The CAP Theorem: A Revisit
The Origin of the Problem
As I delved into the world of distributed databases, I was struck by the dazzling array of characteristics that these systems brought to the table. High availability, resilience, and scalability were just a few of the many traits that made them so appealing. But it was the CAP theorem that caught my attention, and for a while, it became my golden rule. I would apply it to every system I encountered, always wondering whether it was AP or CP. It wasn’t until recently, however, that I began to question the very foundation of this theorem.
The Circumstances Surrounding the CAP Theorem
The CAP theorem, also known as “Brewer’s Conjecture,” has been a topic of discussion for many years. While it is often referred to as a theorem, it can also be seen as an axiom or a conjecture, depending on one’s perspective. The popular argument is that it was first proposed by Brewer, but it was Seth Gilbert and Nancy Lynch who provided the proof in their paper, “Brewer’s Conjecture and the Feasibility of Consistent, Available, Partition-Tolerant Web Services.”
ZooKeeper, MongoDB, and the CAP
When I first encountered the CAP theorem, I thought it was a straightforward concept. However, as I dug deeper, I realized that it was not as simple as I had initially thought. The definition of A, which states that every request received by a non-failing node in the system must result in a response, seemed too simplistic and too diverse to understand. For example, in MongoDB, if a client sends a write request directly to a secondary node, how should the system respond? Should it return an error, or should it count it as a “result in a response”? These kinds of questions made me wonder whether ZooKeeper and MongoDB, both of which are highly available, actually meet the CAP A.
Strict Proof of the CAP Theorem
As I continued to explore the CAP theorem, I began to question its validity. From one perspective, it seems like an axiom, while from another, it appears to be a theorem. I attempted to read “Brewer’s Conjecture and the Feasibility of Consistent, Available, Partition-Tolerant Web Services,” but found it challenging to understand. I also looked at Nancy Lynch’s “Distributed Algorithms,” but was unable to grasp the proof of the CAP theorem.
A Correct Understanding of the CAP
A correct understanding of the CAP theorem requires a deeper examination of its definition and its implications. It is not just a simple concept, but rather a complex idea that has far-reaching consequences. As I continued to explore the topic, I realized that there are many misconceptions about the CAP theorem, and that it is not as straightforward as it seems.
Reference Material
For those interested in learning more about the CAP theorem, I recommend reading “Brewer’s Conjecture and the Feasibility of Consistent, Available, Partition-Tolerant Web Services” and “Distributed Algorithms.” These papers provide a deeper understanding of the CAP theorem and its implications.
The CAP Theorem: A Revisit
In conclusion, the CAP theorem is a complex concept that has far-reaching implications. While it may seem simple at first glance, it requires a deeper examination of its definition and its implications. As I reflect on my journey of understanding the CAP theorem, I realize that it is not just a theorem, but rather a way of thinking about distributed systems.