Every agent in the Consul cluster goes through a lifecycle.
Understanding the lifecycle is useful for building a mental model of an agent's interactions with a cluster and how the cluster treats a node.
The following process describes the agent lifecycle within the context of an existing cluster:
1. **An agent is started** either manually or through an automated or programmatic process.
Newly-started agents are unaware of other nodes in the cluster.
1. **An agent joins a cluster**, which enables the agent to discover agent peers.
Agents join clusters on startup when the [`join`](/commands/join) command is issued or according the [auto-join configuration](/docs/install/cloud-auto-join).
1. **Information about the agent is gossiped to the entire cluster**.
As a result, all nodes will eventually become aware of each other.
1. **Existing servers will begin replicating to the new node** if the agent is a server.
### Failures and Crashes
In the event of a network failure, some nodes may be unable to reach other nodes.
Unreachable nodes will be marked as _failed_.
Distinguishing between a network failure and an agent crash is impossible.
As a result, agent crashes are handled in the same manner is network failures.
Once a node is marked as failed, this information is updated in the service
catalog.
-> **Note:** Updating the catalog 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 the current state.
### Exiting Nodes
When a node leaves a cluster, it communicates its intent and the cluster marks the node as having _left_.
In contrast to changes related to failures, all of the services provided by a node are immediately deregistered.
If a server agent leaves, replication to the exiting server will stop.
To prevent an accumulation of dead nodes (nodes in either _failed_ or _left_
states), Consul will automatically remove dead nodes out of the catalog. This
process is called _reaping_. This is currently done on a configurable
interval of 72 hours (changing the reap interval is _not_ recommended due to
its consequences during outage situations). Reaping is similar to leaving,
causing all associated services to be deregistered.
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:
Consul ships with a `-dev` flag that configures the agent to run in server mode and several additional settings that enable you to quickly get started with Consul.
When starting Consul with the `-dev` flag, the only additional information Consul needs to run is the location of a directory for storing agent state data.
You can specify the location with the `-data-dir` flag or define the location in an external file and point the file with the `-config-file` flag.
You can also point to a directory containing several configuration files with the `-config-dir` flag.
This enables you to logically group configuration settings into separate files. See [Configuring Consul Agents](/docs/agent#configuring-consul-agents) for additional information.
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/config) 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/config/cli-flags#_datacenter) flag.
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/config/cli-flags#_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/config/cli-flags#_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/config/cli-flags#_server) for details. | `false` |
| `datacenter` | String value that specifies which datacenter the agent runs in. <br/>See [-datacenter](/docs/agent/config/cli-flags#_datacenter) for details. | `dc1` |
| `data_dir` | String value that specifies a directory for storing agent state data. <br/>See [`-data-dir`](/docs/agent/config/cli-flags#_data_dir) for details. | none |
| `log_level` | String value that specifies the level of logging the agent reports. <br/>See [`-log-level`](/docs/agent/config/cli-flags#_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/config/cli-flags#_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/config/config-files#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/config/config-files#ports) |
The following example configuration is for a server agent named "`consul-server`". The server is [bootstrapped](/docs/agent/config/cli-flags#_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.
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):
