SWIM Protocol

The Scalable Weakly Consistent Infection-style Process Group Membership (SWIM) Protocol is a group membership protocol based on "outsourced heartbeats"¹ used in distributed systems, first introduced by Abhinandan Das, Indranil Gupta and Ashish Motivala in 2002.²³ It is a hybrid algorithm which combines failure detection with group membership dissemination.

The protocol has two components, the Failure Detector Component and the Dissemination Component.

The Failure Detector Component functions as follows:

1. Every T' time units, each node (${\displaystyle N_{1}}$) sends a ping to random other node (${\displaystyle N_{2}}$) in its membership list.
2. If ${\displaystyle N_{1}}$ receives a response from ${\displaystyle N_{2}}$, ${\displaystyle N_{2}}$ is decided to be healthy and ${\displaystyle N_{1}}$ updates its "last heard from" timestamp for ${\displaystyle N_{2}}$ to be the current time.
3. If ${\displaystyle N_{1}}$ does not receive a response, ${\displaystyle N_{1}}$ contacts k other nodes on its list (${\displaystyle \{N_{3},...,N_{3+k}\}}$), and requests that they ping ${\displaystyle N_{2}}$.
4. If after T' units of time: if no successful response is received, ${\displaystyle N_{1}}$ marks ${\displaystyle N_{2}}$ as failed.

The Dissemination Component functions as follows:

• Upon ${\displaystyle N_{1}}$ detecting a failed node ${\displaystyle N_{2}}$, ${\displaystyle N_{1}}$ sends a multicast message to the rest of the nodes in its membership list, with information about the failed node.
• Voluntary requests for a node to enter/leave the group are also sent via multicast.

The protocol provides the following guarantees:

• Strong Completeness: Full completeness is guaranteed (e.g. the crash-failure of any node in the group is eventually detected by all live nodes).
• Detection Time: The expected value of detection time (from node failure to detection) is ${\displaystyle T'{\dot {}}{\frac {1}{1-e^{-q_{f}}}}}$, where ${\displaystyle T'}$ is the length of the protocol period, and ${\displaystyle q_{f}}$ is the fraction of non-faulty nodes in the group.³

The original SWIM paper lists the following extensions to make the protocol more robust:²

• Suspicion: Nodes that are unresponsive to ping messages are not initially marked as failed. Instead, they are marked as "suspicious"; nodes which discover a "suspicious" node still send a multicast to all other nodes including this mechanism. If a "suspicious" node responds to a ping before some time-out threshold, an "alive" message is sent via multicast to remove the "suspicious" label from the node.
• Infection-Style Dissemination: Instead of propagating node failure information via multicast, protocol messages are piggybacked on the ping messages used to determine node liveness. This is equivalent to gossip dissemination.
• Round-Robin Probe Target Selection: Instead of randomly picking a node to probe during each protocol time step, the protocol is modified so that each node performs a round-robin selection of probe target. This bounds the worst-case detection time of the protocol, without degrading the average detection time.