Software architecture — layered, MVC, event-driven, microservices, serverless: Foundations

This is the foundations page for the parent topic. Before you read about the five styles, you must be able to see what a "module", a "dependency", "coupling", a "process", and an "event" actually are. We build each from a picture, then explain why the topic can't move without it.

0. What is a "module"? (the atom of everything)

Before any architecture, we need the thing we are drawing boundaries around.

The picture: a labelled box. Nothing more. Whether the box is 10 lines or 10,000, architecture treats it the same way — as one thing with a boundary and a name.

Why the topic needs it: every style below is a rule about how boxes may be arranged and connected. Layered stacks boxes vertically; microservices scatter them across a network. If "box" isn't clear, none of that is.

Figure — Software architecture — layered, MVC, event-driven, microservices, serverless

1. What is a "dependency"? (the arrow)

Boxes are useless alone. They talk. When box A calls a function inside box B, or reads B's data, we say A depends on B.

Why the topic needs it: the layered rule "arrows point downward only" and the event-driven idea "producers don't point at consumers" are both statements about arrows. You literally cannot read the topic without this arrow in your head.

2. Coupling and Cohesion (the two master words)

Now we can measure whether an arrangement of boxes-and-arrows is good. Two words do all the work — see Coupling and Cohesion for the full treatment.

Why the topic needs it: the parent says "every architecture style is a strategy to lower coupling while keeping cohesion high." These two words are the scoreboard every style is trying to win.

3. Synchronous vs Asynchronous (waiting, or not)

When box A calls box B, a question appears that the topic returns to again and again: does A stop and wait for B to answer?

4. Process, and "deployment unit" (what actually runs)

Boxes are code at rest. To run, code lives inside a process.

Why the topic needs it: the entire Monolith-vs-Microservices table (one deploy unit vs many, shared database vs per-service) is about how boxes are packed into processes. No "process", no table.

5. Event, Producer, Consumer, Broker (the vocabulary of decoupling)

The event-driven, microservices, and serverless sections all share one small cast of characters.

Now that producers and consumers have names, we can count them. Let us give the counts symbols.

Worked example Why "area vs line" actually matters — put numbers in

Watch what happens as the system grows from small to large, always with equal producers and consumers ( $n = m$ ):

producers $n$	consumers $m$	direct $n \cdot m$	broker $n + m$
2	2	4	4
5	5	25	10
10	10	100	20
50	50	2500	100

At tiny sizes they tie (4 vs 4). But by 50-a-side the direct tangle needs 2500 wires while the broker needs just 100 — a 25× difference, and the gap keeps widening. That is the whole point of "area grows faster than a line": doubling the system roughly quadruples the direct wiring but only doubles the broker wiring. This is the structural reason event systems scale to many teams.

Why the topic needs it: this exact fan-out formula appears in the parent's event-driven section. It is the quantitative reason to prefer a broker, and it rests entirely on the producer/consumer/broker picture above.

6. Two follow-on terms you'll meet (named now, defined in D2)

The parent uses two more phrases in its "mistake" callouts. You only need to recognise them here.

Eventual consistency — when data is copied across independent services, they may briefly disagree, then agree "eventually." Full treatment in Eventual Consistency.
Conway's Law — a system's structure ends up mirroring the communication structure of the org that built it; this is why microservices map to teams. See Conway's Law.

How the foundations feed the topic

Equipment checklist

Cover the right side and answer out loud. If any is shaky, re-read its section before starting the parent topic.

What is a module in architecture terms?

Any named chunk of code treated as one unit — a labelled box with a boundary.

What does an arrow "A → B" (dependency) mean, and why does direction matter?

A needs B to work; changing B can break A, but changing A leaves B untouched.

We want coupling ___ and cohesion ___.

Coupling low (few arrows between boxes), cohesion high (each box does one job).

Difference between synchronous and asynchronous calls?

Sync = caller freezes and waits for the reply; async = caller drops a message and immediately moves on.

Why isn't a layered app automatically scalable?

Layers are usually boxes inside one process (a monolith); you must replicate the whole process, not one layer.

In the fan-out formula, what do

n

and

m

count?

n

= number of producers,

m

= number of consumers.

What is a broker, and what problem does it solve?

A middle-man mailbox between producers and consumers; it cuts connections from

O (n \cdot m)

O (n + m)

An event is phrased in which tense, and does it command anyone?

Past tense (a fact like OrderPlaced); it announces, it does not command.

Does a producer know its consumers in an event system?

No — that ignorance is exactly what makes them decoupled.