Cluster Specification

Cluster Specification


This document describes the design concepts of the clustering. It is divided into two parts, where the first part describes what is currently implemented and the second part describes what is planned as future enhancements/additions. References to unimplemented parts have been marked with the footnote [*]

The Current Cluster


Akka Cluster provides a fault-tolerant decentralized peer-to-peer based cluster membership service with no single point of failure or single point of bottleneck. It does this using gossip protocols and an automatic failure detector.


A logical member of a cluster. There could be multiple nodes on a physical machine. Defined by a hostname:port:uid tuple.
A set of nodes joined together through the membership service.
A single node in the cluster that acts as the leader. Managing cluster convergence, partitions [*], fail-over [*], rebalancing [*] etc.


A cluster is made up of a set of member nodes. The identifier for each node is a hostname:port:uid tuple. An Akka application can be distributed over a cluster with each node hosting some part of the application. Cluster membership and partitioning [*] of the application are decoupled. A node could be a member of a cluster without hosting any actors. Joining a cluster is initiated by issuing a Join command to one of the nodes in the cluster to join.

The node identifier internally also contains a UID that uniquely identifies this actor system instance at that hostname:port. Akka uses the UID to be able to reliably trigger remote death watch. This means that the same actor system can never join a cluster again once it's been removed from that cluster. To re-join an actor system with the same hostname:port to a cluster you have to stop the actor system and start a new one with the same hotname:port which will then receive a different UID.

The cluster membership state is a specialized CRDT, which means that it has a monotonic merge function. When concurrent changes occur on different nodes the updates can always be merged and converge to the same end result.


The cluster membership used in Akka is based on Amazon's Dynamo system and particularly the approach taken in Basho's' Riak distributed database. Cluster membership is communicated using a Gossip Protocol, where the current state of the cluster is gossiped randomly through the cluster, with preference to members that have not seen the latest version.

Vector Clocks

Vector clocks are a type of data structure and algorithm for generating a partial ordering of events in a distributed system and detecting causality violations.

We use vector clocks to reconcile and merge differences in cluster state during gossiping. A vector clock is a set of (node, counter) pairs. Each update to the cluster state has an accompanying update to the vector clock.

Gossip Convergence

Information about the cluster converges locally at a node at certain points in time. This is when a node can prove that the cluster state he is observing has been observed by all other nodes in the cluster. Convergence is implemented by passing a set of nodes that have seen current state version during gossip. This information is referred to as the seen set in the gossip overview. When all nodes are included in the seen set there is convergence.

Gossip convergence cannot occur while any nodes are unreachable. The nodes need to become reachable again, or moved to the down and removed states (see the Membership Lifecycle section below). This only blocks the leader from performing its cluster membership management and does not influence the application running on top of the cluster. For example this means that during a network partition it is not possible to add more nodes to the cluster. The nodes can join, but they will not be moved to the up state until the partition has healed or the unreachable nodes have been downed.

Failure Detector

The failure detector is responsible for trying to detect if a node is unreachable from the rest of the cluster. For this we are using an implementation of The Phi Accrual Failure Detector by Hayashibara et al.

An accrual failure detector decouple monitoring and interpretation. That makes them applicable to a wider area of scenarios and more adequate to build generic failure detection services. The idea is that it is keeping a history of failure statistics, calculated from heartbeats received from other nodes, and is trying to do educated guesses by taking multiple factors, and how they accumulate over time, into account in order to come up with a better guess if a specific node is up or down. Rather than just answering "yes" or "no" to the question "is the node down?" it returns a phi value representing the likelihood that the node is down.

The threshold that is the basis for the calculation is configurable by the user. A low threshold is prone to generate many wrong suspicions but ensures a quick detection in the event of a real crash. Conversely, a high threshold generates fewer mistakes but needs more time to detect actual crashes. The default threshold is 8 and is appropriate for most situations. However in cloud environments, such as Amazon EC2, the value could be increased to 12 in order to account for network issues that sometimes occur on such platforms.

In a cluster each node is monitored by a few (default maximum 5) other nodes, and when any of these detects the node as unreachable that information will spread to the rest of the cluster through the gossip. In other words, only one node needs to mark a node unreachable to have the rest of the cluster mark that node unreachable.

The nodes to monitor are picked out of neighbors in a hashed ordered node ring. This is to increase the likelihood to monitor across racks and data centers, but the order is the same on all nodes, which ensures full coverage.

Heartbeats are sent out every second and every heartbeat is performed in a request/reply handshake with the replies used as input to the failure detector.

The failure detector will also detect if the node becomes reachable again. When all nodes that monitored the unreachable node detects it as reachable again the cluster, after gossip dissemination, will consider it as reachable.

