11.5Pattern 4: Trace Correlation

Use a correlation ID to trace an event chain across multiple services.

import uuid

def test_order_flow_end_to_end(api_client, trace_store):
    """Trace an order through the entire event pipeline."""
    correlation_id = str(uuid.uuid4())

    # Initiate the flow
    response = api_client.post("/api/orders", json={
        **ORDER_PAYLOAD,
        "correlation_id": correlation_id,
    })
    assert response.status_code == 201

    # Wait for all services to process
    time.sleep(15)

    # Query the trace store for all events with this correlation ID
    traces = trace_store.get_traces(correlation_id)

    # Verify the expected event chain
    event_types = [t["event_type"] for t in traces]
    assert "order.created" in event_types
    assert "payment.initiated" in event_types
    assert "payment.completed" in event_types
    assert "inventory.reserved" in event_types
    assert "shipping.label_created" in event_types
    assert "notification.sent" in event_types

    # Verify ordering
    order_idx = event_types.index("order.created")
    payment_idx = event_types.index("payment.completed")
    shipping_idx = event_types.index("shipping.label_created")
    assert order_idx < payment_idx < shipping_idx, (
        f"Events out of order: order@{order_idx}, "
        f"payment@{payment_idx}, shipping@{shipping_idx}"
    )

When to use: Complex flows spanning multiple services. Verifying the entire chain completed correctly.