> For the complete documentation index, see [llms.txt](https://0xgosu.gitbook.io/monolith-framework/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://0xgosu.gitbook.io/monolith-framework/examples/nats-config.md).

# NATS Config File

This example demonstrates how to use `WithNATSConfigFile()` to load NATS server configuration from an external file, and how to combine it with programmatic overrides.

## Overview

The Monolith Framework supports loading NATS server configuration from a standard NATS config file. This is useful for:

* Using organization-standard NATS configurations
* Separating configuration from code
* Managing complex NATS settings (clustering, JetStream, authorization)

## Config Precedence

When both a config file and programmatic options are specified:

1. **Config file provides base settings** - All settings from the file are applied first
2. **Programmatic options override** - Options like `WithNATSPort()` take precedence

```go
app, err := mono.NewMonoApplication(
    mono.WithNATSConfigFile("server.conf"), // Base: port=4222, host=127.0.0.1
    mono.WithNATSPort(4333),                // Override: port becomes 4333
    mono.WithNATSHost("0.0.0.0"),           // Override: host becomes 0.0.0.0
)
```

This pattern allows you to use a standard config file while customizing specific settings at runtime.

## Running the Example

```bash
# From the examples/nats-config directory
cd examples/nats-config
go run .

# Or from the repository root
go run ./examples/nats-config
```

## Expected Output

```
=== Mono-Framework NATS Config File Example ===
Demonstrates: Loading NATS configuration from a file with programmatic overrides

Config file: server.conf
✓ App created with config file + overrides
  - Base config loaded from: server.conf
  - Port overridden: 4222 -> 4333
  - Host overridden: 127.0.0.1 -> 0.0.0.0

✓ Module registered
  → HelloModule started
✓ App started - NATS server is running with merged configuration

Health Status: Healthy=true, NATSHealthy=true

Press Ctrl+C to shutdown...
```

## Files

| File          | Description                                                   |
| ------------- | ------------------------------------------------------------- |
| `main.go`     | Application entry point demonstrating config file + overrides |
| `module.go`   | Simple module implementation                                  |
| `server.conf` | Sample NATS server configuration file                         |
| `README.md`   | This documentation                                            |

## Config File Reference

The `server.conf` file includes:

* **Server settings**: `server_name`, `port`, `host`
* **Connection limits**: `max_connections`, `max_payload`
* **JetStream settings**: Memory and file storage limits
* **Commented examples**: Clustering, leaf nodes, authorization

See the [NATS Server Configuration](https://docs.nats.io/running-a-nats-service/configuration) documentation for all available options.

## When to Use Config Files

**Use config files when:**

* You have complex NATS settings (clustering, TLS, authorization)
* You want to reuse configurations across environments
* You need to share configurations with operations teams
* You want to keep sensitive settings separate from code

**Use programmatic options when:**

* You have simple configurations
* Settings need to be computed at runtime
* You're writing tests with specific port requirements
* You want everything self-contained in code

**Use both together when:**

* You have a base configuration but need runtime overrides
* You want sensible defaults from a file with environment-specific tweaks
* You're running the same app in different environments

## Related Documentation

* [Framework API - WithNATSConfigFile](/monolith-framework/api-reference/framework.md)
* [Quick Start Guide](/monolith-framework/getting-started/quickstart.md)


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