Handling and manipulating a list of http responses - scala

Question

I'm currently trying to implement API logic to fetch multiple images from a server.
This server accepts an image id and return an HTTP response that contains the image in PNG format as an entity.
Right now, we want to add a new endpoint that accepts a list of images IDs and return a list of all the images:
I have done the following:
def getImagesFromIds(IdsList: List[String]): Future[List[HttpResponse]] = {
Future.sequence {
IdsList.map(
id => getImageById(id)
)
}
}
this function will receive a list of ids and will call the getImageById to fetch all the images, it will return a list of HttpResponse.
And for the route definition, I have done the following:
def getImagesByIdsListRoute: Route = get {
path("by-ids-list") {
entity(as[List[String]]){
upcs =>
complete(getImagesFromIds(upcs))
}
}
}
But I'm getting the following error message:
no implicits found for parameter m: marshalling.toresponsemarshallable[list[httpresponse]]
Does Any one know how we can marshall a list of http responses, or if there is any way to improve this logic to fetch multiple http responses ?

Answer 1

If I understand correctly, you want to download multiple images and return them as a HTTP response.
The problems with your current attempt
The call to the API made via getImageById returns a HttpResponse. You can't be sure what is the result of this API call. If it fails, the response won't contain any image at all.
You are trying to return List[HttpResponse] as your response. How should this response be serialized? Akka doesn't know what you mean by that and tries to find a marshaller which will serialize your object (for example to JSON) but can't find one.
Returning a list of images requires zipping them. You can't return multiple entities in a single HTTP response.
Possible approach
You have to change getImageById so that it checks what is in the HttpResponse and returns the entity bytes.
Example:
response match {
case HttpResponse(StatusCodes.OK, _, entity, _) =>
entity.dataBytes
case resp # HttpResponse(code, _, _, _) =>
// Response failed and we don't care about the response entity
// Details: https://doc.akka.io/docs/akka-http/current/implications-of-streaming-http-entity.html
resp.discardEntityBytes()
// Decide yourself how you want to handle failures
throw new RuntimeException("Request failed, response code: " + code)
}
dataBytes returns a Source so you'll end up with a List of Sources. You have to concatenate them via, for example via concat.
The result stream has to be zipped via Compression.gzip.
Finally, the stream can be put in the complete method of getImagesByIdsListRoute.