If system messages cannot be delivered to a node it will be quarantined and then it cannot come back from unreachable. This can happen if the there are too many unacknowledged system messages (e.g. watch, Terminated, remote actor deployment, failures of actors supervised by remote parent). Then the node needs to be moved to the down or removed states (see the Membership Lifecycle section below) and the actor system must be restarted before it can join the cluster again.


After gossip convergence a leader for the cluster can be determined. There is no leader election process, the leader can always be recognised deterministically by any node whenever there is gossip convergence. The leader is just a role, any node can be the leader and it can change between convergence rounds. The leader is simply the first node in sorted order that is able to take the leadership role, where the preferred member states for a leader are up and leaving (see the Membership Lifecycle section below for more information about member states).

The role of the leader is to shift members in and out of the cluster, changing joining members to the up state or exiting members to the removed state. Currently leader actions are only triggered by receiving a new cluster state with gossip convergence.

The leader also has the power, if configured so, to "auto-down" a node that according to the Failure Detector is considered unreachable. This means setting the unreachable node status to down automatically after a configured time of unreachability.

Seed Nodes

The seed nodes are configured contact points for new nodes joining the cluster. When a new node is started it sends a message to all seed nodes and then sends a join command to the seed node that answers first.

The seed nodes configuration value does not have any influence on the running cluster itself, it is only relevant for new nodes joining the cluster as it helps them to find contact points to send the join command to; a new member can send this command to any current member of the cluster, not only to the seed nodes.

Gossip Protocol

A variation of push-pull gossip is used to reduce the amount of gossip information sent around the cluster. In push-pull gossip a digest is sent representing current versions but not actual values; the recipient of the gossip can then send back any values for which it has newer versions and also request values for which it has outdated versions. Akka uses a single shared state with a vector clock for versioning, so the variant of push-pull gossip used in Akka makes use of this version to only push the actual state as needed.

Periodically, the default is every 1 second, each node chooses another random node to initiate a round of gossip with. If less than ½ of the nodes resides in the seen set (have seen the new state) then the cluster gossips 3 times instead of once every second. This adjusted gossip interval is a way to speed up the convergence process in the early dissemination phase after a state change.

The choice of node to gossip with is random but it is biased to towards nodes that might not have seen the current state version. During each round of gossip exchange when no convergence it uses a probability of 0.8 (configurable) to gossip to a node not part of the seen set, i.e. that probably has an older version of the state. Otherwise gossip to any random live node.

This biased selection is a way to speed up the convergence process in the late dissemination phase after a state change.

For clusters larger than 400 nodes (configurable, and suggested by empirical evidence) the 0.8 probability is gradually reduced to avoid overwhelming single stragglers with too many concurrent gossip requests. The gossip receiver also has a mechanism to protect itself from too many simultaneous gossip messages by dropping messages that have been enqueued in the mailbox for too long time.

While the cluster is in a converged state the gossiper only sends a small gossip status message containing the gossip version to the chosen node. As soon as there is a change to the cluster (meaning non-convergence) then it goes back to biased gossip again.

The recipient of the gossip state or the gossip status can use the gossip version (vector clock) to determine whether:

  1. it has a newer version of the gossip state, in which case it sends that back to the gossiper
  2. it has an outdated version of the state, in which case the recipient requests the current state from the gossiper by sending back its version of the gossip state
  3. it has conflicting gossip versions, in which case the different versions are merged and sent back

If the recipient and the gossip have the same version then the gossip state is not sent or requested.

The periodic nature of the gossip has a nice batching effect of state changes, e.g. joining several nodes quickly after each other to one node will result in only one state change to be spread to other members in the cluster.

The gossip messages are serialized with protobuf and also gzipped to reduce payload size.

Membership Lifecycle

A node begins in the joining state. Once all nodes have seen that the new node is joining (through gossip convergence) the leader will set the member state to up.

If a node is leaving the cluster in a safe, expected manner then it switches to the leaving state. Once the leader sees the convergence on the node in the leaving state, the leader will then move it to exiting. Once all nodes have seen the exiting state (convergence) the leader will remove the node from the cluster, marking it as removed.

If a node is unreachable then gossip convergence is not possible and therefore any leader actions are also not possible (for instance, allowing a node to become a part of the cluster). To be able to move forward the state of the unreachable nodes must be changed. It must become reachable again or marked as down. If the node is to join the cluster again the actor system must be restarted and go through the joining process again. The cluster can, through the leader, also auto-down a node after a configured time of unreachability..


If you have auto-down enabled and the failure detector triggers, you can over time end up with a lot of single node clusters if you don't put measures in place to shut down nodes that have become unreachable. This follows from the fact that the unreachable node will likely see the rest of the cluster as unreachable, become its own leader and form its own cluster.

