This page collects brief definitions of some of the technical terms used in the documentation for Consul and Consul Enterprise, as well as some terms that come up frequently in conversations throughout the Consul community.
An agent is the long running daemon on every member of the Consul cluster.
It is started by running
consul agent. The agent is able to run in either client
or server mode. Since all nodes must be running an agent, it is simpler to refer to
the node as being either a client or server, but there are other instances of the agent. All
agents can run the DNS or HTTP interfaces, and are responsible for running checks and
keeping services in sync.
A client is an agent that forwards all RPCs to a server. The client is relatively stateless. The only background activity a client performs is taking part in the LAN gossip pool. This has a minimal resource overhead and consumes only a small amount of network bandwidth.
A server is an agent with an expanded set of responsibilities including participating in the Raft quorum, maintaining cluster state, responding to RPC queries, exchanging WAN gossip with other datacenters, and forwarding queries to leaders or remote datacenters.
We define a datacenter to be a networking environment that is private, low latency, and high bandwidth. This excludes communication that would traverse the public internet, but for our purposes multiple availability zones within a single EC2 region would be considered part of a single datacenter.
When used in our documentation we use consensus to mean agreement upon the elected leader as well as agreement on the ordering of transactions. Since these transactions are applied to a finite-state machine, our definition of consensus implies the consistency of a replicated state machine. Consensus is described in more detail on Wikipedia, and our implementation is described here.
Consul is built on top of Serf which provides a full gossip protocol that is used for multiple purposes. Serf provides membership, failure detection, and event broadcast. Our use of these is described more in the gossip documentation. It is enough to know that gossip involves random node-to-node communication, primarily over UDP.
Refers to the LAN gossip pool which contains nodes that are all located on the same local area network or datacenter.
Refers to the WAN gossip pool which contains only servers. These servers are primarily located in different datacenters and typically communicate over the internet or wide area network.
Remote Procedure Call. This is a request / response mechanism allowing a client to make a request of a server.