What are GraphQL subscriptions, and how do you test them?

Where queries and mutations are request/response, subscriptions are long-lived: the client subscribes and the server pushes messages as events happen, typically over a WebSocket.

subscription {
  orderStatusChanged(orderId: "1001") { status updatedAt }
}

Testing is more involved than request/response:

1. Connection: the subscription establishes and acknowledges.
2. Delivery: trigger the underlying event (e.g. change the order status via a mutation) and assert the subscriber receives the expected message.
3. Filtering: a subscription scoped to orderId: 1001 must NOT receive events for order 1002.
4. Ordering/dedup: if ordering matters, assert messages arrive in order and aren't duplicated.
5. Teardown: unsubscribe/disconnect cleanly, and assert no messages arrive after; check for resource/connection leaks.
6. Reconnection: behaviour on dropped connection — does it resume, and are missed messages handled?

Because they're stateful and timing-dependent, subscription tests are flakier — use explicit waits for expected messages with timeouts rather than fixed sleeps.