Hello there,
I have been assigned the task of rewriting an HTTP api service, initially
written in Spray/Observables, to Akka HTTP/Akka Streams.
While things have gone smoothly for the most part, I have currently reached
a roadblock.
The problem is the following:
There is an API call for fetching a list of categories and the items
that belong to those respective categories. The goal is to return a Source
of Chunked JSON for the following route:
path("categories") {
get {
encodeResponse {
complete(
HttpResponse(entity =
HttpEntity.Chunked(
ContentTypes.`application/json`,
Database.Get.categories.via(asChunkStreamPart)
)
)
)
}
}
Let's pretend that I am returning Plain text rather than JSON in order to
simplify things:
The initial Database query returns vector of KeyPair (String,
List[Strings]), corresponding to Category Name and the Id's of the items
that fall under that category.
I began by wrapping this Observable into a Source[String, List[String]) -
Database.Get.categories: Source[String, List[String]
Next, I need to the content of each item of that list. Now this is where
it gets tricky, I would like for EACH item to ONLY be fetched as there is
demand from the HTTP Response.
Initially I opted for rewriting def getItem(id: String):
Observable[String] as Source[String, Unit] but in order to avoid having a
Source of Sources I wrapped it as Future[String] instead.
This lead to the following signature: Source[Key, List[ Future[String],
Unit]
( So each row has a list of Futures which one invoked will retrieve the
item data)
My question is, is my design fundamentally flawed or am I overlooking
something? Would it make it easier to flatten everything if the DB queries
for the items were Sources themselves?
I need to have the entire list flatten with the Key, so that the method
signature becomes Source[String, Unit].
However, if i am not mistaken, if I were to use flatMapConcat I would be
consuming the entire source ( therefore performing all DB queries for all
the items) correct?
Just to emphasize, I would want to be able to do this:
val dbResult = Database.Get.categories // let's assume there are two
categories with 2 items each : [ "categoy1", "itemCat1A", "itemCat1B",
"categoy2",
"itemCat2A", "itemCat2B"]
dbResult.take(1) // "categoy1"
dbResult.drop(1) // "itemCat1A" // Fires getItem and return's its string
result once available
dbResult.drop(2) // "itemCat1B" // Fires getItem and return's its string
result once available
dbResult.drop(3) // "categoy2"
dbResult.drop(4) // "itemCat2A" // Fires getItem and return's its string
result once available
If anyone could provide some input it would be highly appreciated!
Thanks for reading.
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