Exercise 1: Adding a Fourth Aggregated Service — Possible Solution
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THE NEW SERVICE
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    class ShippingService:
        def get_shipping_estimate(self, product_id):
            time.sleep(0.005)  # matches this chapter's own internal-latency figure
            return {'product_id': product_id, 'eta_days': 3}

Added to ApiGateway following the exact same constructor/method
pattern as the three existing services, and added as a fourth
sequential call inside get_order_page().

RESULTS
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    4 services WITHOUT gateway: 222.5ms
    4 services WITH gateway: 71.4ms
    ratio: 3.12x (chapter's own 3-service ratio was 2.53x)

VERIFYING THE GAP WIDENS
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Without the gateway, each additional service adds a full 50ms external
round trip to the client's own total (166.9ms for 3 services -> 222.5ms
for 4, an increase of roughly 55.6ms - matching the expected 50ms
external cost plus normal timing variance). With the gateway, each
additional service only adds its own small internal cost (66.0ms for 3
services -> 71.4ms for 4, an increase of roughly 5.4ms - matching the
expected 5ms internal cost). The overall speedup ratio grew from 2.53x
(3 services) to 3.12x (4 services), confirming the gap widens as more
services are aggregated, not just holds steady.

WHY THIS CONFIRMS THE GATEWAY'S OWN VALUE SCALES WITH FAN-OUT
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Every additional backend call a client would otherwise need pays the
full external round-trip cost directly; every additional call the
gateway makes internally pays only the much smaller internal cost. This
means the gateway's own relative advantage isn't fixed - it grows
larger the more services a single client-facing page or request
genuinely needs to pull data from, which is exactly the situation real
systems with many small services tend to create over time.

WHY THIS WORKS AS AN ANSWER
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The new service matches this chapter's own established shape and
latency figures exactly, both timings are measured directly rather
than estimated, and the widening gap is confirmed by comparing the
new 4-service ratio against this chapter's own original 3-service
ratio rather than only reporting the new numbers in isolation.