mirror of https://github.com/hashicorp/consul
132 lines
6.5 KiB
Markdown
132 lines
6.5 KiB
Markdown
---
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layout: "docs"
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page_title: "Agent"
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sidebar_current: "docs-agent-running"
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description: |-
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The Consul agent is the core process of Consul. The agent maintains membership information, registers services, runs checks, responds to queries, and more. 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 [consensus quorum](#).
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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 they
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should be run on dedicated instances -- they are more resource intensive than a client
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node. Client nodes make up the majority of the cluster, and they are very lightweight
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as they interface with the server nodes for most operations and maintain very little state
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of their own.
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## Running an Agent
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The agent is started with the `consul agent` command. This command blocks,
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running forever or until told to quit. The agent command takes a variety
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of configuration options, but most have sane defaults.
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When running `consul agent`, you should see output similar to this:
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```text
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$ consul agent -data-dir=/tmp/consul
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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: false (bootstrap: false)
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Client Addr: 127.0.0.1 (HTTP: 8500, DNS: 8600, RPC: 8400)
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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 `consul 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 `-node` flag.
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* **Datacenter**: This is the datacenter in which the agent is configured to run.
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Consul has first-class support for multiple datacenters; however, to work efficiently,
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each node must be configured to report its datacenter. The `-dc` flag
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can be used to set the datacenter. For single-DC configurations, the agent
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will default to "dc1".
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* **Server**: This indicates whether the agent is running in server or client 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" mode. Multiple servers cannot be in bootstrap mode as that would
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put the cluster in an inconsistent 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, DNS, and RPC interfaces. The RPC
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address is used by other `consul` commands (such as `consul members`, `consul join`,
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etc) which query and control a running agent. By default, this binds only to localhost. If you
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change this address or port, you'll have to specify an `-rpc-addr` whenever
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you run commands such as `consul members` to indicate how to reach the
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agent. Other applications can also use the RPC address and port [to control Consul](/docs/agent/rpc.html).
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* **Cluster Addr**: This is the address and set of ports used for communication between
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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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## Stopping an Agent
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An agent can be stopped in two ways: gracefully or forcefully. To gracefully
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halt an agent, send the process an interrupt signal (usually
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`Ctrl-C` from a terminal). When gracefully exiting, the agent first notifies
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the cluster it intends to leave the cluster. This way, other cluster members
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notify the cluster that the node has _left_.
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Alternatively, you can force kill the agent by sending it a kill signal.
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When force killed, the agent ends immediately. The rest of the cluster will
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eventually (usually within seconds) detect that the node has died and
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notify the cluster that the node has _failed_.
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It is especially important that a server node be allowed to leave gracefully
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so that there will be a minimal impact on availability as the server leaves
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the consensus quorum.
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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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## 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 cluster.
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To discover its peers, it must _join_ the cluster. This is done with the `join`
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command or by providing the proper configuration to auto-join on start. Once a node
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joins, this information is gossiped to the entire cluster, meaning all nodes will
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eventually be aware of each other. If the agent is a server, existing servers will
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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 distinguish
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between a network failure and an agent crash, so both cases are handled the same.
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Once a node is marked as failed, this information is updated in the service catalog.
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Mote: there is some nuance here since this update is only possible if the servers can
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still [form a quorum](/docs/internals/consensus.html). Once the network recovers
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or a crashed agent restarts the cluster will repair itself and unmark
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a node as failed. The health check in the catalog will also be updated to reflect
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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. To prevent an accumulation
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of dead nodes, Consul will automatically reap _failed_ nodes out of the
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catalog as well. This is currently done on a non-configurable interval of
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72 hours. Reaping is similar to leaving, causing all associated services
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to be deregistered.
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