This topic provides an overview of the Consul agent, which 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.
Agents run in either client or server mode. Client nodes are lightweight processes that make up the majority of the cluster.
They interface with the server nodes for most operations and maintain very little state of their own.
Clients run on every node where services are running.
In addition to the core agent operations, server nodes participate in the [consensus quorum](/docs/internals/consensus).
The quorum is based on the Raft protocol, which provides strong consistency and availability in the case of failure.
Server nodes should run on dedicated instances because they are more resource intensive than client nodes.
## Requirements
You should run one Consul agent per server or host.
Instances of Consul can run in separate VMs or as separate containers.
At least one server agent per Consul deployment is required, but three to five server agents are recommended.
Refer to the following sections for information about host, port, memory, and other requirements:
- [Server Performance](/docs/install/performance)
- [Required Ports](/docs/install/ports)
The [Datacenter Deploy tutorial](https://learn.hashicorp.com/tutorials/consul/reference-architecture?in=consul/production-deploy#deployment-system-requirements) contains additional information, including licensing configuration, environment variables, and other details.
## Starting the Consul Agent
Start a Consul agent with the `consul` command and `agent` subcommand using the following syntax:
```shell-session
consul agent <options>
```
Consul ships with a `-dev` flag that configures the agent to run with several additional settings.
The `-dev` flag is provided for learning purposes only.
We strongly advise against using it for production environments.
-> **Getting Started Tutorials**: You can test a local agent by following the
The only information Consul needs to run is the location of a directory for storing agent state data, specified with the `-data-dir` flag.
Specifying the data directory and no other options will start a client agent, but the agent will be unable to perform any operations or connect to any other nodes.
The following example starts a Consul agent in dev mode that will store agent state data in the `tmp/consul` directory relative to the current directory:
```shell-session
consul agent -config-file=client.hcl -dev
```
In the following example, the agent configuration file would contain the following setting:
```shell-session
data_dir = "temp/client-data"
```
Agents are highly configurable, which enables you to deploy Consul to any infrastructure. Many of the default options for the `agent` command are suitable for becoming familiar with a local instance of Consul. In practice, however, several additional configuration options must be specified for Consul to function as expected. Refer to [Agent Configuration](/docs/agent/options) topic for a complete list of configuration options.
run. For single-DC configurations, the agent will default to `dc1`, but you can configure which datacenter the agent reports to with the [`-datacenter`](/docs/agent/options#_datacenter) flag.
Consul has first-class support for multiple datacenters, but configuring each node to report its datacenter improves agent efficiency.
Running an agent in server mode requires additional overhead. This is because they participate in the consensus quorum, store cluster state, and handle queries. A server may also be
in ["bootstrap"](/docs/agent/options#_bootstrap_expect) mode, which enables the server to elect itselft as the Raft leader. Multiple servers cannot be in bootstrap mode because it would put the cluster in an inconsistent state.
You can specify many options to configure how Consul operates when issuing the `consul agent` command.
You can also create one or more configuration files and provide them to Consul at startup using either the `-config-file` or `-config-dir` option.
Configuration files must be written in either JSON or HCL format.
-> **Consul Terminology**: Configuration files are sometimes called "service definition" files when they are used to configure client agents.
This is because clients are most commonly used to register services in the Consul catalog.
The following example starts a Consul agent that takes configuration settings from a file called `server.json` located in the current working directory:
```shell-session
consul agent -config-file=server.json
```
The configuration options necessary to successfully use Consul depend on several factors, including the type of agent you are configuring (client or server), the type of environment you are deploying to (e.g., on-premise, multi-cloud, etc.), and the security options you want to implement (ACLs, gRPC encryption).
The following examples are intended to help you understand some of the combinations you can implement to configure Consul.
