mirror of https://github.com/hashicorp/consul
168 lines
7.9 KiB
Markdown
168 lines
7.9 KiB
Markdown
---
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layout: docs
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page_title: Agent
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sidebar_title: Agent
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description: >-
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The Consul agent is the core process of Consul. The agent maintains membership
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information, registers services, runs checks, responds to queries, and more.
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The agent must run on every node that is part of a Consul cluster.
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---
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# Consul Agent
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The Consul agent is the core process of Consul. The agent maintains membership
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information, registers services, runs checks, responds to queries,
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and more. The agent must run on every node that is part of a Consul cluster.
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Any agent may run in one of two modes: client or server. A server
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node takes on the additional responsibility of being part of the
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[consensus quorum](/docs/internals/consensus).
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These nodes take part in Raft and provide strong consistency and availability in
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the case of failure. The higher burden on the server nodes means that usually
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they should be run on dedicated instances -- they are more resource intensive
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than a client node. Client nodes make up the majority of the cluster, and they
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are very lightweight as they interface with the server nodes for most
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operations and maintain very little state of their own.
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## Running an Agent
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The agent is started with the [`consul agent`](/commands/agent) command.
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This command blocks, running forever or until told to quit. You can test a
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local agent by following the
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[Getting Started tutorials](https://learn.hashicorp.com/tutorials/consul/get-started-install?utm_source=consul.io&utm_medium=docs).
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The agent command takes a variety of
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[`configuration options`](/docs/agent/options#command-line-options), but most
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have sane defaults.
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When running [`consul agent`](/commands/agent), you should see output
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similar to this:
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```shell-session
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$ consul agent -data-dir=/tmp/consul
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==> Starting Consul agent...
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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: false (bootstrap: false)
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Client Addr: 127.0.0.1 (HTTP: 8500, DNS: 8600)
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Cluster Addr: 192.168.1.43 (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 192.168.1.43
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...
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```
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There are several important messages that
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[`consul agent`](/commands/agent) outputs:
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- **Node name**: This is a unique name for the agent. By default, this
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is the hostname of the machine, but you may customize it using the
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[`-node`](/docs/agent/options#_node) flag.
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- **Datacenter**: This is the datacenter in which the agent is configured to
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run.
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Consul has first-class support for multiple datacenters; however, to work
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efficiently, each node must be configured to report its datacenter. The
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[`-datacenter`](/docs/agent/options#_datacenter) flag can be used to set the
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datacenter. For single-DC configurations, the agent will default to "dc1".
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- **Server**: This indicates whether the agent is running in server or client
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mode.
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Server nodes have the extra burden of participating in the consensus quorum,
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storing cluster state, and handling queries. Additionally, a server may be
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in ["bootstrap"](/docs/agent/options#_bootstrap_expect) mode. Multiple servers
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cannot be in bootstrap mode as that would put the cluster in an inconsistent
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state.
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- **Client Addr**: This is the address used for client interfaces to the agent.
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This includes the ports for the HTTP and DNS interfaces. By default, this
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binds only to localhost. If you change this address or port, you'll have to
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specify a `-http-addr` whenever you run commands such as
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[`consul members`](/commands/members) to indicate how to reach the
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agent. Other applications can also use the HTTP address and port
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[to control Consul](/api).
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- **Cluster Addr**: This is the address and set of ports used for communication
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between Consul agents in a cluster. Not all Consul agents in a cluster have to
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use the same port, but this address **MUST** be reachable by all other nodes.
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When running under `systemd` on Linux, Consul notifies systemd by sending
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`READY=1` to the `$NOTIFY_SOCKET` when a LAN join has completed. For
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this either the `join` or `retry_join` option has to be set and the
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service definition file has to have `Type=notify` set.
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## Stopping an Agent
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An agent can be stopped in two ways: gracefully or forcefully. Servers and
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Clients both behave differently depending on the leave that is performed. There
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are two potential states a process can be in after a system signal is sent:
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_left_ and _failed_.
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To gracefully halt an agent, send the process an _interrupt signal_ (usually
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`Ctrl-C` from a terminal, or running `kill -INT consul_pid` ). For more
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information on different signals sent by the `kill` command, see
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[here](https://www.linux.org/threads/kill-signals-and-commands-revised.11625/)
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When a Client is gracefully exited, the agent first notifies the cluster it
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intends to leave the cluster. This way, other cluster members notify the
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cluster that the node has _left_.
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When a Server is gracefully exited, the server will not be marked as _left_.
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This is to minimally impact the consensus quorum. Instead, the Server will be
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marked as _failed_. To remove a server from the cluster, the
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[`force-leave`](/commands/force-leave) command is used. Using
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`force-leave` will put the server instance in a _left_ state so long as the
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Server agent is not alive.
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Alternatively, you can forcibly stop an agent by sending it a
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`kill -KILL consul_pid` signal. This will stop any agent immediately. The rest
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of the cluster will eventually (usually within seconds) detect that the node has
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died and notify the cluster that the node has _failed_.
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For client agents, the difference between a node _failing_ and a node _leaving_
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may not be important for your use case. For example, for a web server and load
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balancer setup, both result in the same outcome: the web node is removed
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from the load balancer pool.
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The [`skip_leave_on_interrupt`](/docs/agent/options#skip_leave_on_interrupt) and
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[`leave_on_terminate`](/docs/agent/options#leave_on_terminate) configuration
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options allow you to adjust this behavior.
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## Lifecycle
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Every agent in the Consul cluster goes through a lifecycle. Understanding
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this lifecycle is useful for building a mental model of an agent's interactions
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with a cluster and how the cluster treats a node.
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When an agent is first started, it does not know about any other node in the
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cluster.
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To discover its peers, it must _join_ the cluster. This is done with the
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[`join`](/commands/join)
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command or by providing the proper configuration to auto-join on start. Once a
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node joins, this information is gossiped to the entire cluster, meaning all
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nodes will eventually be aware of each other. If the agent is a server,
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existing servers will begin replicating to the new node.
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In the case of a network failure, some nodes may be unreachable by other nodes.
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In this case, unreachable nodes are marked as _failed_. It is impossible to
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distinguish between a network failure and an agent crash, so both cases are
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handled the same.
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Once a node is marked as failed, this information is updated in the service
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catalog.
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-> **Note:** There is some nuance here since this update is only possible if the servers can still [form a quorum](/docs/internals/consensus). Once the network recovers or a crashed agent restarts the cluster will repair itself and unmark a node as failed. The health check in the catalog will also be updated to reflect this.
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When a node _leaves_, it specifies its intent to do so, and the cluster
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marks that node as having _left_. Unlike the _failed_ case, all of the
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services provided by a node are immediately deregistered. If the agent was
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a server, replication to it will stop.
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To prevent an accumulation of dead nodes (nodes in either _failed_ or _left_
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states), Consul will automatically remove dead nodes out of the catalog. This
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process is called _reaping_. This is currently done on a configurable
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interval of 72 hours (changing the reap interval is _not_ recommended due to
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its consequences during outage situations). Reaping is similar to leaving,
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causing all associated services to be deregistered.
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