Consul uses a gossip protocol to manage membership and broadcast messages to the cluster. All of this is provided through the use of the Serf library. The gossip protocol used by Serf is based on "SWIM: Scalable Weakly-consistent Infection-style Process Group Membership Protocol", with a few minor adaptations. There are more details about Serf's protocol here.
»Gossip in Consul
Consul makes use of two different gossip pools. We refer to each pool as the LAN or WAN pool respectively. Each datacenter Consul operates in has a LAN gossip pool containing all members of the datacenter, both clients and servers. The LAN pool is used for a few purposes. Membership information allows clients to automatically discover servers, reducing the amount of configuration needed. The distributed failure detection allows the work of failure detection to be shared by the entire cluster instead of concentrated on a few servers. Lastly, the gossip pool allows for reliable and fast event broadcasts.
The WAN pool is globally unique, as all servers should participate in the WAN pool regardless of datacenter. Membership information provided by the WAN pool allows servers to perform cross datacenter requests. The integrated failure detection allows Consul to gracefully handle an entire datacenter losing connectivity, or just a single server in a remote datacenter.
All of these features are provided by leveraging Serf. It is used as an embedded library to provide these features. From a user perspective, this is not important, since the abstraction should be masked by Consul. It can be useful however as a developer to understand how this library is leveraged.
SWIM makes the assumption that the local node is healthy in the sense
that soft real-time processing of packets is possible. However, in cases
where the local node is experiencing CPU or network exhaustion this assumption
can be violated. The result is that the
serfHealth check status can
occasionally flap, resulting in false monitoring alarms, adding noise to
telemetry, and simply causing the overall cluster to waste CPU and network
resources diagnosing a failure that may not truly exist.
Lifeguard completely resolves this issue with novel enhancements to SWIM.
For more details about Lifeguard, please see the Making Gossip More Robust with Lifeguard blog post, which provides a high level overview of the HashiCorp Research paper Lifeguard : SWIM-ing with Situational Awareness. The Serf gossip protocol guide also provides some lower-level details about the gossip protocol and Lifeguard.