LUC #54: CAP Theorem Explained In Simple Terms

Plus, API testing types, tips and strategies for effective debugging, and 8 popular network protocols explained

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CAP Theorem Simplified

Do I keep the system available even though the data is incorrect?

Or do I wait for the data to become consistent throughout the system, even if it means the system is unavailable in the meantime?

This is a classic conundrum faced in distributed systems.

It’s the core dilemma that CAP theorem explores.

Let’s dive in.

The CAP Theorem explained

The CAP theorem is a fundamental principle in distributed computing that outlines the trade-offs a distributed system must make when dealing with three key properties—consistency, availability, and partition tolerance.

CAP theorem asserts that a distributed system cannot simultaneously provide consistency, availability, and partition tolerance.

Consistency ensures all nodes display the same data simultaneously, which is crucial for systems that need all clients to receive up-to-date and accurate information.

Availability means that every request (read or write) receives a response, even if it’s not the most recent write. The system remains operational and responsive at all times.

Partition tolerance refers to the system’s ability to continue operating despite message losses or failure within the system.

Given that network partitions are inevitable, systems must choose between consistency and availability.

It’s important to note that CAP theorem assumes ideal conditions of 100% availability and 100% consistency.

In the real world, it’s not so black and white.

The real world is complex, dynamic, and messy, with varying degrees of consistency and availability.

While CAP theorem underscores a crucial aspect of system design—balancing trade-offs—the simplistic model can be misleading. It’s best to think of it as a guide or tool rather than a strict rule.

Practical Implications and Trade-offs

The CAP theorem highlights the need for trade-offs in distributed system design. Different systems must prioritize specific aspects based on their requirements.

Consider an online retail store with multiple inventory databases across different locations.

Consistency vs Availability


Ensures all customers see the same inventory information.

For example, if a customer in San Francisco sees that there are 5 units of a product available, a customer in Sydney will see the same.

This prevents overselling but if one database becomes unreachable, the system may deny all sales transactions to maintain consistency, affecting availability.


Ensures customers can always place orders, even if the inventory databases are not perfectly synchronized. This means that if the San Francisco database is temporarily unreachable, customers can still place orders based on the Sydney database.

This improves customer experience but risks inconsistencies, such as two customers purchasing the same product simultaneously, leading to overselling.

The store must decide which aspect is more critical.

If preventing overselling is paramount, consistency should be prioritized.

If ensuring customers can always place orders is more important, availability should take precedence.

Understanding these trade-offs helps us design a system that best meets their operational needs.

Modern Interpretations and Applications

The principles of CAP theorem remain highly applicable today, as cloud computing, big data, and microservices dominate the tech landscape.

Given that modern workloads are highly dynamic, systems in these environments must continually reevaluate the balance between consistency and availability.

Adopting adaptable models that offer the best balance between these components in real-time is generally advisable.

The CAP theorem continues to serve as a guide for building resilient distributed systems capable of managing unanticipated issues.

While it’s a good starting point, it doesn’t provide a complete picture of the trade-offs to consider when designing robust distributed systems. Distributed systems are complex, and consistency and availability are just two qualities to consider when designing a robust system.

Final Thoughts

The CAP theorem, while simple in its formulation, offers profound insights into the design and operation of distributed systems.

It provides a framework that helps us understand the trade-offs involved in creating robust systems.

Since Eric Brewer introduced it in the year 2,000, technologies have evolved immensely. However, the principles of the CAP theorem continue to guide us in making informed trade-offs and building robust systems that meet requirements.

API Testing Types (Recap)

Six of the most important forms of API testing:

Validation Testing — tests an API's adherence to system requirements and standards, establishing a baseline for further testing.

Performance Testing — evaluates speed, responsiveness, and stability.

Security Testing — identifies vulnerabilities, preventing unauthorized access and data breaches with techniques like penetration testing and fuzz testing.

Functional Testing — verifies that the API performs its intended operations correctly and responds accurately to requests.

Reliability Testing — assesses consistent performance over time, exposing and correcting stability issues.

Integration Testing — assesses API communication with other systems, confirming stable performance and smooth integration across different components.

Tips and Strategies For Effective Debugging

1) Define the problem

Identify the problem’s symptoms, and compare expected versus actual outcomes. Determine its scope, assess its severity and impact, and note steps to reproduce it. This clarity streamlines the troubleshooting process.

2) Reproduce it

Reproducing the bug is often the most effective way to pinpoint its cause. However, if this can't be done, try checking the environment where it occurred, search the error message online, assess the system's state at the time, note how often it happens, and identify any recurring patterns. These steps can offer vital clues.

3) Identify the cause

Logs are a big help in the debugging process; if they're insufficient, add more logs and reproduce the issue. Some additional strategies are to use debugging tools for insights, test components in smaller chunks, and try commenting out code sections to pinpoint the problem area.

4) Provide a postmortem

When a bug's cause is identified and resolved, thoroughly document the issue, the fix, and ways to prevent it in the future. Sharing this knowledge with the team is important to ensure everyone is informed and can benefit from the lessons learned, promoting a proactive approach to future challenges.

  • HTTP (Hypertext Transfer Protocol) — Used by web browsers and servers to communicate and exchange.

  • HTTPS (Hypertext Transfer Protocol Secure) — An extension of HTTP that offers secure and encrypted communication.

  • FTP (File Transfer Protocol) — Used to transfer files between a client and server.

  • TCP Transmission Control Protocol — Delivers a stream of ordered bytes from one computer to another.

  • IP (Internet Protocol) — Addresses and routes packets of data sent between networked devices.

  • UDP (User Datagram Protocol) — A simple and connectionless protocol that does not divide messages into packets and send them in order.

  • SMTP (Simple Mail Transfer Protocol) — Used to transmit emails across IP networks.

  • SSH (Secure Shell) — A cryptographic network protocol for secure data communication, remote command-line login, and remote command execution between two networked computers.

That wraps up this week’s issue of Level Up Coding’s newsletter!

Join us again next week where we’ll explore clean architecture, database caching strategies, cookies vs sessions, and more.