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Consul Architecture
Consul is a complex system that has many different moving parts. To help users and developers of Consul form a mental model of how it works, this page documents the system architecture.
Glossary
Before describing the architecture, we provide a glossary of terms to help clarify what is being discussed:
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Agent - 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 either being 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. -
Client - 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 of LAN gossip pool. This has a minimal resource overhead and consumes only a small amount of network bandwidth.
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Server - An agent that is server mode. When in server mode, there is an expanded set of responsibilities including participating in the Raft quorum, maintaining cluster state, responding to RPC queries, WAN gossip to other datacenters, and forwarding queries to leaders or remote datacenters.
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Datacenter - A datacenter seems obvious, but there are subtle details such as multiple availability zones in EC2. 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.
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Consensus - 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 FSM, we implicitly include the consistency of a replicated state machine. Consensus is described in more detail on Wikipedia, as well as our implementation here.
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Gossip - 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 mechanisms. Our use of these is described more in the gossip documentation. It is enough to know gossip involves random node-to-node communication, primarily over UDP.
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LAN Gossip - This is used to mean that there is a gossip pool, containing nodes that are all located on the same local area network or datacenter.
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WAN Gossip - This is used to mean that there is a gossip pool, containing servers that are primary located in different datacenters and must communicate over the internet or wide area network.
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RPC - RPC is short for a Remote Procedure Call. This is a request / response mechanism allowing a client to make a request from a server.
10,000 foot view
From a 10,000 foot altitude the architecture of Consul looks like this:
Lets break down this image and describe each piece. First of all we can see that there are two datacenters, one and two respectively. Consul has first class support for multiple datacenters and expects this to be the common case.
Within each datacenter we have a mixture of clients and servers. It is expected that there be between three and five servers. This strikes a balance between availability in the case of failure and performance, as consensus gets progressively slower as more machines are added. However, there is no limit to the number of clients, and they can easily scale into the thousands or tens of thousands.
All the nodes that are in a datacenter participate in a gossip protocol. This means there is a gossip pool that contains all the nodes for a given datacenter. This serves a few purposes: first, there is no need to configure clients with the addresses of servers, discovery is done automatically. Second, the work of detecting node failures is not placed on the servers but is distributed. This makes the failure detection much more scalable than naive heartbeating schemes. Thirdly, it is used as a messaging layer to notify when important events such as leader election take place.
The servers in each datacenter are all part of a single Raft peer set. This means that they work together to elect a leader, which has extra duties. The leader is responsible for processing all queries and transactions. Transactions must also be replicated to all peers as part of the consensus protocol. Because of this requirement, when a non-leader server receives an RPC request it forwards it to the cluster leader.
The server nodes also operate as part of a WAN gossip. This pool is different from the LAN pool, as it is optimized for the higher latency of the internet, and is expected to only contain other Consul server nodes. The purpose of this pool is to allow datacenters to discover each other in a low touch manner. Bringing a new datacenter online is as easy as joining the existing WAN gossip. Because the servers are all operating in this pool, it also enables cross-datacenter requests. When a server receives a request for a different datacenter, it forwards it to a random server in the correct datacenter. That server may then forward to the local leader.
This results in a very low coupling between datacenters, but because of failure detection, connection caching and multiplexing, cross-datacenter requests are relatively fast and reliable.
Getting in depth
At this point we've covered the high level architecture of Consul, but there are much more details to each of the sub-systems. The consensus protocol is documented in detail, as is the gossip protocol. The documentation for the security model and protocols used are also available.
For other details, either consult the code, ask in IRC or reach out to the mailing list.