Personal Catalogue: React & Firebase — Chapter 9, Exercise 3
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TASK
Explain, for each of CORS, PM2, and a reverse proxy, precisely why
this course never needed to build it — tying each answer back to a
specific earlier architectural decision rather than a general
"Firebase handles it" claim.

SOLUTION

CORS: Cross-Origin Resource Sharing is a browser-enforced policy that
governs whether a plain fetch()/XHR request from one origin is allowed
to read a response from a different origin. This app never makes that
kind of request — the Firestore client SDK, initialized back in
Chapter 1, talks to Firestore's own backend using its own long-lived
connection and protocol, not a same-origin-restricted browser fetch
call the way the MongoDB sibling's own React app talks to its Express
API. With no cross-origin fetch of that kind ever happening, there was
never a CORS policy of this app's own to configure in the first place.

PM2: PM2 exists to keep a long-running Node process alive, restart it
if it crashes, and survive a server reboot. This course made the real
decision, back in Chapter 3, to have no backend server process of its
own at all — React talks directly to Firestore, a fully managed
service Firebase itself keeps running. There is no process belonging
to this project for PM2 to supervise.

Reverse proxy: An nginx reverse proxy in the MongoDB sibling's own
setup exists to put a real public domain and TLS certificate in front
of a raw Node process listening on a plain port, and to avoid exposing
that Node process directly to the internet. This course has no Node
process to protect or terminate TLS in front of — Firebase Hosting,
configured in this very chapter, already serves the built frontend
directly over HTTPS from Firebase's own infrastructure, with no
internal service address that would ever need a proxy sitting in front
of it.

In all three cases, the common thread is the same: each of these tools
solves a problem created specifically by running a backend server
process of one's own. Chapter 3's decision to never run one didn't just
simplify the app's own runtime architecture — it also eliminated every
production concern that only exists because a server process exists.

WHY THIS WORKS AS AN ANSWER
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It explains the real purpose each tool actually serves before
explaining why this app doesn't have the underlying problem that tool
solves, ties every answer back to the specific Chapter 3 decision
rather than a vague "Firebase is managed," and identifies the shared
root cause connecting all three rather than treating them as three
unrelated coincidences.