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| layout | page_title | sidebar_current | description |
|---|---|---|---|
| intro | Run the Agent | gettingstarted-agent | After Consul is installed, the agent must be run. The agent can either run in a server or client mode. Each data center must have at least one server, although 3 or 5 is recommended. A single server deployment is highly discouraged as data loss is inevitable in a failure scenario. |
Run the Consul Agent
After Consul is installed, the agent must be run. The agent can run either in server or client mode. Each data center must have at least one server, although 3 or 5 is recommended. A single server deployment is highly discouraged as data loss is inevitable in a failure scenario. This guide covers bootstrapping a new data center.
All other agents run in client mode, a very lightweight process that registers services, runs health checks, and forwards queries to servers. The agent must be run on every node that is part of the cluster.
Starting the Agent
For simplicity, we'll run a single Consul agent in server mode right now:
$ consul agent -server -bootstrap-expect 1 -data-dir /tmp/consul
==> WARNING: BootstrapExpect Mode is specified as 1; this is the same as Bootstrap mode.
==> WARNING: Bootstrap mode enabled! Do not enable unless necessary
==> WARNING: It is highly recommended to set GOMAXPROCS higher than 1
==> Starting Consul agent...
==> Starting Consul agent RPC...
==> Consul agent running!
Node name: 'Armons-MacBook-Air'
Datacenter: 'dc1'
Server: true (bootstrap: true)
Client Addr: 127.0.0.1 (HTTP: 8500, DNS: 8600, RPC: 8400)
Cluster Addr: 10.1.10.38 (LAN: 8301, WAN: 8302)
==> Log data will now stream in as it occurs:
[INFO] serf: EventMemberJoin: Armons-MacBook-Air.local 10.1.10.38
[INFO] raft: Node at 10.1.10.38:8300 [Follower] entering Follower state
[INFO] consul: adding server for datacenter: dc1, addr: 10.1.10.38:8300
[ERR] agent: failed to sync remote state: rpc error: No cluster leader
[WARN] raft: Heartbeat timeout reached, starting election
[INFO] raft: Node at 10.1.10.38:8300 [Candidate] entering Candidate state
[INFO] raft: Election won. Tally: 1
[INFO] raft: Node at 10.1.10.38:8300 [Leader] entering Leader state
[INFO] consul: cluster leadership acquired
[INFO] consul: New leader elected: Armons-MacBook-Air
[INFO] consul: member 'Armons-MacBook-Air' joined, marking health alive
As you can see, the Consul agent has started and has output some log data. From the log data, you can see that our agent is running in server mode and has claimed leadership of the cluster. Additionally, the local member has been marked as a healthy member of the cluster.
~> Note for OS X Users: Consul uses your hostname as the
default node name. If your hostname contains periods, DNS queries to
that node will not work with Consul. To avoid this, explicitly set
the name of your node with the -node flag.
Cluster Members
If you run consul members in another terminal, you can see the members of
the Consul cluster. We'll cover joining clusters in the next section, but for now,
you should only see one member (yourself):
$ consul members
Node Address Status Type Build Protocol
Armons-MacBook-Air 10.1.10.38:8301 alive server 0.5.1 2
The output shows our own node, the address it is running on, its
health state, its role in the cluster, and some version information.
Additional metadata can be viewed by providing the -detailed flag.
The output of the members command is based on the
gossip protocol and is eventually consistent.
That is, at any point in time, the view of the world as seen by your local
agent may not exactly match the state on the servers. For a strongly consistent
view of the world, use the HTTP API as it forwards the
request to the Consul servers:
$ curl localhost:8500/v1/catalog/nodes
[{"Node":"Armons-MacBook-Air","Address":"10.1.10.38"}]
In addition to the HTTP API, the DNS interface can be used to query the node. Note that you have to make sure to point your DNS lookups to the Consul agent's DNS server which runs on port 8600 by default. The format of the DNS entries (such as "Armons-MacBook-Air.node.consul") will be covered in more detail later.
$ dig @127.0.0.1 -p 8600 Armons-MacBook-Air.node.consul
...
;; QUESTION SECTION:
;Armons-MacBook-Air.node.consul. IN A
;; ANSWER SECTION:
Armons-MacBook-Air.node.consul. 0 IN A 10.1.10.38
Stopping the Agent
You can use Ctrl-C (the interrupt signal) to gracefully halt the agent.
After interrupting the agent, you should see it leave the cluster
and shut down.
By gracefully leaving, Consul notifies other cluster members that the node left. If you had forcibly killed the agent process, other members of the cluster would have detected that the node failed. When a member leaves, its services and checks are removed from the catalog. When a member fails, its health is simply marked as critical, but it is not removed from the catalog. Consul will automatically try to reconnect to failed nodes, allowing it to recover from certain network conditions, while left nodes are no longer contacted.
Additionally, if an agent is operating as a server, a graceful leave is important to avoid causing a potential availability outage affecting the consensus protocol. See the guides section for details on how to safely add and remove servers.