mirror of https://github.com/status-im/consul.git
143 lines
8.3 KiB
Markdown
143 lines
8.3 KiB
Markdown
---
|
|
layout: "docs"
|
|
page_title: "Consul Architecture"
|
|
sidebar_current: "docs-internals-architecture"
|
|
description: |-
|
|
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.
|
|
---
|
|
|
|
# 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.
|
|
|
|
~> **Advanced Topic!** 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.
|
|
|
|
## 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 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.
|
|
|
|
* 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 in the LAN gossip
|
|
pool. This has a minimal resource overhead and consumes only a small amount of network
|
|
bandwidth.
|
|
|
|
* Server - 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.
|
|
|
|
* Datacenter - While the definition of a datacenter seems obvious, there are subtle details
|
|
that must be considered. For example, in EC2, are multiple availability zones considered
|
|
to comprise a single datacenter? 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.
|
|
|
|
* 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
|
|
[finite-state machine](https://en.wikipedia.org/wiki/Finite-state_machine), our definition
|
|
of consensus implies the consistency of a replicated state machine. Consensus is described
|
|
in more detail on [Wikipedia](https://en.wikipedia.org/wiki/Consensus_(computer_science)),
|
|
and our implementation is described [here](/docs/internals/consensus.html).
|
|
|
|
* Gossip - Consul is built on top of [Serf](https://www.serf.io/) which provides a full
|
|
[gossip protocol](https://en.wikipedia.org/wiki/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](/docs/internals/gossip.html). It is enough to know
|
|
that gossip involves random node-to-node communication, primarily over UDP.
|
|
|
|
* LAN Gossip - Refers to the LAN gossip pool which contains nodes that are all
|
|
located on the same local area network or datacenter.
|
|
|
|
* WAN Gossip - 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.
|
|
|
|
* RPC - Remote Procedure Call. This is a request / response mechanism allowing a
|
|
client to make a request of a server.
|
|
|
|
## 10,000 foot view
|
|
|
|
From a 10,000 foot altitude the architecture of Consul looks like this:
|
|
|
|
<div class="center">
|
|
[](/assets/images/consul-arch.png)
|
|
</div>
|
|
|
|
Let's break down this image and describe each piece. First of all, we can see
|
|
that there are two datacenters, labeled "one" and "two". Consul has first
|
|
class support for [multiple datacenters](/docs/guides/datacenters.html) 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 to 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 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 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 single leader, a selected server 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 pool. This pool is different from the LAN pool
|
|
as it is optimized for the higher latency of the internet and is expected to contain only
|
|
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 pool. 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.
|
|
|
|
In general, data is not replicated between different Consul datacenters. When a
|
|
request is made for a resource in another datacenter, the local Consul servers forward
|
|
an RPC request to the remote Consul servers for that resource and return the results.
|
|
If the remote datacenter is not available, then those resources will also not be
|
|
available, but that won't otherwise affect the local datacenter. There are some special
|
|
situations where a limited subset of data can be replicated, such as with Consul's built-in
|
|
[ACL replication](/docs/guides/acl.html#outages-and-acl-replication) capability, or
|
|
external tools like [consul-replicate](https://github.com/hashicorp/consul-replicate).
|
|
|
|
In some places, client agents may cache data from the servers to make it
|
|
available locally for performance and reliability. Examples include Connect
|
|
certificates and intentions which allow the client agent to make local decisions
|
|
about inbound connection requests without a round trip to the servers. Some API
|
|
endpoints also support optional result caching. This helps reliability because
|
|
the local agent can continue to respond to some queries like service-discovery
|
|
or Connect authorization from cache even if the connection to the servers is
|
|
disrupted or the servers are temporarily unavailable.
|
|
|
|
## Getting in depth
|
|
|
|
At this point we've covered the high level architecture of Consul, but there are many
|
|
more details for each of the subsystems. 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 are also available.
|
|
|
|
For other details, either consult the code, ask in IRC, or reach out to the mailing list.
|