apophis-fastify/docs/chaos.md

# Chaos Mode

Inject controlled failures into contract tests to validate resilience guarantees.

Chaos testing applies invariant-driven verification under adverse conditions: if a contract must hold, it should still hold when dependencies fail, responses are delayed, or payloads are corrupted.

## Usage

```javascript
const result = await fastify.apophis.contract({
  runs: 50,
  chaos: {
    delay: { probability: 0.1, minMs: 100, maxMs: 500 },
    error: { probability: 0.1, statusCode: 503 },
    dropout: { probability: 0.05 },
    corruption: { probability: 0.1 },
  },
});
```

## Event Types

### Delay

Adds artificial latency. Tests timeout contracts:

```apostl
response_time(this) < 1000
```

**Note**: Delay events are generated by the chaos arbitrary but the inbound delay handler is currently a no-op. Use this for timeout contract documentation; actual delay injection requires the outbound delay strategy or a custom handler.

### Error

Forces HTTP status codes. Tests error-handling contracts:

```apostl
// Behavioral: when the service is unavailable, the client receives a valid retry signal
if status:503 then response_headers(this).retry-after > 0
```

### Dropout

Simulates network failure (status 0). Tests fallback contracts:

```apostl
// Behavioral: partial failure must still return previously cached data
if status:0 then response_body(this).cached_data == previous(response_body(GET /cache/{request_params(this).key}))
```

### Corruption

Mutates response bodies. Tests parsing robustness:

```apostl
// Behavioral: corrupted requests maintain traceability for debugging
if status:400 then response_body(this).request_id == request_headers(this).x-request-id
```

## Corruption Strategies

Built-in strategies are content-type agnostic:

| Strategy | Effect |
|----------|--------|
| `truncate` | Cuts response body short |
| `malformed` | Invalidates structural boundaries (e.g., unclosed JSON, bad headers) |
| `field-corrupt` | Replaces a random field value with corrupted data |

Extension strategies can add content-type-specific behavior if needed.

## Custom Corruption via Extensions

```javascript
const myExtension = {
  name: 'custom-corrupt',
  corruptionStrategies: {
    'application/vnd.api+json': (data) => ({
      ...data,
      corrupted: true,
    }),
    'text/*': (data) => `CORRUPTED:${String(data)}`,
  },
};

await fastify.register(apophis, {
  extensions: [myExtension],
});
```

Extension strategies take precedence over built-ins. Wildcard patterns (`text/*`) match any subtype.

## Environment Guard

Low-level contract chaos APIs require `NODE_ENV=test`. For CLI qualification, environment policy controls whether chaos gates may run.

```
Error: chaos is only available in test environment. Set NODE_ENV=test to enable quality features.
```

## Interpreting Results

Failed tests include chaos events in diagnostics:

```json
{
  "statusCode": 503,
  "error": "Contract violation: if status:503 then response_headers(this).retry-after > 0",
  "chaosEvents": [
    {
      "type": "error",
      "injected": true,
      "details": {
        "statusCode": 503,
        "reason": "Chaos error: overridden 200 with 503"
      }
    }
  ]
}
```

## Best Practices

1. **Start small**: `probability: 0.05` (5% of requests)
2. **Test one failure mode at a time**: Comment out other chaos types
3. **Verify contracts handle chaos**: `if status:503 then response_code(GET /health) == 200`
4. **Use seeds for reproducibility**: `seed: 42` makes chaos deterministic

## Example: Testing Retry Logic

```javascript
fastify.get('/data', {
  schema: {
    'x-ensures': [
      'if status:503 then response_headers(this).retry-after > 0',
      'redirect_count(this) <= 3',
    ],
  },
}, handler);

// Test
const result = await fastify.apophis.contract({
  chaos: {
    error: { probability: 0.2, statusCode: 503 },
  },
});
```