Understanding Eventual Consistency
Tags: database, tutorial, performance, webdev
Published: 2024-01-01 19:42:31

## Introduction
In the world of distributed systems, data management and synchronization are critical aspects that ensure the smooth operation of the system. One of the models that has gained popularity in this domain is **Eventual Consistency**.

## What is Eventual Consistency?
Eventual consistency is a model that ensures that data will eventually become consistent across all nodes or replicas in the system. It allows for temporary states of inconsistency during the propagation of updates.

When a change is made to a piece of data, that change is first applied locally. The new version is then propagated to other nodes or replicas in the system over time. However, there could be a period where different nodes or replicas may have different versions of the same data due to network latency or concurrent updates.

The key concept here is that, as time progresses and all updates are eventually propagated, the system will become consistent across all nodes. This model is also known as “Brewer’s Conjecture” or the CAP theorem. It’s a trade-off between consistency (always reflecting the latest data) and availability (ensuring that data is always accessible), making it suitable for applications where eventual consistency works well.

## Is Eventual Consistency limited to NoSQL databases?

Eventual consistency is not limited to NoSQL databases. The concept can be applied in various types of distributed systems, including both SQL and NoSQL databases, as well as other distributed applications.

The choice between eventual consistency and strong consistency depends on the specific requirements of an application. Eventual consistency is often chosen when performance is a priority, the system is designed to tolerate temporary inconsistencies, or the application allows eventual consistency because it has built-in mechanisms to handle data conflicts.

On the other hand, strong consistency is preferred when data accuracy and real-time consistency are essential, the application doesn’t tolerate temporary inconsistencies or data conflicts, or the system is designed to prioritize data integrity over other considerations.


## Example
Let's consider a simple example of an eventual consistent system using a NoSQL database, like MongoDB. We have a collection of documents representing user profiles. Each document stores the user's
name, email address, and profile picture:
```
{
    "_id": "user1",
    "name": "John Doe",
    "email": "john@example.com",
    "profilePicture": "https://example.com/john.jpg"
}
{
    "_id": "user2",
    "name": "Jane Smith",
    "email": "jane@example.com",
    "profilePicture": "https://example.com/jane.jpg"
}
```
Suppose that we have two nodes (let's call them Node A and Node B) in our MongoDB cluster, each storing a replica of this collection. When an update is made on one node, it might not be immediately propagated
to the other node due to network latency or other factors.

Let's say that we receive an update to change Jane Smith's email address from "jane @ example.com" to "new_email @ example.com". This update is first applied on Node A, but not yet propagated to Node B. If we
were to query the data from both nodes at this moment, we would see different versions of Jane Smith's profile:

Node A:
```
{
    "_id": "user2",
    "name": "Jane Smith",
    "email": "new_email@example.com",
    "profilePicture": "https://example.com/jane.jpg"
}
```
Node B:
```
{
    "_id": "user2",
    "name": "Jane Smith",
    "email": "jane@example.com",
    "profilePicture": "https://example.com/jane.jpg"
}
```
In this situation, the system is in a state of temporary inconsistency. If we perform the same query on both Node A and Node B after some time has passed, the data will eventually be consistent:

Node A (eventually):
```
{
    "_id": "user2",
    "name": "Jane Smith",
    "email": "new_email@example.com",
    "profilePicture": "https://example.com/jane.jpg"
}
```
Node B (eventually):
```
{
    "_id": "user2",
    "name": "Jane Smith",
    "email": "new_email@example.com",
    "profilePicture": "https://example.com/jane.jpg"
}
```

## Conclusion
Eventual consistency is a powerful model for managing data in distributed systems. It provides a balance between data consistency and system availability, making it a popular choice for many large-scale applications. However, it’s important to understand the trade-offs involved and choose the right consistency model based on the specific requirements of your application.

[Wikipedia Article](https://en.wikipedia.org/wiki/Eventual_consistency)