memberlist is a Go library that manages cluster membership and member failure detection using a gossip based protocol.
The use cases for such a library are far-reaching: all distributed systems require membership, and memberlist is a re-usable solution to managing cluster membership and node failure detection.
memberlist is eventually consistent but converges quickly on average. The speed at which it converges can be heavily tuned via various knobs on the protocol. Node failures are detected and network partitions are partially tolerated by attempting to communicate to potentially dead nodes through multiple routes.
If you wish to build memberlist you'll need Go version 1.2+ installed.
Please check your installation with:
go version
Memberlist is surprisingly simple to use. An example is shown below:
/* Create the initial memberlist from a safe configuration. Please reference the godoc for other default config types. http://godoc.org/github.com/hashicorp/memberlist#Config */ list, err := memberlist.Create(memberlist.DefaultLocalConfig()) if err != nil { panic("Failed to create memberlist: " + err.Error()) } // Join an existing cluster by specifying at least one known member. n, err := list.Join([]string{"1.2.3.4"}) if err != nil { panic("Failed to join cluster: " + err.Error()) } // Ask for members of the cluster for _, member := range list.Members() { fmt.Printf("Member: %s %s\n", member.Name, member.Addr) } // Continue doing whatever you need, memberlist will maintain membership // information in the background. Delegates can be used for receiving // events when members join or leave.
The most difficult part of memberlist is configuring it since it has many available knobs in order to tune state propagation delay and convergence times. Memberlist provides a default configuration that offers a good starting point, but errs on the side of caution, choosing values that are optimized for higher convergence at the cost of higher bandwidth usage.
For complete documentation, see the associated Godoc.
memberlist is based on “SWIM: Scalable Weakly-consistent Infection-style Process Group Membership Protocol”, with a few minor adaptations, mostly to increase propagation speed and convergence rate.
A high level overview of the memberlist protocol (based on SWIM) is described below, but for details please read the full SWIM paper followed by the memberlist source. We welcome any questions related to the protocol on our issue tracker.
memberlist begins by joining an existing cluster or starting a new cluster. If starting a new cluster, additional nodes are expected to join it. New nodes in an existing cluster must be given the address of at least one existing member in order to join the cluster. The new member does a full state sync with the existing member over TCP and begins gossiping its existence to the cluster.
Gossip is done over UDP to a with a configurable but fixed fanout and interval. This ensures that network usage is constant with regards to number of nodes, as opposed to exponential growth that can occur with traditional heartbeat mechanisms. Complete state exchanges with a random node are done periodically over TCP, but much less often than gossip messages. This increases the likelihood that the membership list converges properly since the full state is exchanged and merged. The interval between full state exchanges is configurable or can be disabled entirely.
Failure detection is done by periodic random probing using a configurable interval. If the node fails to ack within a reasonable time (typically some multiple of RTT), then an indirect probe as well as a direct TCP probe are attempted. An indirect probe asks a configurable number of random nodes to probe the same node, in case there are network issues causing our own node to fail the probe. The direct TCP probe is used to help identify the common situation where networking is misconfigured to allow TCP but not UDP. Without the TCP probe, a UDP-isolated node would think all other nodes were suspect and could cause churn in the cluster when it attempts a TCP-based state exchange with another node. It is not desirable to operate with only TCP connectivity because convergence will be much slower, but it is enabled so that memberlist can detect this situation and alert operators.
If both our probe, the indirect probes, and the direct TCP probe fail within a configurable time, then the node is marked “suspicious” and this knowledge is gossiped to the cluster. A suspicious node is still considered a member of cluster. If the suspect member of the cluster does not dispute the suspicion within a configurable period of time, the node is finally considered dead, and this state is then gossiped to the cluster.
This is a brief and incomplete description of the protocol. For a better idea, please read the SWIM paper in its entirety, along with the memberlist source code.
As mentioned earlier, the memberlist protocol is based on SWIM but includes minor changes, mostly to increase propagation speed and convergence rates.
The changes from SWIM are noted here:
memberlist does a full state sync over TCP periodically. SWIM only propagates changes over gossip. While both eventually reach convergence, the full state sync increases the likelihood that nodes are fully converged more quickly, at the expense of more bandwidth usage. This feature can be totally disabled if you wish.
memberlist has a dedicated gossip layer separate from the failure detection protocol. SWIM only piggybacks gossip messages on top of probe/ack messages. memberlist also piggybacks gossip messages on top of probe/ack messages, but also will periodically send out dedicated gossip messages on their own. This feature lets you have a higher gossip rate (for example once per 200ms) and a slower failure detection rate (such as once per second), resulting in overall faster convergence rates and data propagation speeds. This feature can be totally disabed as well, if you wish.
memberlist stores around the state of dead nodes for a set amount of time, so that when full syncs are requested, the requester also receives information about dead nodes. Because SWIM doesn't do full syncs, SWIM deletes dead node state immediately upon learning that the node is dead. This change again helps the cluster converge more quickly.