--- layout: "intro" page_title: "Run the Agent" sidebar_current: "gettingstarted-agent" description: |- After Consul is installed, the agent must be run. The agent can either run 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. --- # 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](/docs/guides/bootstrapping.html). ## Starting the Agent For simplicity, we'll run a single Consul agent in server mode: ```text $ 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 ==> 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`](/docs/commands/members.html) 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): ```text $ 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`](/docs/commands/members.html) command is based on the [gossip protocol](/docs/internals/gossip.html) 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](/docs/agent/http.html) as it forwards the request to the Consul servers: ```text $ curl localhost:8500/v1/catalog/nodes [{"Node":"Armons-MacBook-Air","Address":"10.1.10.38"}] ``` In addition to the HTTP API, the [DNS interface](/docs/agent/dns.html) 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. ```text $ 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](/docs/internals/consensus.html). See the [guides section](/docs/guides/index.html) 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](services.html)!