apophis-fastify/docs/chaos.md

Chaos Mode

Inject controlled failures into contract tests to validate resilience guarantees.

Usage

const result = await fastify.apophis.contract({
  depth: 'standard',
  chaos: {
    probability: 0.1,  // 10% of requests get chaos
    delay: { probability: 1, minMs: 100, maxMs: 500 },
    error: { probability: 1, statusCode: 503 },
    dropout: { probability: 1 },
    corruption: { probability: 1 },
  },
})

Event Types

Delay

Adds artificial latency. Tests timeout contracts:

timeout_occurred(this) == false
response_time(this) < 1000

Error

Forces HTTP status codes. Tests error-handling contracts:

if status:503 then response_body(this).retry_after != null

Dropout

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

status:200 || status:0

Corruption

Mutates response bodies. Tests parsing robustness:

response_body(this).id != null

Content-Type Aware Corruption

Built-in strategies for common formats:

Content-Type	Strategy	Effect
application/json	Truncate or null field	Removes fields or sets random field to null
application/x-ndjson	Chunk corrupt	Corrupts one NDJSON chunk
text/event-stream	Event corrupt	Adds malformed SSE line
multipart/form-data	Field corrupt	Replaces field with corrupted data
text/plain	Truncate	Cuts string in half

Custom Corruption via Extensions

const myExtension = {
  name: 'custom-corrupt',
  corruptionStrategies: {
    'application/vnd.api+json': (data) => ({
      ...data as object,
      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 mode is only available in test environment.

Interpreting Results

Failed tests include chaos events in diagnostics:

{
  "statusCode": 503,
  "error": "Contract violation: status:200",
  "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_body(this).error != null
4. Use seeds for reproducibility: seed: 42 makes chaos deterministic

Example: Testing Retry Logic

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

// Test
const result = await fastify.apophis.contract({
  chaos: {
    probability: 0.2,
    error: { probability: 1, statusCode: 503 },
  },
})