### Common Configuration Settings
The following settings are commonly used in the configuration file (also called a service definition file when registering services with Consul) to configure Consul agents:
| `node_name` | String value that specifies a name for the agent node. <br/>See [`-node-id`](/docs/agent/options#_node_id) for details. | Hostname of the machine |
| `server` | Boolean value that determines if the agent runs in server mode. <br/>See [`-server`](/docs/agent/options#_server) for details. | `false` |
| `datacenter` | String value that specifies which datacenter the agent runs in. <br/>See [-datacenter](/docs/agent/options#_datacenter) for details. | `dc1` |
| `data_dir` | String value that specifies a directory for storing agent state data. <br/>See [`-data-dir`](/docs/agent/options#_data_dir) for details. | none |
| `log_level` | String value that specifies the level of logging the agent reports. <br/>See [`-log-level`](docs/agent/options#_log_level) for details. | `info` |
| `retry_join` | Array of string values that specify one or more agent addresses to join after startup. The agent will continue trying to join the specified agents until it has successfully joined another member. <br/>See [`-retry-join`](/docs/agent/options#_retry_join) for details. | none |
| `addresses` | Block of nested objects that define addresses bound to the agent for internal cluster communication. | `"http": "0.0.0.0"` See the Agent Configuration page for [default address values](/docs/agent/options#addresses) |
| `ports` | Block of nested objects that define ports bound to agent addresses. <br/>See (link to addresses option) for details. | See the Agent Configuration page for [default port values](/docs/agent/options#ports) |
### Server Node in a Service Mesh
The following example configuration is for a server agent named "`consul-server`". The server is [bootstrapped](/docs/agent/options#_bootstrap) and the Consul GUI is enabled.
The reason this server agent is configured for a service mesh is that the `connect` configuration is enabled. Connect is Consul's service mesh component that provides service-to-service connection authorization and encryption using mutual Transport Layer Security (TLS). Applications can use sidecar proxies in a service mesh configuration to establish TLS connections for inbound and outbound connections without being aware of Connect at all. See [Connect](/docs/connect) for details.
<Tabs>
<Tab heading="HCL">
```hcl
node_name = "consul-server"
server = true
bootstrap = true
ui_config {
enabled = true
}
datacenter = "dc1"
data_dir = "/consul/data"
log_level = "INFO"
addresses {
http = "0.0.0.0"
}
connect {
enabled = true
}
```
</Tab>
<Tab heading="JSON">
```json
{
"node_name": "consul-server",
"server": true,
"bootstrap": true,
"ui_config": {
"enabled": true
},
"datacenter": "dc1",
"data_dir": "consul/data",
"log_level": "INFO",
"addresses": {
"http": "0.0.0.0"
},
"connect": {
"enabled": true
}
}
```
</Tab>
</Tabs>
### Client Node Registering a Service
Using Consul as a central service registry is a common use case.
The following example configuration includes common settings to register a service with a Consul agent and enable health checks (see [Checks](/docs/discovery/checks) to learn more about health checks):
<Tabs>
<Tab heading="HCL">
```hcl
node_name = "consul-client"
server = false
datacenter = "dc1"
data_dir = "consul/data"
log_level = "INFO"
retry_join = [ "consul-server" ]
service {
id = "dns"
name = "dns"
tags = [ "primary" ]
address = "localhost"
port = 8600
check {
id = "dns"
name = "Consul DNS TCP on port 8600"
tcp = "localhost:8600"
interval = "10s"
timeout = "1s"
}
}
```
</Tab>
<Tab heading="JSON">
```json
{
"node_name": "consul-client",
"server": false,
"datacenter": "dc1",
"data_dir": "consul/data",
"log_level": "INFO",
"retry_join": ["consul-server"],
"service": {
"id": "dns",
"name": "dns",
"tags": ["primary"],
"address": "localhost",
"port": 8600,
"check": {
"id": "dns",
"name": "Consul DNS TCP on port 8600",
"tcp": "localhost:8600",
"interval": "10s",
"timeout": "1s"
}
}
}
```
</Tab>
</Tabs>
### Server Node with Encryption Enabled
The following example shows a server node configured with encryption enabled.
Refer to the [Security](/docs/security) chapter for additional information about how to configure security options for Consul.
-> **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.
