Implications of the streaming nature of Request/Response Entities
Akka HTTP is streaming all the way through, which means that the back-pressure mechanisms enabled by Akka Streams are exposed through all layers–from the TCP layer, through the HTTP server, all the way up to the user-facing HttpRequest
and HttpResponse
and their HttpEntity
APIs.
This has surprising implications if you are used to non-streaming / not-reactive HTTP clients. Specifically it means that: “lack of consumption of the HTTP Entity, is signaled as back-pressure to the other side of the connection”. This is a feature, as it allows one only to consume the entity, and back-pressure servers/clients from overwhelming our application, possibly causing unnecessary buffering of the entity in memory.
Put another way: Streaming all the way through is a feature of Akka HTTP that allows consuming entities (and pulling them through the network) in a streaming fashion, and only on demand when the client is ready to consume the bytes. Therefore, you have to explicitly consume or discard the entity.
On a client, for example, if the application doesn’t subscribe to the response entity within akka.http.host-connection-pool.response-entity-subscription-timeout
, the stream will fail with a TimeoutException: Response entity was not subscribed after ...
.
Consuming (or discarding) the Entity of a request is mandatory! If accidentally left neither consumed or discarded Akka HTTP will assume the incoming data should remain back-pressured, and will stall the incoming data via TCP back-pressure mechanisms. A client should consume the Entity regardless of the status of the HttpResponse
.
Client-Side handling of streaming HTTP Entities
Consuming the HTTP Response Entity (Client)
There are two use-cases to consume the entity of a response:
- process the bytes as the response arrives from the network buffer
- load all the bytes in memory first, and process them afterwards
The most common use-case, and recommended, of course, is consuming the response entity as a stream, which can be done via running the underlying dataBytes
Source.
It is encouraged to use various streaming techniques to utilise the underlying infrastructure to its fullest, for example by framing the incoming chunks, parsing them line-by-line and then connecting the flow into another destination Sink, such as a File or other Akka Streams connector:
- Scala
-
source
import java.io.File import akka.actor.ActorSystem import akka.http.scaladsl.model._ import akka.stream.scaladsl.{ FileIO, Framing } import akka.util.ByteString implicit val system: ActorSystem = ActorSystem() val response: HttpResponse = ??? response.entity.dataBytes .via(Framing.delimiter(ByteString("\n"), maximumFrameLength = 256)) .map(transformEachLine) .runWith(FileIO.toPath(new File("/tmp/example.out").toPath)) def transformEachLine(line: ByteString): ByteString = ???
- Java
However, sometimes the need may arise to consume the entire entity as Strict
entity (which means that it is completely loaded into memory). Akka HTTP provides a special toStrict(timeout)
method which can be used to eagerly consume the entity and make it available in memory. Once in memory, data can be consumed as a ByteString
or as a Source
:
- Scala
-
source
import scala.concurrent.ExecutionContext import scala.concurrent.Future import scala.concurrent.duration._ import akka.actor.ActorSystem import akka.http.scaladsl.model._ import akka.util.ByteString implicit val system: ActorSystem = ActorSystem() implicit val dispatcher: ExecutionContext = system.dispatcher case class ExamplePerson(name: String) def parse(line: ByteString): ExamplePerson = ??? val response: HttpResponse = ??? // toStrict to enforce all data be loaded into memory from the connection val strictEntity: Future[HttpEntity.Strict] = response.entity.toStrict(3.seconds) // You can now use the `data` directly... val person1: Future[ExamplePerson] = strictEntity.map(e => parse(e.data)) // Though it is also still possible to use the streaming API to consume dataBytes, // even though now they're in memory: val person2: Future[ExamplePerson] = strictEntity flatMap { e => e.dataBytes .runFold(ByteString.empty) { case (acc, b) => acc ++ b } .map(parse) }
- Java
Integrating with Akka Streams
In some cases, it is necessary to process the results of a series of Akka HTTP calls as Akka Streams. In order to ensure that the HTTP Response Entity is consumed in a timely manner, the Akka HTTP stream for each request must be executed and completely consumed, then sent along for further processing.
