处理和操作 http 响应列表

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

1. 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.
2. 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.
3. Returning a list of images requires zipping them. You can't return multiple entities in a single HTTP response.

Possible approach

1. 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)
}

2. dataBytes returns a Source so you'll end up with a List of Sources . You have to concatenate them via, for example via concat .
3. The result stream has to be zipped via Compression.gzip .
4. Finally, the stream can be put in the complete method of getImagesByIdsListRoute .