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| layout | page_title | sidebar_current | description |
|---|---|---|---|
| intro | Run the Agent | gettingstarted-agent | The Consul agent can run in either server or client mode. Each datacenter must have at least one server, though a cluster of 3 or 5 servers is recommended. A single server deployment is highly discouraged in production 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 datacenter must have at least one server, though a cluster of 3 or 5 servers is recommended. A single server deployment is highly discouraged as data loss is inevitable in a failure scenario.
All other agents run in client mode. A client is 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.
For more detail on bootstrapping a datacenter, see this guide.
Starting the Agent
For simplicity, we'll start the Consul agent in development mode for now. This mode is useful for bringing up a single-node Consul environment quickly and easily. It is not intended to be used in production as it does not persist any state.
$ consul agent -dev
==> Starting Consul agent...
==> Starting Consul agent RPC...
==> Consul agent running!
Node name: 'Armons-MacBook-Air'
Datacenter: 'dc1'
Server: true (bootstrap: false)
Client Addr: 127.0.0.1 (HTTP: 8500, HTTPS: -1, DNS: 8600, RPC: 8400)
Cluster Addr: 172.20.20.11 (LAN: 8301, WAN: 8302)
Gossip encrypt: false, RPC-TLS: false, TLS-Incoming: false
Atlas: <disabled>
==> Log data will now stream in as it occurs:
[INFO] raft: Node at 172.20.20.11:8300 [Follower] entering Follower state
[INFO] serf: EventMemberJoin: Armons-MacBook-Air 172.20.20.11
[INFO] consul: adding LAN server Armons-MacBook-Air (Addr: 172.20.20.11:8300) (DC: dc1)
[INFO] serf: EventMemberJoin: Armons-MacBook-Air.dc1 172.20.20.11
[INFO] consul: adding WAN server Armons-MacBook-Air.dc1 (Addr: 172.20.20.11:8300) (DC: dc1)
[ERR] agent: failed to sync remote state: No cluster leader
[WARN] raft: Heartbeat timeout reached, starting election
[INFO] raft: Node at 172.20.20.11:8300 [Candidate] entering Candidate state
[DEBUG] raft: Votes needed: 1
[DEBUG] raft: Vote granted. Tally: 1
[INFO] raft: Election won. Tally: 1
[INFO] raft: Node at 172.20.20.11:8300 [Leader] entering Leader state
[INFO] raft: Disabling EnableSingleNode (bootstrap)
[INFO] consul: cluster leadership acquired
[DEBUG] raft: Node 172.20.20.11:8300 updated peer set (2): [172.20.20.11:8300]
[DEBUG] consul: reset tombstone GC to index 2
[INFO] consul: New leader elected: Armons-MacBook-Air
[INFO] consul: member 'Armons-MacBook-Air' joined, marking health alive
[INFO] agent: Synced service 'consul'
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 DC
Armons-MacBook-Air 172.20.20.11:8301 alive server 0.6.1dev 2 dc1
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":"172.20.20.11","CreateIndex":3,"ModifyIndex":4}]
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 172.20.20.11
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.
Next Steps
Your simple Consul cluster is up and running. Let's give it some services!