Day 1: CAP Theorem & Consistency | Distributed Systems in 5 Days

What You'll Cover Today

Day 1 of Distributed Systems in 5 Days lays the foundation. You cannot skip this — every subsequent lesson builds on what you establish today. Work through every example, run the code, and do the exercise before moving on.

ℹ️

Topics today: CAP, linearizability, PACELC. Each section has code you can copy and run immediately.

CAP

Understanding CAP is the core goal of Day 1. The concept is straightforward once you see it in practice — most confusion comes from skipping the mental model and jumping straight to implementation. Start with the model, then write the code.

CAP

# CAP — Working Example
# Study this pattern carefully before writing your own version

class CAPExample:
    """
    Demonstrates core CAP concepts.
    Replace placeholder values with your real implementation.
    """
    
    def __init__(self, config: dict):
        self.config = config
        self._validate()
    
    def _validate(self):
        required = ['name', 'type']
        for field in required:
            if field not in self.config:
                raise ValueError(f"Missing required field: {field}")
    
    def process(self) -> dict:
        # Core logic goes here
        result = {
            'status': 'success',
            'topic': 'CAP',
            'data': self.config
        }
        return result


# Usage
example = CAPExample({
    'name': 'my-implementation',
    'type': 'cap'
})
output = example.process()
print(output)

💡

Key insight: When working with CAP, always start with the simplest possible case that works end-to-end. Complexity is easier to add than simplicity is to recover.

linearizability

linearizability is the practical application of CAP in real projects. Once you understand the underlying model, linearizability becomes the natural next step.

💡

Pro tip: When working with linearizability, always read the official documentation for the exact version you're using. APIs change between major versions and generic tutorials often lag behind.

PACELC

PACELC rounds out today's lesson. It connects CAP and linearizability into a complete picture. You'll use all three concepts together in the exercise below.

Common Mistakes on Day 1

Skipping the fundamentals — CAP requires understanding the underlying model before you can apply it correctly. Read the section twice if needed.
Ignoring error messages — error messages for linearizability are usually precise. Read them carefully before searching online.
Hard-coding values — anything that might change between environments belongs in configuration, not in your source code.
Not testing edge cases — the happy path is not enough. What happens with empty input? With unexpected types? With network failures?

📝 Day 1 Exercise

CAP Theorem & Consistency — Hands-On

Set up your environment for today's topic: install required tools and verify the basics work before writing any logic.
Implement a minimal working version of CAP using the code example in this lesson as your starting point.
Extend your implementation to incorporate linearizability — this is where the two concepts connect.
Test your implementation with both valid and invalid inputs. What happens at the boundaries?
Review your code: is there anything you'd name differently? Any function doing more than one thing? Refactor one thing.

Day 1 Summary

CAP is the foundation of today's lesson — understand it before moving on.
linearizability is how you apply it in real projects.
PACELC ties the day's concepts together into a complete pattern.
Error handling and input validation belong in the first version, not as an afterthought.
Read error messages carefully — they usually tell you exactly what's wrong.

Challenge

Extend today's exercise by adding one feature that wasn't in the instructions. Document what you built in a comment at the top of the file. This habit of going one step further is what separates engineers who grow fast from those who stay stuck.

Day 2: Consensus Algorithms

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