akka/akka-http

Access to websocket server materialized value

Open

#1167 opened on May 29, 2017

View on GitHub
 (6 comments) (3 reactions) (0 assignees)Scala (1,311 stars) (598 forks)batch import
1 - triagedhelp wantedt:coret:servert:websocket

Description

Websocket server methods do not allow accessing the materialized value of the flow it is given. They take a flow as argument and return the HTTP response to send back:

HttpResponse response = WebSocket.handleWebSocketRequestWith(request, greeterFlow);

The server takes care of materializing the flow and ignores both values. This prevents users from getting them, which can be problematic. For example if the flow source is a queue, one needs to get its materialized value in order to push to the queue. A workaround is to setup a callback on mapMaterializedValue to send the queue to whoever is interested in it:

Source.<Message>queue(5, OverflowStrategy.backpressure())
    .mapMaterializedValue(sourceQueue -> { /* send queue to whoever is interested */; return sourceQueue; });

But this is a workaround rather than a clean solution, as mapMaterializedValue is meant to transform the materialized value rather than broadcast it. The client has methods that return this materialized value:

Pair<CompletionStage<WebSocketUpgradeResponse>, SourceQueueWithComplete<Message>> pair =
        http.singleWebSocketRequest(WebSocketRequest.create(connect), flow, materializer);

It would be great if the server had methods for doing something similar. This might be tricky since contrary to the client, the server takes care of materializing the value after the response it sent.

(mailing list pointer: Graph materialized value of websocket upgrade)

Contributor guide

Tech stack
java
Domain
backend
Issue type
feature
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
4
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
3-5 days
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
mostly clear
Prerequisites
Akka HTTPAkka StreamsJava or Scala
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
30
Research direction
The issue requests server side methods that return the materialized value of the websocket flow, similar to the client side singleWebSocketRequest. The challenge is that the server materializes the flow after sending the response. Investigate the current WebSocket server API in akka http, particularly WebSocket.handleWebSocketRequestWith and related classes. Look at how the client handles materialized values (e.g., singleWebSocketRequest returns a Pair). Propose an API design that allows the server to expose the materialized value, perhaps by altering the method signature or introducing a new method that returns both the response and the materialized value. Consider using a Future or callback pattern. Reference the linked GitHub issue and the mailing list discussion for additional context.