Middleware

Middlewares a functions that run before anything in the application. They are mostly used to modify or add parameters to the request object.

Creating middlewares

Similar to request handlers, middlewares take the Request and Response objects as arguments.

show_time <- \(req, res) {
  print(Sys.time())
}

The use() method employs a middleware to your app instance.

app$use(show_time)

Here’s a full reprex:

library(ambiorix)

app <- Ambiorix$new(log = FALSE)

app$use(show_time)

app$get("/", \(req, res){
  res$send("Using {ambiorix}!")
})

app$get("/about", \(req, res){
  res$text("About")
})

app$start()

Unlike other request handlers which must return a response, middlewares may but do not have to.

Multiple middleware can also be used. They are run in the order in which they’re used:

library(ambiorix)

app <- Ambiorix$new()

app$use(\(req, res){
  req$x <- 1 # set x to 1
})

app$use(\(req, res){
  print(req$x)
})

app$get("/", \(req, res){
  res$sendf("x set to %s", req$x)
})

app$start()

Common Pattern

Existing middlewares tend to use function factories, which is useful if you want to package reusable middleware.

factory <- \(prefix){
  \(req, res){
    cat(prefix, "-log\n")
  }
}

m1 <- factory("HELLO")
m2 <- factory("WORLD")

app$use(m1)$use(m2)

Existing middlewares

See some Existing Middleware and how you can use them.