akka/akka-core

Node never joins a cluster when a list of seed nodes consists exclusively of more than one "selfAddress"

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#25,820 opened on Oct 23, 2018

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Description

The issue occurs when the list of seed nodes contains multiple occurrences of selfAddress and there are no other addresses in that list. Here is a reproducible example:

val cluster = Cluster(system)
val seedNodes = Seq(cluster.selfAddress, cluster.selfAddress)
cluster.joinSeedNodes(seedNodes)

The root cause of the issue is the following:

  1. We successfully pass the condition here: https://github.com/akka/akka/blob/master/akka-cluster/src/main/scala/akka/cluster/ClusterDaemon.scala#L605-L607 and instantiate the FirstSeedNodeProcess actor to join the cluster.
  2. In the FirstSeedNodeProcess we filter out all seed nodes that match the selfAddress instance: https://github.com/akka/akka/blob/master/akka-cluster/src/main/scala/akka/cluster/ClusterDaemon.scala#L1418 . The remainingSeedNodesends up empty.
  3. As a result the FirstSeedNodeProcess actor never sends the InitJoin message.

Despite the minority of this issue, it may cause some unexpected behavior in case when the seed-node-timeout is high enough.

The quick solution for this issue is to use Set here: https://github.com/akka/akka/blob/master/akka-cluster/src/main/scala/akka/cluster/ClusterDaemon.scala#L599. And if possible pass this Set instances into the FirstSeedNodeProcess and JoinSeedNodeProcess constructors.

I'd be happy to submit a PR if the problem and the proposed solution make sense.

Contributor guide

Research direction
Reproduce the bug by creating a seed node list with duplicate selfAddress. Study the code in ClusterDaemon.scala around lines 599, 605-607, and 1418. Implement the fix by converting the seed node list to a Set before passing to FirstSeedNodeProcess and JoinSeedNodeProcess. Ensure that the join process works correctly with duplicates removed.
Tech stack
scala
Domain
backend
Issue type
Bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
2
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
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
GitScalaAkka
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
70