State Diagram for the Member States
Member States
  • joining

    transient state when joining a cluster

  • up

    normal operating state

  • leaving / exiting

    states during graceful removal

  • down

    marked as down (no longer part of cluster decisions)

  • removed

    tombstone state (no longer a member)

User Actions
  • join

    join a single node to a cluster - can be explicit or automatic on startup if a node to join have been specified in the configuration

  • leave

    tell a node to leave the cluster gracefully

  • down

    mark a node as down

Leader Actions

The leader has the following duties:

  • shifting members in and out of the cluster
    • joining -> up
    • exiting -> removed
Failure Detection and Unreachability
  • fd*

    the failure detector of one of the monitoring nodes has triggered causing the monitored node to be marked as unreachable

  • unreachable*

    unreachable is not a real member states but more of a flag in addition to the state signaling that the cluster is unable to talk to this node, after beeing unreachable the failure detector may detect it as reachable again and thereby remove the flag

Future Cluster Enhancements and Additions


In addition to membership also provide automatic partitioning [*], handoff [*], and cluster rebalancing [*] of actors.

Additional Terms

These additional terms are used in this section.

partition [*]
An actor or subtree of actors in the Akka application that is distributed within the cluster.
partition point [*]
The actor at the head of a partition. The point around which a partition is formed.
partition path [*]
Also referred to as the actor address. Has the format actor1/actor2/actor3
instance count [*]
The number of instances of a partition in the cluster. Also referred to as the N-value of the partition.
instance node [*]
A node that an actor instance is assigned to.
partition table [*]
A mapping from partition path to a set of instance nodes (where the nodes are referred to by the ordinal position given the nodes in sorted order).



Actor partitioning is not implemented yet [*].

Each partition (an actor or actor subtree) in the actor system is assigned to a set of nodes in the cluster. The actor at the head of the partition is referred to as the partition point. The mapping from partition path (actor address of the format "a/b/c") to instance nodes is stored in the partition table and is maintained as part of the cluster state through the gossip protocol. The partition table is only updated by the leader node. Currently the only possible partition points are routed actors.

Routed actors can have an instance count greater than one. The instance count is also referred to as the N-value. If the N-value is greater than one then a set of instance nodes will be given in the partition table.

Note that in the first implementation there may be a restriction such that only top-level partitions are possible (the highest possible partition points are used and sub-partitioning is not allowed). Still to be explored in more detail.

The cluster leader determines the current instance count for a partition based on two axes: fault-tolerance and scaling.

Fault-tolerance determines a minimum number of instances for a routed actor (allowing N-1 nodes to crash while still maintaining at least one running actor instance). The user can specify a function from current number of nodes to the number of acceptable node failures: n: Int => f: Int where f < n.

Scaling reflects the number of instances needed to maintain good throughput and is influenced by metrics from the system, particularly a history of mailbox size, CPU load, and GC percentages. It may also be possible to accept scaling hints from the user that indicate expected load.

The balancing of partitions can be determined in a very simple way in the first implementation, where the overlap of partitions is minimized. Partitions are spread over the cluster ring in a circular fashion, with each instance node in the first available space. For example, given a cluster with ten nodes and three partitions, A, B, and C, having N-values of 4, 3, and 5; partition A would have instances on nodes 1-4; partition B would have instances on nodes 5-7; partition C would have instances on nodes 8-10 and 1-2. The only overlap is on nodes 1 and 2.

The distribution of partitions is not limited, however, to having instances on adjacent nodes in the sorted ring order. Each instance can be assigned to any node and the more advanced load balancing algorithms will make use of this. The partition table contains a mapping from path to instance nodes. The partitioning for the above example would be:

A -> { 1, 2, 3, 4 }
B -> { 5, 6, 7 }
C -> { 8, 9, 10, 1, 2 }

If 5 new nodes join the cluster and in sorted order these nodes appear after the current nodes 2, 4, 5, 7, and 8, then the partition table could be updated to the following, with all instances on the same physical nodes as before:

A -> { 1, 2, 4, 5 }
B -> { 7, 9, 10 }
C -> { 12, 14, 15, 1, 2 }

When rebalancing is required the leader will schedule handoffs, gossiping a set of pending changes, and when each change is complete the leader will update the partition table.

Additional Leader Responsibilities

After moving a member from joining to up, the leader can start assigning partitions [*] to the new node, and when a node is leaving the leader will reassign partitions [*] across the cluster (it is possible for a leaving node to itself be the leader). When all partition handoff [*] has completed then the node will change to the exiting state.

On convergence the leader can schedule rebalancing across the cluster, but it may also be possible for the user to explicitly rebalance the cluster by specifying migrations [*], or to rebalance [*] the cluster automatically based on metrics from member nodes. Metrics may be spread using the gossip protocol or possibly more efficiently using a random chord method, where the leader contacts several random nodes around the cluster ring and each contacted node gathers information from their immediate neighbours, giving a random sampling of load information.


