# NATS Client Module - Documentation

## Overview

The `natsClient.js` module provides a comprehensive NATS WebSocket client with automatic reconnection, request/reply messaging, and msghandler integration.

## Features

- **Automatic Connection**: Connects to NATS server on demand
- **Auto-Reconnection**: Exponential backoff reconnection on connection loss
- **Request/Reply**: NATS request/reply pattern for synchronous communication
- **Publish/Subscribe**: NATS pub/sub pattern for broadcast messaging
- **Msghandler Integration**: Automatic envelope wrapping/unwrapping for message serialization

## API Reference

### Connection Management

#### `getNats(opts)`

Establishes connection to NATS server.

```javascript
import { getNats } from './natsClient.js';

// Connect to default server
await getNats();

// Connect to custom server
await getNats({ servers: 'wss://nats.yiem.cc' });
```

**Parameters:**
- `opts.servers` (string): NATS server URL (default: `wss://nats.yiem.cc`)

**Returns:** `Promise<nc>` - NATS connection object

#### `connectNATS(brokerUrl, config)`

Connects to NATS broker using the provided URL or config.

```javascript
import { connectNATS } from './natsClient.js';

await connectNATS('wss://nats.yiem.cc');
// or
await connectNATS(null, { nats_server_info: { url: 'wss://nats.yiem.cc' } });
```

**Parameters:**
- `brokerUrl` (string): NATS broker URL
- `config` (object): Configuration object with `nats_server_info.url`

#### `closeNats()`

Closes the NATS connection and unsubscribes from all topics.

```javascript
import { closeNats } from './natsClient.js';

await closeNats();
```

### Subscription

#### `subscribe(subject, onMessage)`

Subscribes to a NATS subject and registers a message handler.

```javascript
import { subscribe } from './natsClient.js';

const subscription = await subscribe('sommpanion.backend.dbapi.v1.inbox', (message) => {
  console.log('Received message:', message);
});

// Later, to unsubscribe:
await subscription.unsubscribe();
```

**Parameters:**
- `subject` (string): NATS subject to subscribe to
- `onMessage` (function): Callback function to handle incoming messages

**Returns:** `Promise<{unsubscribe: Function}>` - Subscription object with unsubscribe method

### Publishing

#### `publish(subject, payload, opts)`

Publishes a message to a NATS subject.

```javascript
import { publish } from './natsClient.js';

await publish('sommpanion.backend.dbapi.v1.inbox', { cmd: 'search', data: { keyword: 'chardonnay' } });
```

**Parameters:**
- `subject` (string): NATS subject to publish to
- `payload` (object|string): Message payload
- `opts` (object): Optional configuration

### Request/Reply

#### `request(subject, payload, opts)`

Sends a request and waits for a reply using NATS request/reply pattern.

```javascript
import { request } from './natsClient.js';

const response = await request(
  'sommpanion.backend.dbapi.v1.inbox',
  { functioncall: 'search_wine_table', args: { searchkeyword: 'chardonnay' } }
);
```

**Parameters:**
- `subject` (string): NATS subject to send request to
- `payload` (string|object): Request payload
- `opts` (object): Optional configuration

**Returns:** `Promise<object>` - Response from the server

#### `reply(subject, handler, opts)`

Sets up a reply handler for request/reply pattern.

```javascript
import { reply } from './natsClient.js';

const subscription = await reply('sommpanion.backend.dbapi.v1.inbox', (request, msg) => {
  return { functioncall: request.functioncall, result: 'success', type: 'text' };
});

// Later, to unsubscribe:
await subscription.unsubscribe();
```

**Parameters:**
- `subject` (string): NATS subject to listen on
- `handler` (function): Handler function that returns response
- `opts` (object): Optional configuration

**Returns:** `Promise<{unsubscribe: Function}>` - Subscription object with unsubscribe method

### Msghandler Integration

#### `sendMsghandlerRequest(subject, functioncall, args, config)`

Sends a request using msghandler envelope format for proper message serialization.

```javascript
import { sendMsghandlerRequest } from './natsClient.js';
import msghandlerCSR from './msghandler-csr.js';

const response = await sendMsghandlerRequest(
  'sommpanion.backend.dbapi.v1.inbox',
  'search_wine_table',
  { searchkeyword: 'chardonnay', searchcolumn: 'wine_name' },
  { nats_server_info: { url: 'wss://nats.yiem.cc' } }
);

console.log(response.result); // Array of wine records
```

**Parameters:**
- `subject` (string): NATS subject to send request to
- `functioncall` (string): Backend function to invoke (e.g., `search_wine_table`)
- `args` (object): Function arguments
- `config` (object): Configuration object containing `nats_server_info.url`

**Returns:** `Promise<object>` - Response with `functioncall`, `result`, and `type` fields

### Configuration

#### Connection Status Store

```javascript
import { connectionStatus } from './natsClient.js';

connectionStatus.subscribe(status => {
  console.log('NATS status:', status);
});
```

