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hardSystem Design

What are event sourcing and CQRS?

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01

Understand the problem

Explain event sourcing and CQRS.

event-sourcingcqrs
02

Attempt it yourself

Sketch your approach before reading the solution — that's what interviews test.

Nudge consolestandby

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03

Study the solution

Step 1: Outline use cases and constraints

Gather requirements and scope the problem. Ask questions to clarify use cases and constraints. Discuss assumptions.

Use cases

We'll scope the problem to handle only the following use cases

  • User needs to understand event-sourcing to make architecture de

Solution ready — 2 min read

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04

Read the code

Derive state by folding events
Run Playground
# Event sourcing: events are the source of truth; state is a fold of them.
events = [
    {"type": "Opened",    "balance": 0},
    {"type": "Deposited", "amount": 100},
    {"type": "Withdrew",  "amount": 30},
    {"type": "Deposited", "amount": 50},
]

def project(events):                       # build a read model from the log
    balance = 0
    for e in events:
        if e["type"] == "Opened":     balance = e["balance"]
        elif e["type"] == "Deposited": balance += e["amount"]
        elif e["type"] == "Withdrew":  balance -= e["amount"]
    return balance

print("current balance:", project(events))   # 120
# You can replay to any point in time, audit every change, or build a
# brand-new projection from the same events.
05

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