Handoff for an actor-based system is different than for a data-based system. The most important point is that message ordering (from a given node to a given actor instance) may need to be maintained. If an actor is a singleton actor (only one instance possible throughout the cluster) then the cluster may also need to assure that there is only one such actor active at any one time. Both of these situations can be handled by forwarding and buffering messages during transitions.

A graceful handoff (one where the previous host node is up and running during the handoff), given a previous host node N1, a new host node N2, and an actor partition A to be migrated from N1 to N2, has this general structure:

  1. the leader sets a pending change for N1 to handoff A to N2
  2. N1 notices the pending change and sends an initialization message to N2
  3. in response N2 creates A and sends back a ready message
  4. after receiving the ready message N1 marks the change as complete and shuts down A
  5. the leader sees the migration is complete and updates the partition table
  6. all nodes eventually see the new partitioning and use N2

There are transition times in the handoff process where different approaches can be used to give different guarantees.

Migration Transition

The first transition starts when N1 initiates the moving of A and ends when N1 receives the ready message, and is referred to as the migration transition.

The first question is; during the migration transition, should:

  • N1 continue to process messages for A?
  • Or is it important that no messages for A are processed on N1 once migration begins?

If it is okay for the previous host node N1 to process messages during migration then there is nothing that needs to be done at this point.

If no messages are to be processed on the previous host node during migration then there are two possibilities: the messages are forwarded to the new host and buffered until the actor is ready, or the messages are simply dropped by terminating the actor and allowing the normal dead letter process to be used.

Update Transition

The second transition begins when the migration is marked as complete and ends when all nodes have the updated partition table (when all nodes will use N2 as the host for A, i.e. we have convergence) and is referred to as the update transition.

Once the update transition begins N1 can forward any messages it receives for A to the new host N2. The question is whether or not message ordering needs to be preserved. If messages sent to the previous host node N1 are being forwarded, then it is possible that a message sent to N1 could be forwarded after a direct message to the new host N2, breaking message ordering from a client to actor A.

In this situation N2 can keep a buffer for messages per sending node. Each buffer is flushed and removed when an acknowledgement (ack) message has been received. When each node in the cluster sees the partition update it first sends an ack message to the previous host node N1 before beginning to use N2 as the new host for A. Any messages sent from the client node directly to N2 will be buffered. N1 can count down the number of acks to determine when no more forwarding is needed. The ack message from any node will always follow any other messages sent to N1. When N1 receives the ack message it also forwards it to N2 and again this ack message will follow any other messages already forwarded for A. When N2 receives an ack message, the buffer for the sending node can be flushed and removed. Any subsequent messages from this sending node can be queued normally. Once all nodes in the cluster have acknowledged the partition change and N2 has cleared all buffers, the handoff is complete and message ordering has been preserved. In practice the buffers should remain small as it is only those messages sent directly to N2 before the acknowledgement has been forwarded that will be buffered.

Graceful Handoff

A more complete process for graceful handoff would be:

  1. the leader sets a pending change for N1 to handoff A to N2
  2. N1 notices the pending change and sends an initialization message to N2. Options:
    1. keep A on N1 active and continuing processing messages as normal
    2. N1 forwards all messages for A to N2
    3. N1 drops all messages for A (terminate A with messages becoming dead letters)
  3. in response N2 creates A and sends back a ready message. Options:
    1. N2 simply processes messages for A as normal
    2. N2 creates a buffer per sending node for A. Each buffer is opened (flushed and removed) when an acknowledgement for the sending node has been received (via N1)
  4. after receiving the ready message N1 marks the change as complete. Options:
    1. N1 forwards all messages for A to N2 during the update transition
    2. N1 drops all messages for A (terminate A with messages becoming dead letters)
  5. the leader sees the migration is complete and updates the partition table
  6. all nodes eventually see the new partitioning and use N2
    1. each node sends an acknowledgement message to N1
    2. when N1 receives the acknowledgement it can count down the pending acknowledgements and remove forwarding when complete
    3. when N2 receives the acknowledgement it can open the buffer for the sending node (if buffers are used)

The default approach is to take options 2a, 3a, and 4a - allowing A on N1 to continue processing messages during migration and then forwarding any messages during the update transition. This assumes stateless actors that do not have a dependency on message ordering from any given source.

  • If an actor has persistent (durable) state then nothing needs to be done, other than migrating the actor.
  • If message ordering needs to be maintained during the update transition then option 3b can be used, creating buffers per sending node.
  • If the actors are robust to message send failures then the dropping messages approach can be used (with no forwarding or buffering needed).
  • If an actor is a singleton (only one instance possible throughout the cluster) and state is transferred during the migration initialization, then options 2b and 3b would be required.

Stateful Actor Replication


Stateful actor replication is not implemented yet [*].

[*] Not Implemented Yet

  • Actor partitioning
  • Actor handoff
  • Actor rebalancing
  • Stateful actor replication