The `connectionStatus` store emits objects with:
- `state`: `"connecting"`, `"connected"`, `"error"`, `"disconnected"`
- `message`: Human-readable status message

## End-to-End Examples

### Example 1: Search with Msghandler

```javascript
import { sendMsghandlerRequest, connectNATS } from './natsClient.js';

async function searchWines(keyword) {
  await connectNATS(null, { nats_server_info: { url: 'wss://nats.yiem.cc' } });
  
  const response = await sendMsghandlerRequest(
    'sommpanion.backend.dbapi.v1.inbox',
    'search_wine_table',
    { 
      searchkeyword: keyword,
      searchcolumn: 'seo_name'
    },
    { nats_server_info: { url: 'wss://nats.yiem.cc' } }
  );
  
  if (response && response.result) {
    console.log('Found wines:', response.result);
    return response.result;
  }
  
  return [];
}
```

### Example 2: Complete Search Component Integration

```javascript
import { sendMsghandlerRequest, getNats } from './natsClient.js';

export async function searchWines(keyword, config) {
  try {
    await getNats();
    
    const searchPayload = {
      searchkeyword: keyword || '*',
      searchcolumn: 'seo_name'
    };
    
    const response = await sendMsghandlerRequest(
      config?.dbservice_subject || 'sommpanion.backend.dbapi.v1.inbox',
      'search_wine_table',
      searchPayload,
      config
    );
    
    return response?.result || [];
    
  } catch (error) {
    console.error('Search failed:', error);
    return [];
  }
}
```

### Example 3: Basic Publish/Subscribe (No Msghandler)

```javascript
import { getNats, subscribe, publish } from './natsClient.js';

async function basicPubSubExample() {
  // Connect to NATS
  await getNats();
  
  // Subscribe to a topic
  const subscription = await subscribe('my.topic', (message) => {
    console.log('Received:', message);
  });
  
  // Publish a message
  await publish('my.topic', { data: 'hello world' });
  
  // Later, unsubscribe
  await subscription.unsubscribe();
}
```

### Example 4: Direct Request/Reply (No Msghandler)

```javascript
import { getNats, request } from './natsClient.js';

async function directRequestExample() {
  // Connect to NATS
  await getNats();
  
  // Send a direct request (no msghandler envelope)
  const response = await request(
    'sommpanion.backend.dbapi.v1.inbox',
    JSON.stringify({ 
      cmd: 'status',
      data: {}
    })
  );
  
  console.log('Response:', response);
}
```

## Error Handling

### Connection Errors

```javascript
import { connectNATS } from './natsClient.js';

try {
  await connectNATS('wss://nats.yiem.cc');
} catch (error) {
  console.error('NATS connection failed:', error);
}
```

### Request/Reply Errors

```javascript
import { sendMsghandlerRequest, connectNATS } from './natsClient.js';

try {
  const response = await sendMsghandlerRequest(
    'sommpanion.backend.dbapi.v1.inbox',
    'search_wine_table',
    { searchkeyword: '*', searchcolumn: 'seo_name' },
    { nats_server_info: { url: 'wss://nats.yiem.cc' } }
  );
  
  console.log(response.result);
} catch (error) {
  console.error('Request failed:', error.message);
}
```

## Best Practices

1. **Reuse Connection**: Don't connect/disconnect for each request. Connect once on app startup.
2. **Handle Errors**: Always wrap NATS operations in try-catch blocks.
3. **Use Msghandler**: Always use `sendMsghandlerRequest()` for backend API calls.
4. **Clean Up**: Unsubscribe from topics when components are destroyed.
5. **Configuration**: Read NATS server URL from config for environment-specific configuration.

## Msghandler Message Format

### Request Envelope

When using `sendMsghandlerRequest()`, the module automatically:
1. Wraps the payload in msghandler format
2. Uses `dictionary` payload type with `db_request` data name
3. Automatically selects transport based on payload size

### Response Format

The response from backend follows this structure:

```json
{
  "correlation_id": "uuid-v4",
  "msg_id": "uuid-v4",
  "timestamp": "2026-05-26T...",
  "send_to": "sommpanion.backend.dbapi.v1.inbox",
  "msg_purpose": "chat",
  "sender_name": "agent-backend",
  "payloads": [
    ["db_request", {
      "functioncall": "search_wine_table",
      "result": [...],
      "type": "jsontable"
    }, "dictionary"]
  ]
}
```

The `sendMsghandlerRequest()` function automatically:
- Unwraps the envelope using `smartunpack()`
- Extracts the result from `payloads[0][1]`
- Returns `{functioncall, result, type}` object

## Troubleshooting

### Connection Issues

- Verify NATS server URL in config
- Check network connectivity
- Ensure NATS server is running

### Request Timeouts

- Check backend service is running
- Verify subject matches backend listener

### Response Parsing Errors

- Ensure backend returns msghandler format
- Check payload type matches expected format
- Verify `functioncall` and `result` fields exist in response