# Configuration

## HOCON

EMQX Edge uses Human-Optimized Config Object Notation (HOCON) as its default configuration file format. HOCON is a superset of JSON, designed to be more human-friendly and easier to read and write, making it ideal for managing configuration data.

All main configuration files are located in the `etc` directory. In addition to the primary configuration file, EMQX Edge supports modular configuration through the `include` directive, allowing you to separate settings for specific features such as authentication.

The main configuration files include:

| Configuration File       | Description                                        |
| ------------------------ | -------------------------------------------------- |
| `etc/nanomq.conf`        | Core EMQX Edge configuration                       |
| `etc/nanomq_pwd.conf`    | Username and password authentication settings      |
| `etc/nanomq_acl.conf`    | Access control (ACL) settings                      |
| `etc/nanomq_bridge.conf` | MQTT bridge configuration (used with `nanomq_cli`) |

The sections below reference configuration syntax and examples using the HOCON format.

## Syntax

In the configuration file, the values can be notated as JSON-like objects, such as:

```bash
log {
    dir = "/tmp"
    file = "nanomq.log"
}
```

Another equivalent representation is flat, such as:

```bash
log.dir = "/tmp"
log.file = "nanomq.log"
```

This flat format is almost backward compatible (the so-called 'cuttlefish' format).

It is not fully compatible because HOCON requires strings to be quoted, while cuttlefish treats all characters to the right of the `=` mark as the value.

For example, cuttlefish：`websocket.bind = 0.0.0.0:8083/mqtt`，HOCON：`websocket.bind = "0.0.0.0:8083/mqtt"`.

### Configuration Overlay Rules

In HOCON, configuration objects are overlaid, meaning that later definitions can override earlier ones. This overlay behavior follows two main principles:

- Within one file, when the same object is defined multiple times, later definitions recursively override earlier ones.
- In layered configurations, objects from higher-priority layers override those in lower-priority layers.

**Example**

In the example below, the `log.level=debug` line appears after the initial `log` object definition. This causes the `level` field in the `log` object to be updated from `error` to `debug`, while the `to` field remains unchanged:

```bash
log {
    to=[file,console]
    level=error
}

## ... more configs ...

log.level=debug
```

As a result, the final merged `log` configuration is:

```bash
log {
    to = [file, console]
    level = debug
}
```

This behavior allows fine-grained overrides without repeating the entire configuration structure.

**Attention:** The configuration method of the conf file no longer includes the "enable" option. Any configuration that appears in the configuration file is enabled by default.