routr is a simple and versatile router for R based web servers. For people not familiar with back-end development, a router is a piece of middleware that delegates HTTP requests to the correct handler function. The delegation is based in the URL of the request and in essence means that requests directed at /persons/thomas/ ends up in another handler than /packages/routr/.

routr is heavily inspired by other routers build for other platforms, especially those for Express.js and Ruby on Rails, though it doesn’t mimick either.

routr is available on CRAN and can be installed in the regular way.

install.packages('routr')

Alternatively you can grab the development version from Github with

# install.packages('devtools')
devtools::install_github('thomasp85/routr')

A router is build up of several seperate routes that are collected in a route stack. The stack recieves the request and passes it on to the first route in the stack. Depending on whether the route can handle the request and whether the handler signals a fall-through, the request is passed along the stack until a handler signals that no further processing should be done. This means that it is possible to stack different functionality like user verification, static ressource serving, etc. on top of each other.

A handler is a function that accepts the arguments request, response, keys, and .... The handler must return a boolean indicating if the request should be passed down the stack (TRUE) or not (FALSE). routr uses the reqres package to provide powerful request and response classes that makes it easy to work with an HTTP exchange. An example of a simple handler is:

h <- function(request, response, keys, ...) {
    response$status <- 200L
    response$type <- 'html'
    response$body <- '<h1>Hello World!</h1>'
    return(FALSE)
}

No matter the content of the request passed to this handler it will return a “Hello World!” to the client. Because it returns FALSE it block any other handlers below it to modify the response.

A route is a collection of handlers. For any given request, only one handler in the route will be called. A route is an object of the R6 Route class and can be created as so:

route <- Route$new()
route$add_handler('get', '/hello/:what/', h)

The first argument to add_handler defines the request type while the second defines the path that the handler responds to. The path need not be static. In the above example the :what defines a variable meaning that the handler will respond to any /hello/<something>/ variation. The variable and the value is available to the handler in the keys argument. For instance, if a request with the URL /hello/mars/ were passed through the route, the keys argument passed to the handler would contain list(what = 'mars'). Variables can only span a single level, meaning that the above handler would not respond to /hello/jupiter/saturn/. To match to anything use /hello/* for responding to any sub-URL to hello. Matches to * will not end up in the keys list. If several paths in a route matches a URL the most specific will be used, meaning that /* will match everything but will always chosen last. With all that in mind lets change the handler to respond to the what variable:

h <- function(request, response, keys, ...) {
    response$status <- 200L
    response$type <- 'html'
    response$body <- paste0('<h1>Hello ', keys$what, '!</h1>')
    return(FALSE)
}
route$add_handler('get', '/hello/:what/', h)

Let’s also add a fallback handler that captures everything:

hFallback <- function(request, response, keys, ...) {
    response$status <- 200L
    response$type <- 'html'
    response$body <- '<h1>I\'m not saying hello to you</h1>'
    return(FALSE)
}
route$add_handler('get', '/*', hFallback)

The route stack manages several routes and takes care of receiving a request and returning a response. A route stack is an object of the R6 class RouteStack and is created like this:

router <- RouteStack$new()
router$add_route(route, 'test')

The order in which routes are added to the stack determines the calling order, with those added first taking precedence over those added later. Request are handled by the dispatch method like so:

router$dispatch(request)

A RouteStack is a fiery-compliant plugin meaning that it can be passed to the attach() method of a fiery server. This will set the server up to pass requests through the route stack and use the resulting response automatically

app <- fiery::Fire$new()
app$attach(router)
app$ignite(block = FALSE)
# In Terminal (or visit in browser)
# curl http://127.0.0.1:8080/hello/mars/
# <h1>Hello Mars!</h1>
app$extinguish()

By default the router responds to request events but can also be used to dispatch on header and message events. In the latter case the request that is send through the handlers is a modified version of the request used to establish the WebSocket version. If used as a WebSocket router a way to extract the path to dispatch on must be provided as part of the RouteStack construction.

Please note that the ‘routr’ project is released with a Contributor Code of Conduct. By contributing to this project, you agree to abide by its terms.