feat: Down all when indirectly connected, #32862

akka-cluster/src/main/resources/reference.conf

@@ -423,6 +423,14 @@ akka.cluster.split-brain-resolver {
   # 4 seconds.
   down-all-when-unstable = on
 
+  # In a malfunctioning network there can be situations where nodes are observed as unreachable
+  # via some network links, but they are still indirectly connected via other nodes, i.e. it's
+  # not a clean network partition.
+  # By default it will keep fully connected nodes and down all the indirectly connected nodes,
+  # but when this flag is enabled it will down all nodes as precaution for the possible
+  # uncertainty that indirectly connected nodes can cause.
+  down-all-when-indirectly-connected = off
+
 }
 #//#split-brain-resolver
 

akka-cluster/src/main/scala/akka/cluster/sbr/SplitBrainResolver.scala

@@ -435,12 +435,16 @@ import akka.remote.artery.ThisActorSystemQuarantinedEvent
    */
   def actOnDecision(decision: Decision): Set[UniqueAddress] = {
     val nodesToDown =
-      try {
-        strategy.nodesToDown(decision)
-      } catch {
-        case e: IllegalStateException =>
-          log.warning(e.getMessage)
-          strategy.nodesToDown(DownAll)
+      if (settings.DownAllWhenIndirectlyConnected && decision.isIndirectlyConnected) {
+        strategy.nodesToDown(DownAll)
+      } else {
+        try {
+          strategy.nodesToDown(decision)
+        } catch {
+          case e: IllegalStateException =>
+            log.warning(e.getMessage)
+            strategy.nodesToDown(DownAll)
+        }
       }
 
     observeDecision(decision, nodesToDown, unreachableDataCenters)

akka-cluster/src/main/scala/akka/cluster/sbr/SplitBrainResolverSettings.scala

@@ -67,6 +67,9 @@ import akka.util.Helpers.Requiring
     }
   }
 
+  val DownAllWhenIndirectlyConnected: Boolean =
+    cc.getBoolean("down-all-when-indirectly-connected")
+
   // the individual sub-configs below should only be called when the strategy has been selected
 
   def keepMajorityRole: Option[String] = role(strategyConfig(KeepMajorityName))

akka-cluster/src/multi-jvm/scala/akka/cluster/sbr/DownAllIndirectlyConnected3NodeSpec.scala

@@ -0,0 +1,98 @@
+/*
+ * Copyright (C) 2020-2025 Lightbend Inc. <https://www.lightbend.com>
+ */
+
+package akka.cluster.sbr
+
+import scala.concurrent.duration._
+
+import com.typesafe.config.ConfigFactory
+
+import akka.cluster.Cluster
+import akka.cluster.MemberStatus
+import akka.cluster.MultiNodeClusterSpec
+import akka.remote.testkit.Direction
+import akka.remote.testkit.MultiNodeConfig
+
+object DownAllIndirectlyConnected3NodeSpec extends MultiNodeConfig {
+  val node1 = role("node1")
+  val node2 = role("node2")
+  val node3 = role("node3")
+
+  commonConfig(ConfigFactory.parseString("""
+    akka {
+      loglevel = INFO
+      cluster {
+        downing-provider-class = "akka.cluster.sbr.SplitBrainResolverProvider"
+        split-brain-resolver.active-strategy = keep-majority
+        split-brain-resolver.stable-after = 6s
+        split-brain-resolver.down-all-when-indirectly-connected = on
+
+        run-coordinated-shutdown-when-down = off
+      }
+
+      actor.provider = cluster
+
+      test.filter-leeway = 10s
+    }
+  """))
+
+  testTransport(on = true)
+}
+
+class DownAllIndirectlyConnected3NodeSpecMultiJvmNode1 extends DownAllIndirectlyConnected3NodeSpec
+class DownAllIndirectlyConnected3NodeSpecMultiJvmNode2 extends DownAllIndirectlyConnected3NodeSpec
+class DownAllIndirectlyConnected3NodeSpecMultiJvmNode3 extends DownAllIndirectlyConnected3NodeSpec
+
+class DownAllIndirectlyConnected3NodeSpec extends MultiNodeClusterSpec(DownAllIndirectlyConnected3NodeSpec) {
+  import DownAllIndirectlyConnected3NodeSpec._
+
+  "A 3-node cluster with down-all-when-indirectly-connected=on" should {
+    "down all when two unreachable but can talk via third" in {
+      val cluster = Cluster(system)
+
+      runOn(node1) {
+        cluster.join(cluster.selfAddress)
+      }
+      enterBarrier("node1 joined")
+      runOn(node2, node3) {
+        cluster.join(node(node1).address)
+      }
+      within(10.seconds) {
+        awaitAssert {
+          cluster.state.members.size should ===(3)
+          cluster.state.members.foreach {
+            _.status should ===(MemberStatus.Up)
+          }
+        }
+      }
+      enterBarrier("Cluster formed")
+
+      runOn(node1) {
+        testConductor.blackhole(node2, node3, Direction.Both).await
+      }
+      enterBarrier("Blackholed")
+
+      within(10.seconds) {
+        awaitAssert {
+          runOn(node3) {
+            cluster.state.unreachable.map(_.address) should ===(Set(node(node2).address))
+          }
+          runOn(node2) {
+            cluster.state.unreachable.map(_.address) should ===(Set(node(node3).address))
+          }
+          runOn(node1) {
+            cluster.state.unreachable.map(_.address) should ===(Set(node(node3).address, node(node2).address))
+          }
+        }
+      }
+      enterBarrier("unreachable")
+
+      // all downed
+      awaitCond(cluster.isTerminated, max = 15.seconds)
+
+      enterBarrier("done")
+    }
+  }
+
+}

akka-cluster/src/test/scala/akka/cluster/sbr/SplitBrainResolverSpec.scala

@@ -590,7 +590,6 @@ class SplitBrainResolverSpec
       // from side2 of the partition, majority
       assertDowningSide(side2, Set(memberA, memberB, memberC, memberD, memberE, memberF, memberG))
     }
-
   }
 
   "KeepOldest" must {

akka-docs/src/main/paradox/split-brain-resolver.md

@@ -392,6 +392,17 @@ If there is a combination of indirectly connected nodes and a clean network part
 above decision with the ordinary decision, e.g. keep majority, after excluding suspicious failure detection
 observations.
 
+### Down all when indirectly connected
+
+Addition precaution can be enabled for the uncertainty that may follow from indirectly connected nodes
+by enabling downing of all nodes if indirectly connected nodes are detected:
+
+```
+akka.cluster.split-brain-resolver {
+  down-all-when-indirectly-connected = on
+}
+```
+
 ## Down all when unstable
 
 When reachability observations by the failure detector are changed the SBR decisions