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The problem: your services are holding hands
Synchronous call — checkout waits on emaillatency ~20ms
Checkout
POST /order → email
waiting…
Email
SMTP relay
200 in 20ms
healthy — checkout calls email synchronously and waits for the reply
Checkout calls the email service directly and synchronously. That one design decision couples their fates:
- ▸Email is slow → checkout is slow — your revenue path waits on a marketing email
- ▸Email is down → checkout is down — a non-critical service kills a critical one
- ▸Traffic spikes hit every service at once — no buffer anywhere, everything falls together
★
Direct synchronous calls couple services' fates — if the downstream is slow or down, you are too.
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