Failing to account for this behavior can result in seemingly non-deterministic failures due to complex interactions between http and stream buffering. This manifests as errors such as the following:
Response entity was not subscribed after 1 second. Make sure to read the response `entity` body or call `entity.discardBytes()` on it -- in case you deal with `HttpResponse`, use the shortcut `response.discardEntityBytes()`.
This error indicates that the http response has been available for too long without being consumed. It can be partially worked around by increasing the subscription timeout, but you will still run the risk of running into network level timeouts and could still exceed the timeout under load so it’s best to resolve the issue properly such as in the examples below:
- Scala
-
source
import scala.concurrent.ExecutionContext import scala.concurrent.Future import akka.NotUsed import akka.actor.ActorSystem import akka.http.scaladsl.Http import akka.http.scaladsl.model._ import akka.util.ByteString import akka.stream.scaladsl.{ Flow, Sink, Source } implicit val system: ActorSystem = ActorSystem() implicit val dispatcher: ExecutionContext = system.dispatcher case class ExamplePerson(name: String) def parse(line: ByteString): Option[ExamplePerson] = line.utf8String.split(" ").headOption.map(ExamplePerson.apply) val requests: Source[HttpRequest, NotUsed] = Source .fromIterator(() => Range(0, 10).map(i => HttpRequest(uri = Uri(s"https://localhost/people/$i"))).iterator ) val processorFlow: Flow[Option[ExamplePerson], Int, NotUsed] = Flow[Option[ExamplePerson]].map(_.map(_.name.length).getOrElse(0)) // Run and completely consume a single akka http request def runRequest(req: HttpRequest): Future[Option[ExamplePerson]] = Http() .singleRequest(req) .flatMap { response => response.entity.dataBytes .runReduce(_ ++ _) .map(parse) } // Run each akka http flow to completion, then continue processing. You'll want to tune the `parallelism` // parameter to mapAsync -- higher values will create more cpu and memory load which may or may not positively // impact performance. requests .mapAsync(2)(runRequest) .via(processorFlow) .runWith(Sink.ignore)
- Java
Discarding the HTTP Response Entity (Client)
Sometimes when calling HTTP services we do not care about their response payload (e.g. all we care about is the response code), yet as explained above entity still has to be consumed in some way, otherwise we’ll be exerting back-pressure on the underlying TCP connection.
The discardEntityBytes
convenience method serves the purpose of easily discarding the entity if it has no purpose for us. It does so by piping the incoming bytes directly into an Sink.ignore
.
The two snippets below are equivalent, and work the same way on the server-side for incoming HTTP Requests:
- Scala
-
source
import scala.concurrent.ExecutionContext import akka.actor.ActorSystem import akka.http.scaladsl.model.HttpMessage.DiscardedEntity import akka.http.scaladsl.model._ implicit val system: ActorSystem = ActorSystem() implicit val dispatcher: ExecutionContext = system.dispatcher val response1: HttpResponse = ??? // obtained from an HTTP call (see examples below) val discarded: DiscardedEntity = response1.discardEntityBytes() discarded.future.onComplete { done => println("Entity discarded completely!") }
- Java
Or the equivalent low-level code achieving the same result:
- Scala
-
source
val response1: HttpResponse = ??? // obtained from an HTTP call (see examples below) val discardingComplete: Future[Done] = response1.entity.dataBytes.runWith(Sink.ignore) discardingComplete.onComplete(done => println("Entity discarded completely!"))
- Java
Server-Side handling of streaming HTTP Entities
HTTP Entities of a request are directly linked to Streams fed by the underlying TCP connection. Thus, if request entities remain not consumed, the server will back-pressure the connection, expecting the user-code to eventually decide what to do with the incoming data.
The most common use-case is to consume the request entity using directives such as BasicDirectives.extractDataBytes
. Some directives force an implicit toStrict
operation, such as entity(as[String])
.
