# CAP Theorem

## Definition of CAP Theorem

The CAP Theorem, also known as Brewer’s Theorem, is a fundamental principle in distributed computing that states that a system cannot simultaneously provide all three of the following properties: Consistency (C), Availability (A), and Partition Tolerance (P). In simpler terms, a distributed system can achieve only two of these properties at once. This theorem helps engineers make decisions when designing and maintaining distributed systems based on the necessary trade-offs between these properties.

### Phonetic

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## Key Takeaways

1. CAP Theorem states that it is impossible for a distributed data store to simultaneously provide Consistency, Availability, and Partition Tolerance.
2. Consistency means every node sees the same data at the same time, Availability ensures that the system continues to operate and respond to requests despite failures, and Partition Tolerance means the system can withstand network partitions between nodes.
3. When designing a distributed system, developers need to make trade-offs between the three principles, often by selecting two out of the three, based on the specific requirements of their application.

## Importance of CAP Theorem

The CAP Theorem, also known as Brewer’s Theorem, is of vital importance in the field of distributed computing as it helps developers and system architects understand the fundamental trade-offs and limitations when designing large-scale, fault-tolerant, and distributed systems.

The theorem states that it is impossible for a distributed data store to simultaneously guarantee Consistency, Availability, and Partition Tolerance, meaning only two of these attributes can be ensured at a time.

This understanding allows architects to prioritize and make informed decisions about which attributes to emphasize according to their specific use case and system requirements, leading to better performance, reliability, and overall user experience.

## Explanation

The CAP Theorem, also known as Brewer’s Theorem, is essential to understand and appreciate the design trade-offs made while architecting distributed systems.

It revolves around three highly desirable characteristics in a distributed system: Consistency (C), Availability (A), and Partition Tolerance (P). In simple terms, consistency refers to the practice of ensuring that every request to read data results in the latest written value; availability reflects the system’s ability to serve the maximum number of read and write requests, even if some part of the system fails; and partition tolerance refers to how well the system continues to operate even when it is divided into separate partitions due to network outages or errors.

The essence of the CAP Theorem lies in stating that it is impossible to achieve all three of these desirable characteristics simultaneously in a distributed system.

A system can only prioritize two out of the three at any given time.

This theorem compels system architects and developers to make difficult decisions, depending on their application’s specific use case and requirements, as they select the combinations of Consistency and Partition Tolerance (CP), Availability and Partition Tolerance (AP), or Consistency and Availability (CA). By understanding the CAP Theorem’s inherent trade-offs and carefully choosing the most suitable combination, developers can design and build robust distributed systems that prove to be resilient, scalable, and functional within the bounds of their intended purposes.

## Examples of CAP Theorem

The CAP Theorem, also known as Brewer’s Theorem, states that in a distributed data storage system, it is impossible to simultaneously guarantee all three of the following properties: Consistency (C) every read receives the most recent write; Availability (A) every request receives a response; and Partition Tolerance (P) the system continues to operate despite network partitioning. Real-world examples of the CAP Theorem in use include:

Amazon’s DynamoDB: A highly available and scalable NoSQL managed database service designed for applications that require a fast and flexible data model. To provide high availability, it sacrifices consistency; when a network partition occurs, it continues to operate on both sides, with data inconsistencies resolved after the partition is resolved. As a result, it prioritizes Availability and Partition Tolerance over Consistency (AP).

Google’s Bigtable: A distributed database system used to power several of Google’s services like Google Search and Google Analytics. Bigtable is designed to provide a high level of consistency and partition tolerance while maintaining availability. However, if a network partition occurs, it might suffer from reduced availability in some regions, making it more CP-oriented.

MongoDB: A popular NoSQL database that offers high performance, high availability, and easy scalability. By default, MongoDB leans towards providing Consistency and Partition Tolerance (CP), as writes must be acknowledged by a majority of replicas. However, MongoDB allows users to adjust the system’s behavior using read and write concerns, allowing them to prioritize consistency or availability based on their specific application needs.These examples illustrate how different distributed databases and storage systems choose to balance the competing constraints of the CAP Theorem to meet the requirements of their specific applications.

## FAQ: CAP Theorem

### 1. What is the CAP Theorem?

The CAP Theorem, also known as Brewer’s Theorem, is a concept in distributed computing systems that states it is impossible for a database or system to simultaneously provide all three of the following guarantees: Consistency, Availability, and Partition Tolerance. The theorem implies that only two of these three properties can be achieved at any given time.

### 2. What do Consistency, Availability, and Partition Tolerance mean?

Consistency: Ensures that all nodes in a distributed system see the same data at the same time. When data is changed on one node, it is updated across all nodes.

Availability: Ensures that every request to the system receives a response, whether the data is accurate or not. The system is always accessible and operational, even if some nodes are down.

Partition Tolerance: Ensures that the system continues to operate despite network failures or communication breakdowns between nodes. The system can handle partitions and still maintain functionality.

### 3. How does CAP Theorem impact distributed database design?

CAP Theorem presents a significant challenge in designing distributed databases because it forces developers to prioritize only two of the three guarantees. The choice depends on the specific requirements and use case for the system. For some applications, consistency and partition tolerance might be more crucial, while for others, availability and partition tolerance might be the main priority. Understanding the trade-offs between the chosen properties will help developers create more robust and reliable distributed systems.

### 4. Are there any practical examples of databases that prioritize different aspects of CAP Theorem?

Yes, various databases prioritize different aspects of CAP Theorem. For example, Apache Cassandra favors availability and partition tolerance (AP), sacrificing consistency. In contrast, Google’s Bigtable and HBase prioritize consistency and partition tolerance (CP) while sacrificing availability. Both choices depend on the specific needs of the applications utilizing these databases.

### 5. Is it possible to achieve a balance of all three CAP Theorem properties?

In strictly theoretical terms, it is impossible to achieve perfect consistency, availability, and partition tolerance all at the same time. However, many distributed systems strive to find a practical balance between these properties by using techniques like eventual consistency, tunable consistency, or employing different configurations depending on the use case. The goal in many systems is to minimize the trade-offs as much as possible while still providing a robust and reliable solution.

## Related Technology Terms

• Consistency
• Availability
• Partition Tolerance
• Distributed Systems

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