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docs: internals/architecture: minor fixes

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Dan Frost 2014-11-26 12:31:38 +00:00
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@ -23,20 +23,20 @@ Before describing the architecture, we provide a glossary of terms to help
clarify what is being discussed: clarify what is being discussed:
* Agent - An agent is the long running daemon on every member of the Consul cluster. * 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*, 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 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 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 agents can run the DNS or HTTP interfaces, and are responsible for running checks and
keeping services in sync. keeping services in sync.
* Client - A client is an agent that forwards all RPCs to a server. The client is relatively * 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. stateless. The only background activity a client performs is taking part in the LAN gossip
This has a minimal resource overhead and consumes only a small amount of network bandwidth. 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 * Server - A server is an agent with an expanded set of responsibilities including
of responsibilities including participating in the Raft quorum, maintaining cluster state, participating in the Raft quorum, maintaining cluster state, responding to RPC queries,
responding to RPC queries, WAN gossip to other datacenters, and forwarding queries to leaders WAN gossip to other datacenters, and forwarding queries to leaders or remote datacenters.
or remote datacenters.
* Datacenter - A datacenter seems obvious, but there are subtle details such as multiple * 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 availability zones in EC2. We define a datacenter to be a networking environment that is
@ -47,13 +47,13 @@ the public internet.
the elected leader as well as agreement on the ordering of transactions. Since these 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 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)), 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). and our implementation is described [here](/docs/internals/consensus.html).
* Gossip - Consul is built on top of [Serf](http://www.serfdom.io/), which provides a full * 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. [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 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 is described more in the [gossip documentation](/docs/internals/gossip.html). It is enough to know
gossip involves random node-to-node communication, primarily over UDP. 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 * LAN Gossip - Refers to the LAN gossip pool, which contains nodes that are all
located on the same local area network or datacenter. located on the same local area network or datacenter.
@ -62,8 +62,8 @@ located on the same local area network or datacenter.
servers are primarily located in different datacenters and typically communicate servers are primarily located in different datacenters and typically communicate
over the internet or wide area network. over the internet or wide area network.
* RPC - RPC is short for a Remote Procedure Call. This is a request / response mechanism * RPC - Remote Procedure Call. This is a request / response mechanism allowing a
allowing a client to make a request from a server. client to make a request of a server.
## 10,000 foot view ## 10,000 foot view
@ -73,7 +73,7 @@ From a 10,000 foot altitude the architecture of Consul looks like this:
![Consul Architecture](consul-arch.png) ![Consul Architecture](consul-arch.png)
</div> </div>
Lets break down this image and describe each piece. First of all we can see Let's 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 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. class support for multiple datacenters and expects this to be the common case.
@ -85,9 +85,9 @@ 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). 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 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, 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 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 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 scalable than naive heartbeating schemes. Thirdly, it is used as a messaging layer to notify
when important events such as leader election take place. when important events such as leader election take place.
@ -97,8 +97,8 @@ processing all queries and transactions. Transactions must also be replicated to
as part of the [consensus protocol](/docs/internals/consensus.html). Because of this requirement, 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. 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, 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 only contain 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 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 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. WAN gossip. Because the servers are all operating in this pool, it also enables cross-datacenter requests.
@ -110,8 +110,8 @@ connection caching and multiplexing, cross-datacenter requests are relatively fa
## Getting in depth ## Getting in depth
At this point we've covered the high level architecture of Consul, but there are much At this point we've covered the high level architecture of Consul, but there are many
more details to each of the sub-systems. The [consensus protocol](/docs/internals/consensus.html) is more details for 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) 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 the security model and protocols used are also available.