118 lines
6.6 KiB
Markdown
118 lines
6.6 KiB
Markdown
---
|
|
layout: "docs"
|
|
page_title: "Consul Architecture"
|
|
sidebar_current: "docs-internals-architecture"
|
|
---
|
|
|
|
# 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.
|
|
|
|
<div class="alert alert-block alert-warning">
|
|
<strong>Advanced Topic!</strong> This page covers technical details of
|
|
the internals of Consul. You don't need to know these details to effectively
|
|
operate and use Consul. These details are documented here for those who wish
|
|
to learn about them without having to go spelunking through the source code.
|
|
</div>
|
|
|
|
## Glossary
|
|
|
|
Before describing the architecture, we provide a glossary of terms to help
|
|
clarify what is being discussed:
|
|
|
|
* 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 other are 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 RPC's 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.
|
|
|
|
* Server - An agent that is server mode. When in server mode, there is an expanded set
|
|
of responsibilities including participated in the Raft quorum, maintaining cluster state,
|
|
responding to RPC queries, WAN gossip to other datacenters, forwarding of queries to leaders
|
|
or remote datacenters.
|
|
|
|
* Datacenter - A data center seems obvious, but there are subtle details such as multiple
|
|
availability zones in EC2. We define a data center to be a networking environment that is
|
|
private, low latency, and high badwidth. This excludes communication that would traverse
|
|
the public internet.
|
|
|
|
* 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](http://en.wikipedia.org/wiki/Consensus_(computer_science)),
|
|
as well as our [implementation here](/docs/internals/consensus.html).
|
|
|
|
* Gossip - Consul is built on top of [Serf](http://www.serfdom.io/), which provides a full
|
|
[gossip protocol](http://en.wikipedia.org/wiki/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](/docs/internals/gossip.html). It is enough to know
|
|
gossip involves random node-to-node communication, primary over UDP.
|
|
|
|
* 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.
|
|
|
|
* 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.
|
|
|
|
* 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 data centers 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](/docs/internals/gossip.html).
|
|
This means is there is a Serf cluster 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,
|
|
that discovery is done automatically using Serf. 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](/docs/internals/consensus.html). 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-dc 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 a Serf failure detection,
|
|
connection caching and multiplexing, cross-dc 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](/docs/internals/consensus.html) is
|
|
documented in detail, as is the [gossip protocol](/docs/internals/gossip.html). The [documentation](/docs/internals/security.html)
|
|
for the security model and protocols used for is also available.
|
|
|
|
For other details, either consult the code, ask in IRC or reach out to the mailing list.
|
|
|