Call multiple webservices from play 2 - web-services

Question

I am a play2.0-Scala-beginner and have to call several Webservices to generate a HTML page.
After reading the The Play WS API page and a very interesting article from Sadek Drobi I am still unsure what's the best way to accomplish this.
The article shows some code snippets which I don't fully understand as a Play beginner.
Figure 2 on page 4:
val response: Either[Response,Response] =
WS.url("http://someservice.com/post/123/comments").focusOnOk
val responseOrUndesired: Either[Result,Response] = response.left.map {
case Status(4,0,4) => NotFound
case Status(4,0,3) => NotAuthorized
case _ => InternalServerError
}
val comments: Either[Result,List[Comment]] =
responseOrUndesired.right.map(r => r.json.as[List[Comment]])
// in the controller
comment.fold(identity, cs => Ok(html.showComments(cs)))
What does the last line with the fold do? Should comment be comments? Haven't I group the last statement in an Async block?
Figure 4 shows how to combine several IO calls with a single for-expression:
for {
profile <- profilePromise
events <- attachedEventsPromise
articles <- topArticlesPromise
} yield Json.obj(
"profile" -> profile,
"events" -> events,
"articles" -> articles )
}
// in the controller
def showInfo(...) = Action { rq =>
Async {
actorInfo(...).map(info => Ok(info))
}
}
How can I use this snippet? (I am a bit confused by the extra-} after the for-expression.)
Should I write something like this?
var actorInfo = for { // Model
profile <- profilePromise
events <- attachedEventsPromise
articles <- topArticlesPromise
} yield Json.obj(
"profile" -> profile,
"events" -> events,
"articles" -> articles )
def showInfo = Action { rq => // Controller
Async {
actorInfo.map(info => Ok(info))
}
}
What's the best way to combine the snippets from figure 2 and 4 (error handling + composition of IO non-blocking calls)? (f.ex. I want to produce a Error 404 status code if any of the called webservice produce an Error 404).
Maybe someone knows a complete example of calling webservices in the play framework (cannot find an example in the play Sample applications or anywhere else).

Answer 1

I have to say that the article is wrong in the example you show in Figure 2. The method focusOnOk does not exist in Play 2.0. I assume the author of the article used a pre-release version of Play 2 then.
Regarding comment, yes it should be comments. The fold in the statement is operating on an Either. It takes 2 functions as parameters. The first is a function to apply if it is a left value. The second is a function to apply if it is a right value. A more detailed explanation can be found here: http://daily-scala.blogspot.com/2009/11/either.html
So what the line does is. If I have a left value (which meant I got an undesired response), apply the built-in identity function which just gives you back the value. If it has a right value (which means I got an OK response), make a new result that shows the comments somehow.
Regarding Async, it's not actually asynchronous. focusOnOk is a blocking function (a remnant from the old Java days of Play 1.x). But remember, that's not valid Play 2 code.
As for Figure 4, the trailing } is actually because it's a partial alternative of what's in Figure 3. Instead of the numerous promise flatMaps. You can do a for comprehension instead. Also, I think it should be userInfo(...).map instead of actorInfo(...).map.
The Play documentation you linked to actually already shows you a full example.
def feedTitle(feedUrl: String) = Action {
Async {
WS.url(feedUrl).get().map { response =>
Ok("Feed title: " + (response.json \ "title").as[String])
}
}
}
will get whatever is at feedUrl, and you map it to do something with the response which has a status field you can check to see if it was a 404 or something else.
To that end, the Figure 3 and 4 of your linked article should give you a starting point. So you'd have something like,
def getInfo(...) : Promise[String] = {
val profilePromise = WS.url(...).get()
val attachedEventsPromise = WS.url(...).get()
val topArticlesPromise = WS.url(...).get()
for {
profile <- profilePromise
events <- attachedEventsPromise
articles <- topArticlesPromise
} yield {
// or return whatever you want
// remember to change String to something else in the return type
profile.name
}
}
def showInfo(...) = Action { rq =>
Async {
getInfo(...).map { info =>
// convert your info to a Result
Ok(info)
}
}
}