Consuming the HTTP Request Entity (Server)
The simplest way of consuming the incoming request entity is to transform it into an actual domain object, for example by using the entity directive:
- Scala
-
source
import akka.actor.ActorSystem import akka.http.scaladsl.server.Directives._ import akka.http.scaladsl.marshallers.sprayjson.SprayJsonSupport._ import spray.json.DefaultJsonProtocol._ import spray.json.RootJsonFormat implicit val system: ActorSystem = ActorSystem() // needed for the future flatMap/onComplete in the end implicit val executionContext: ExecutionContext = system.dispatcher final case class Bid(userId: String, bid: Int) // these are from spray-json implicit val bidFormat: RootJsonFormat[Bid] = jsonFormat2(Bid.apply) val route = path("bid") { put { entity(as[Bid]) { (bid: Bid) => // incoming entity is fully consumed and converted into a Bid complete("The bid was: " + bid) } } }
- Java
You can also access the raw dataBytes
and run the underlying stream. For example, you could pipe the raw dataBytes
into a FileIO Sink
. The FileIO Sink
signals completion via a Future[IoResult]
once all the data has been written into the file:
- Scala
-
source
import akka.actor.ActorSystem import akka.stream.scaladsl.FileIO import akka.http.scaladsl.server.Directives._ import java.io.File implicit val system = ActorSystem() // needed for the future flatMap/onComplete in the end implicit val executionContext = system.dispatcher val route = (put & path("lines")) { withoutSizeLimit { extractDataBytes { bytes => val finishedWriting = bytes.runWith(FileIO.toPath(new File("/tmp/example.out").toPath)) // we only want to respond once the incoming data has been handled: onComplete(finishedWriting) { ioResult => complete("Finished writing data: " + ioResult) } } } }
- Java
Discarding the HTTP Request Entity (Server)
You may want to discard the uploaded entity. For example, depending on some validation (e.g. “is user authorized to upload files?”).
Please note that “discarding the HTTP Request Entity” means that the entire upload will proceed, even though you are not interested in the data being streamed to the server. This is useful if you are simply not interested in the entity.
In order to discard the dataBytes
explicitly you can invoke the discardEntityBytes
bytes of the incoming HttpRequest
:
- Scala
-
source
import akka.actor.ActorSystem import akka.http.scaladsl.server.Directives._ import akka.http.scaladsl.model.HttpRequest implicit val system = ActorSystem() // needed for the future flatMap/onComplete in the end implicit val executionContext = system.dispatcher val route = (put & path("lines")) { withoutSizeLimit { extractRequest { (r: HttpRequest) => val finishedWriting = r.discardEntityBytes().future // we only want to respond once the incoming data has been handled: onComplete(finishedWriting) { done => complete("Drained all data from connection... (" + done + ")") } } } }
- Java
A related concept is cancelling the incoming entity.dataBytes
stream. Cancellation results in Akka HTTP abruptly closing the connection from the Client. This may be useful when you detect that the given user should not be allowed to make any uploads at all, and you want to drop the connection (instead of reading and ignoring the incoming data). This can be done by attaching the incoming entity.dataBytes
to a Sink.cancelled()
which will cancel the entity stream, which in turn will cause the underlying connection to be shut-down by the server – effectively hard-aborting the incoming request:
- Scala
-
source
import akka.actor.ActorSystem import akka.stream.scaladsl.Sink import akka.http.scaladsl.server.Directives._ import akka.http.scaladsl.model.headers.Connection implicit val system = ActorSystem() // needed for the future flatMap/onComplete in the end implicit val executionContext = system.dispatcher val route = (put & path("lines")) { withoutSizeLimit { extractDataBytes { data => // Closing connections, method 1 (eager): // we deem this request as illegal, and close the connection right away: data.runWith(Sink.cancelled) // "brutally" closes the connection // Closing connections, method 2 (graceful): // consider draining connection and replying with `Connection: Close` header // if you want the client to close after this request/reply cycle instead: respondWithHeader(Connection("close")) complete(StatusCodes.Forbidden -> "Not allowed!") } } }
- Java
See also the Closing a connection section for an in-depth explanation on closing connection.
Pending: Automatic discarding of not used entities
Under certain conditions it is possible to detect an entity is very unlikely to be used by the user for a given request, and issue warnings or discard the entity automatically. This advanced feature has not been implemented yet, see the below note and issues for further discussion and ideas.
An advanced feature code named “auto draining” has been discussed and proposed for Akka HTTP, and we’re hoping to implement or help the community implement it.
You can read more about it in issue #183 as well as issue #117 ; as always, contributions are very welcome!