mirror of https://github.com/hashicorp/consul
119 lines
4.9 KiB
Markdown
119 lines
4.9 KiB
Markdown
---
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layout: "intro"
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page_title: "Run the Agent"
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sidebar_current: "gettingstarted-agent"
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---
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# Run the Consul Agent
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After Consul is installed, the agent must be run. The agent can either run
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in a server or client mode. Each datacenter must have at least one server,
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although 3 or 5 is recommended. A single server deployment is _**highly**_ discouraged
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as data loss is inevitable in a failure scenario. [This guide](/docs/guides/bootstrapping.html)
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covers bootstrapping a new datacenter. All other agents run in client mode, which
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is a very lightweight process that registers services, runs health checks,
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and forwards queries to servers. The agent must be run for every node that
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will be part of the cluster.
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## Starting the Agent
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For simplicity, we'll run a single Consul agent in server mode right now:
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```
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$ consul agent -server -bootstrap -data-dir /tmp/consul
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==> WARNING: Bootstrap mode enabled! Do not enable unless necessary
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==> WARNING: It is highly recommended to set GOMAXPROCS higher than 1
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==> Starting Consul agent...
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==> Starting Consul agent RPC...
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==> Consul agent running!
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Node name: 'Armons-MacBook-Air'
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Datacenter: 'dc1'
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Server: true (bootstrap: true)
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Client Addr: 127.0.0.1 (HTTP: 8500, DNS: 8600, RPC: 8400)
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Cluster Addr: 10.1.10.38 (LAN: 8301, WAN: 8302)
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==> Log data will now stream in as it occurs:
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[INFO] serf: EventMemberJoin: Armons-MacBook-Air.local 10.1.10.38
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[INFO] raft: Node at 10.1.10.38:8300 [Follower] entering Follower state
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[INFO] consul: adding server for datacenter: dc1, addr: 10.1.10.38:8300
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[ERR] agent: failed to sync remote state: rpc error: No cluster leader
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[WARN] raft: Heartbeat timeout reached, starting election
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[INFO] raft: Node at 10.1.10.38:8300 [Candidate] entering Candidate state
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[INFO] raft: Election won. Tally: 1
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[INFO] raft: Node at 10.1.10.38:8300 [Leader] entering Leader state
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[INFO] consul: cluster leadership acquired
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[INFO] consul: New leader elected: Armons-MacBook-Air
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[INFO] consul: member 'Armons-MacBook-Air' joined, marking health alive
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```
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As you can see, the Consul agent has started and has output some log
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data. From the log data, you can see that our agent is running in server mode,
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and has claimed leadership of the cluster. Additionally, the local member has
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been marked as a healthy member of the cluster.
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## Cluster Members
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If you run `consul members` in another terminal, you can see the members of
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the Consul cluster. You should only see one member (yourself). We'll cover
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joining clusters in the next section.
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```
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$ consul members
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Armons-MacBook-Air 10.1.10.38:8301 alive role=consul,dc=dc1,vsn=1,vsn_min=1,vsn_max=1,port=8300,bootstrap=1
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```
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The output shows our own node, the address it is running on, its
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health state, and some metadata associated with the node. Some important
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metadata keys to recognize are the `role` and `dc` keys. These tell you
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the service name and the datacenter that member is within. These can be
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used to lookup nodes and services using the DNS interface, which is covered
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shortly.
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The output from the `members` command is generated based on the
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[gossip protocol](/docs/internals/gossip.html) and is eventually consistent.
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For a strongly consistent view of the world, use the
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[HTTP API](/docs/agent/http.html), which forwards the request to the
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Consul servers:
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```
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$ curl localhost:8500/v1/catalog/nodes
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[{"Node":"Armons-MacBook-Air","Address":"10.1.10.38"}]
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```
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In addition to the HTTP API, the
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[DNS interface](/docs/agent/dns.html) can be used to query the node. Note
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that you have to make sure to point your DNS lookups to the Consul agent's
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DNS server, which runs on port 8600 by default. The format of the DNS
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entries (such as "Armons-MacBook-Air.node.consul") will be covered later.
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```
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$ dig @127.0.0.1 -p 8600 Armons-MacBook-Air.node.consul
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...
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;; QUESTION SECTION:
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;Armons-MacBook-Air.node.consul. IN A
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;; ANSWER SECTION:
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Armons-MacBook-Air.node.consul. 0 IN A 10.1.10.38
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```
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## Stopping the Agent
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You can use `Ctrl-C` (the interrupt signal) to gracefully halt the agent.
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After interrupting the agent, you should see it leave the cluster gracefully
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and shut down.
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By gracefully leaving, Consul notifies other cluster members that the
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node _left_. If you had forcibly killed the agent process, other members
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of the cluster would have detected that the node _failed_. When a member leaves,
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it's services and checks are removed from the catalog. When a member fails,
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it's health is simply marked as critical, but is not removed from the catalog.
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Consul will automatically try to reconnect to _failed_ nodes, which allows it
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to recover from certain network conditions, while _left_ nodes are no longer contacted.
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Additionally, if an agent is operating as a server, a graceful leave is important
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to avoid causing a potential availability outage affecting the [consensus protocol](/docs/internals/consensus.html).
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See the [guides section](/docs/guides/index.html) to safely add and remove servers.
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