akka/akka-core

Clarification for typed GroupRouter comment on sending messages to unreachable cluster nodes

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#29,389 opened on Jul 16, 2020

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1 - triagedhelp wantedt:docst:typed

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Description

In the documentation for the GroupRouter, it states the following:

https://github.com/akka/akka/blob/6d290d66992429ed54af35e7354cb38770b449e6/akka-actor-typed/src/main/scala/akka/actor/typed/scaladsl/Routers.scala#L18

Taking a closer look at the implementation, when the GroupRouter receives the collection of possible routees from the receptionist, it uses an unapply method that filters based on reachable nodes (at least based on the associated comment indicates). As such, it seems the router tries, at least somewhat, to avoid unreachable nodes.

Is this a correct understanding of the GroupRouter's implementation?

New documentation could instead say:

The GroupRouter routes messages to the currently known reachable nodes, but there is always a chance that a node becomes unreachable as the router sends a message.

References for the GroupRouter's use of unapply, the definition of that unapply, and the associated reference to the serviceInstances method used in the unapply:

https://github.com/akka/akka/blob/6d290d66992429ed54af35e7354cb38770b449e6/akka-actor-typed/src/main/scala/akka/actor/typed/internal/routing/GroupRouterImpl.scala#L77-L84

https://github.com/akka/akka/blob/6d290d66992429ed54af35e7354cb38770b449e6/akka-actor-typed/src/main/scala/akka/actor/typed/receptionist/Receptionist.scala#L56-L75

https://github.com/akka/akka/blob/6d290d66992429ed54af35e7354cb38770b449e6/akka-actor-typed/src/main/scala/akka/actor/typed/receptionist/Receptionist.scala#L354-L368

Contributor guide

Research direction
Review the referenced code snippets and update the GroupRouter documentation comment to reflect that it filters reachable nodes.
Tech stack
scala
Domain
backend
Issue type
Documentation
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
1
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
Under 1 hour
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
None
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
95