# CAP Theorem

CAP is acronym for Consistency, Availability, and Partition Tolerance. It is a theorem that states that it is impossible for a distributed data store to simultaneously provide more than two out of the following three guarantees:

* CA (Consistency and Availability) - All nodes see the same data at the same time. This guarantee is not applied for distributed systems because they have to be Partition Tolerance
* AP (Availability and Partition Tolerance) - Every request receives a (non-error) response, without the guarantee that the data is the most recent.
* CP (Consistency and Partition Tolerance) - Every request receives a (non-error) response, with the guarantee that the data is the most recent.

## Consistency

**Strong Consistency** - during the write operation, all replicas in the system should be updated before the write operation is acknowledged as performed. In other words, any read operation always returns the most recent write. Achieving strong consistency requires a high level of coordination and synchronization between nodes, which can affect availability.

**Eventual Consistency** – during the write operation, the replicas in the system are updated asynchronously. In that case, replicas in the system could return stale data for a period of time until the change is propagated to all replicas.

## Links

* [Video by IBM Technology: What is CAP Theorem?](https://youtu.be/eWMgsk7mpFc)
* [Video by ByteByteGo: CAP Theorem Simplified | System Design Fundamentals](https://youtu.be/BHqjEjzAicA)
* [Video by Studying with Alex: Friendly Intro To the CAP Theorem (Consistency vs Availability vs Partition Tolerance)](https://youtu.be/gkg-FAEXIkY) – I liked the examples which were used to explain the CAP theorem.
* [Article by Vivek Kumar Singh: Eventual Consistency vs Strong Consistency](https://medium.com/system-design-blog/eventual-consistency-vs-strong-consistency-b4de1f92534d)
* [Article by Peter Bailis: Linearizability versus Serializability](http://www.bailis.org/blog/linearizability-versus-serializability/)


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://wiki.dmmax.me/system-design/cap.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.