59 lines
3.3 KiB
Markdown
59 lines
3.3 KiB
Markdown
---
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layout: "docs"
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page_title: "Gossip Protocol"
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sidebar_current: "docs-internals-gossip"
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description: |-
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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.
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---
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# Gossip Protocol
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Consul uses a [gossip protocol](https://en.wikipedia.org/wiki/Gossip_protocol)
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to manage membership and broadcast messages to the cluster. All of this is provided
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through the use of the [Serf library](https://www.serf.io/). The gossip protocol
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used by Serf is based on
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["SWIM: Scalable Weakly-consistent Infection-style Process Group Membership Protocol"](http://www.cs.cornell.edu/~asdas/research/dsn02-swim.pdf),
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with a few minor adaptations. There are more details about [Serf's protocol here](https://www.serf.io/docs/internals/gossip.html).
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~> **Advanced Topic!** This page covers technical details of
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the internals of Consul. You don't need to know these details to effectively
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operate and use Consul. These details are documented here for those who wish
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to learn about them without having to go spelunking through the source code.
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## Gossip in Consul
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Consul makes use of two different gossip pools. We refer to each pool as the
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LAN or WAN pool respectively. Each datacenter Consul operates in has a LAN gossip pool
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containing all members of the datacenter, both clients and servers. The LAN pool is
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used for a few purposes. Membership information allows clients to automatically discover
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servers, reducing the amount of configuration needed. The distributed failure detection
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allows the work of failure detection to be shared by the entire cluster instead of
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concentrated on a few servers. Lastly, the gossip pool allows for reliable and fast
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event broadcasts for events like leader election.
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The WAN pool is globally unique, as all servers should participate in the WAN pool
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regardless of datacenter. Membership information provided by the WAN pool allows
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servers to perform cross datacenter requests. The integrated failure detection
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allows Consul to gracefully handle an entire datacenter losing connectivity, or just
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a single server in a remote datacenter.
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All of these features are provided by leveraging [Serf](https://www.serf.io/). It
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is used as an embedded library to provide these features. From a user perspective,
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this is not important, since the abstraction should be masked by Consul. It can be useful
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however as a developer to understand how this library is leveraged.
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<a name="lifeguard"></a>
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## Lifeguard Enhancements
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SWIM makes the assumption that the local node is healthy in the sense
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that soft real-time processing of packets is possible. However, in cases
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where the local node is experiencing CPU or network exhaustion this assumption
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can be violated. The result is that the `serfHealth` check status can
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occasionally flap, resulting in false monitoring alarms, adding noise to
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telemetry, and simply causing the overall cluster to waste CPU and network
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resources diagnosing a failure that may not truly exist.
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Lifeguard completely resolves this issue with novel enhancements to SWIM.
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Please see the [Serf's gossip protocol guide](https://www.serf.io/docs/internals/gossip.html#lifeguard)
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section on Lifeguard for more details.
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