Cluster Specification

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.

Leader

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.

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

Transitions

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

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.