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Lithium - A split-brain resolver for Akka-Cluster
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README.md

Build Status

Lithium - A Split-Brain Resolver for Akka-Cluster

When a cluster member becomes unreachable the leader cannot perform its duties anymore. Members cannot change state, singletons cannot be moved to a different member. In such situations the cluster administrator has to manually down members so the leader can continue its duties. This library provides a few strategies that will automatically down members without needing any intervention.

Getting started

resolvers += Resolver.bintrayRepo("swissborg", "maven")
libraryDependencies += "com.swissborg" %% "lithium" % "0.9.5"

Stable-After

The strategy is run only after the cluster has been stable for a configured amount of time. The stability is affected by members changing state and failure detections. However, the stability is not affected by members becoming "joining" or "weakly-up" as they are not counted in decisions. The reason for this is that a node can move to those states during network partitions and as result potentially not be seen by all the partitions.

The stable-after duration should be chosen longer than the time it takes to gossip cluster membership changes. Additionally, since a partition cannot communicate together, the duration should be large enough so that persistent actor have the time to stop in one partition before being instantiated somewhere on the surviving partition.

Down-when-unstable

The down-all-when-unstable flag when not set to off will down the partition if the cluster has been unstable for longer than stable-after + 3/4 * stable-after. This stops the situation where a persistent actor is started in the surviving partition before being stopped in its original partition because the stable-after timeout in the original is never hit. The duration can be overriden and should be chosen as less than 2 * stable-after. It can also be disabled by setting it to off but this is not recommended.

akka.cluster {
  downing-provider-class = "com.swissborg.lithium.DowningProviderImpl"
}

com.swissborg.lithium {
  # The name of the strategy to use for split-brain resolution.
  # Available: static-quorum, keep-majority, keep-referee, keep-oldest.
  active-strategy = null

  # Duration during which the cluster must be stable before taking
  # action on the network-partition. The duration must chose large
  # enough to allow for membership events to be gossiped and persistent
  # actor to be migrated.
  stable-after = 30s
  
  # Down the partition if it has been unstable for too long. If the cluster
  # wait too long before downing itself persistent actors might already be
  # restarted on another partition, leading to two instances of the same
  # persistent actor.
  # It is by default derived from 'stable-after' to be 'stable-after' + 3/4 'stable-after'.
  # If overriden, it must be less than 2 * 'stable-after'. To disable the downing, set 
  # it to 'off', however this is not recommended.
  #down-all-when-unstable = on
}

Strategies

Static-Quorum

Keeps the partition that contains at least quorum-size nodes and downs all the other partitions.

The quorum-size should be chosen as strictly more than half the nodes in the cluster, only counting nodes with the configured role.

In a cluster with 5 nodes, in the case of a split of two partitions. One with 3 nodes and the other with 2 nodes, the 3 node partition will survive. With only 3 nodes still in the cluster there is the risk that the cluster will downed while fixing the next failure. Hence, new nodes should be added to the cluster after resolutions.

The cluster can down itself in a few cases:

  • When the cluster grows too large it will down itself to avoid creating a split-brain when multiple partitions form a quorum.
  • When multiple partitions occur and none of them forms a quorum.
  • When a large amount of nodes crash and not enough nodes remain to form a quorum.

This strategy is useful when the size of the cluster is fixed or the number of nodes with the given role is fixed.

By design this strategy cannot lead to a split-brain.

Configuration

com.swissborg.lithium {
  active-strategy = "static-quorum"
  static-quorum {
    # Minimum number of nodes in the surviving partition.
    quorum-size = null
    
    # Only take in account nodes with this role.
    role = ""
  }
}

Keep-Majority

Keeps the side of partition that contains the majority of nodes and downs the other partitions. In the case where they have the same size the one containing the member with the lowest address is kept.

The cluster can down itself in a few cases:

  • When multiple partitions occur and none of them forms a majority.
  • When a majority of nodes crash, the remaining nodes do not form a majority and down themselves.

This strategy is useful when the size of the cluster is dynamic

This strategy can on rare occasions lead to a split-brain when a network partition occurs during a membership dissemination and a partition ends up seeing certain nodes as Up while others do not.

Configuration

com.swissborg.lithium {
  active-strategy = "keep-majority"
  keep-majority {
    # Only take in account nodes with this role.
    role = ""
  }
}

Keep-Referee

Keeps the partition that can contains the specified member and downs the other partitions.

The cluster can down itself in a few cases:

  • When the referee crashes.
  • When the number of nodes is below down-all-if-less-than-nodes.

This strategy is useful when the cluster has a node without which it cannot run.

By design this strategy cannot lead to a split-brain.

Configuration

com.swissborg.lithium {
  active-strategy = "keep-referee"
  keep-referee {
    # Address of the member in the format "akka.tcp://system@host:port"
    referee = null
    
    # Minimum number of nodes in the surviving partition.
    down-all-if-less-than-nodes = 1
  }
}

Keep-Oldest

Keeps the partition that can contains the oldest member in the cluster and downs the other partitions.

By enabling down-if-alone the other partitions will down the oldest node if it is cut-off from the rest. This can prevent the scenario where you have a 100 node cluster, the oldest node become unreachable. 99 nodes will be downed and leave the cluster heavily crippled.

When down-if-alone is enabled, multiple partitions occur, and the oldest node is alone the cluster will down itself as a partition cannot know if the unreachable nodes are in a single partition or form multiple ones. Hence, it then assumes that there is only one other partition and downs itself. On the other side, the oldest node (who is alone) knows it is alone and will down itself.

This strategy can on rare occasions lead to a split-brain when a network partition occurs during a membership dissemination and a partition ends up seeing the oldest nodes as Removed and selecting the second-oldest as the oldest.

Configuration

com.swissborg.lithium {
  active-strategy = "keep-oldest"
  keep-oldest {
    # Down the oldest member when alone.
    down-if-alone = no
    
    # Only take in account nodes with this role.
    role = ""
  }
}

Indirectly-Connected Members

An indirectly-connected member is one that has been detected or has detected another member as unreachable but that is still connected to via some other nodes.

These nodes will always be downed in combination with the ones downed by the configured strategy.

Indirectly connected nodes are downed as they sit on the intersection of two partitions. As result both partitions will assume it is in theirs leading to all sorts of trouble. By downing them the partitions become clean partitions that do not overlap.

License

Lithium is Open Source and available under the Apache 2 License.

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