akka-http 'Directive0' to force the inner route be evaluated separately for each request? - akka-http

Question

Within akka-http, routes are evaluated only once (at build time), unless they rely on some dynamic, extracted parameter (client IP, request header, anything).
One of my custom directives depends on a side effect (a potentially changing system file), so I wish to execute it separately, each time. I'm looking for an extractNothing that I could wrap it with.
Is there such?
Ability to consume a parameter would also work, then I can (extract[Unit](_) & consume).

Answer 1

Routes Are Still Dynamic
You are correct that routes are evaluated only once. However, the body of any Route can be dynamic in nature.
As an example, suppose we have a function that returns the current time:
val getTime = () => System.currentTimeMillis().toString
This function can be embedded within a Route:
val route =
get {
path("currentTime") {
complete(getTime())
}
}
Updating File Route
Now each HttpRequest will get a different time value back even though the route is a val.
Similarly, you could use streaming file io in akka to create a dynamic HttpResponse, the entity of which is the contents of your changing file:
val fileResponse = () => {
val file = Paths get "sourceFile.txt"
val chunkSource = FileIO fromPath file
val chunkEntity =
Chunked.fromData(ContentTypes.`text/plain(UTF-8)`, chunkSource)
HttpResponse(entity = chunkEntity)
}
val fileRoute =
get {
path("sourceFile") {
complete(fileResponse())
}
}
Inner & Outer Routes
Extending the example 1 step further to dynamic Routes, you could write a function that fails every other time:
val halfBaked = () => {
var isGood = true
() => {
if(isGood) {
isGood = false
complete(fileResponse())
}
else {
isGood = true
complete(StatusCodes.InternalServerError)
}
}
}
And wrap this in an outer Route :
val innerRoute = halfBaked()
val outerRoute =
get {
path("feelingLucky") {
innerRoute()
}
}