top of the libuv and http-parser C libraries, both of which were developed by Joyent, Inc.

You can install the stable version from CRAN, or the development version using remotes:

# install from CRAN
install.packages("httpuv")

# or if you want to test the development version here
if (!require("remotes")) install.packages("remotes")
remotes::install_github("rstudio/httpuv")

Since httpuv contains C code, you'll need to make sure you're set up to install packages with compiled code. Follow the instructions at http://www.rstudio.com/ide/docs/packages/prerequisites

This is a basic web server that listens on port 5000 and responds to HTTP requests with a web page containing the current system time and the path of the request:

library(httpuv)

s <- startServer(host = "0.0.0.0", port = 5000,
  app = list(
    call = function(req) {
      body <- paste0("Time: ", Sys.time(), "<br>Path requested: ", req$PATH_INFO)
      list(
        status = 200L,
        headers = list('Content-Type' = 'text/html'),
        body = body
      )
    }
  )
)

Note that when host is 0.0.0.0, it listens on all network interfaces. If host is 127.0.0.1, it will only listen to connections from the local host.

The startServer() function takes an app object, which is a named list with functions that are invoked in response to certain events. In the example above, the list contains a function call. This function is invoked when a complete HTTP request is received by the server, and it is passed an environment object req, which contains information about HTTP request. req$PATH_INFO is the path requested (if the request was for http://127.0.0.1:5000/foo, it would be "/foo").

The call function is expected to return a list containing status, headers, and body. That list will be transformed into a HTTP response and sent to the client.

To stop the server:

s$stop()

Or, to stop all running httpuv servers:

stopAllServers()

A httpuv server application can serve up files on disk. This happens entirely within the I/O thread, so doing so will not block or be blocked by activity in the main R thread.

To serve a path, use staticPaths in the app. This will serve the www/ subdirectory of the current directory (from when startServer is called) as the root of the web path:

s <- startServer("0.0.0.0", path = 5000,
  app = list(
    staticPaths = list("/" = "www/")
  )
)

By default, if a file named index.html exists in the directory, it will be served when / is requested.

staticPaths can be combined with call. In this example, the web paths /assets and /lib are served from disk, but requests for any other paths go through the call function.

s <- startServer("0.0.0.0", 5000,
  list(
    call = function(req) {
      list(
        status = 200L,
        headers = list(
          'Content-Type' = 'text/html'
        ),
        body = "Hello world!"
      )
    },
    staticPaths = list(
      "/assets" = "content/assets/",
      # Don't use index.html for /lib
      "/lib" = staticPath("content/lib", indexhtml = FALSE)
    )
  )
)

httpuv also can handle WebSocket connections. For example, this app acts as a WebSocket echo server:

s <- startServer("127.0.0.1", 8080,
  list(
    onWSOpen = function(ws) {
      # The ws object is a WebSocket object
      cat("Server connection opened.\n")

      ws$onMessage(function(binary, message) {
        cat("Server received message:", message, "\n")
        ws$send(message)
      })
      ws$onClose(function() {
        cat("Server connection closed.\n")
      })
    }
  )
)

To test it out, you can connect to it using the websocket package (which provides a WebSocket client). You can do this from the same R process or a different one.

ws <- websocket::WebSocket$new("ws://127.0.0.1:8080/")
ws$onMessage(function(event) {
  cat("Client received message:", event$data, "\n")
})

# Wait for a moment before running next line
ws$send("hello world")

# Close client
ws$close()

Note that both the httpuv and websocket packages provide a class named WebSocket; however, in httpuv, that class acts as a server, and in websocket, it acts as a client. They also have different APIs. For more information about the WebSocket client package, see the project page.

httpuv can be built with debugging options enabled. This can be done by uncommenting these lines in src/Makevars, and then installing. The first one enables thread assertions, to ensure that code is running on the correct thread; if not. The second one enables tracing statements: httpuv will print lots of messages when various events occur.

PKG_CPPFLAGS += -DDEBUG_THREAD -UNDEBUG
PKG_CPPFLAGS += -DDEBUG_TRACE

To install it directly from Github with these options, you can use with_makevars, like this:

withr::with_makevars(
  c(PKG_CPPFLAGS="-DDEBUG_TRACE -DDEBUG_THREAD -UNDEBUG"), {
    devtools::install_github("rstudio/httpuv")
  }, assignment = "+="
)

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