API Conditional Requests Handler

You are a caching specialist implementing efficient conditional request handling.

## CONTEXT
I need to implement conditional requests for my API.

**Requirements:**
- Resource Types: {{RESOURCES}}
- Update Frequency: {{FREQUENCY}}
- Concurrency Needs: {{CONCURRENCY}}
- Cache Strategy: {{CACHE}}

## TASK
Design conditional request handling:

### 1. ETAG GENERATION
```typescript
function generateETag(data: any): string {
  const hash = crypto
    .createHash('md5')
    .update(JSON.stringify(data))
    .digest('hex');
  return `"${hash}"`;
}
```

### 2. GET WITH CONDITIONAL
```
Request:
GET /api/users/123
If-None-Match: "abc123"

Response (not modified):
HTTP/1.1 304 Not Modified
ETag: "abc123"

Response (modified):
HTTP/1.1 200 OK
ETag: "def456"
{...new data...}
```

### 3. PUT WITH CONDITIONAL (Optimistic Locking)
```
Request:
PUT /api/users/123
If-Match: "abc123"
{...update data...}

Response (conflict):
HTTP/1.1 412 Precondition Failed
{
  "error": "Resource was modified",
  "currentETag": "def456"
}
```

### 4. LAST-MODIFIED
```
Response:
Last-Modified: Wed, 15 Jan 2024 10:30:00 GMT

Request:
If-Modified-Since: Wed, 15 Jan 2024 10:30:00 GMT
```

### 5. IMPLEMENTATION
```typescript
async function handleConditional(req, res, resource) {
  const etag = generateETag(resource);

  if (req.headers['if-none-match'] === etag) {
    return res.status(304).end();
  }

  res.setHeader('ETag', etag);
  return res.json(resource);
}
```

### 6. WEAK VS STRONG ETAGS
```
Strong: "abc123"
Weak: W/"abc123"
```