Eventual consistency and strict consistency

1. Overview

Let’s define eventual consistency vs strict consistency.

 

2. Description

2.1 Strict consistency(or immediate or strong)

• For any incoming write operation, once a write is acknowledged to the client, the updated value is visible on reading from any replicated node (server) in the system
• Guaranteed data resiliency
  • Once a write is acknowledged to the client the update is protected from node failure with redundancy
• This effectively means that all readers are blocked until replication of the new data to all the nodes is complete

2.1.1 Where strict consistency is needed

• Scale-out file storage
  • Scenario: The data is written to only one node (on NVRAM) and acknowledged. An enterprise customer once explained to me that under heavy load, a node may get marked offline. Effectively it’s down, resulting in clients getting “File-Not-Found” errors for files they had successfully written just a few seconds prior. This wreaks havoc on their applications.
• Instant recovery from backup
  • If a failure happens on the recovery node, the application fails and the system loses live production VM data
• Data Recovery and Protection

 

2.1.2 Limits of strict consistency

• Being able to have a detrimental effect on system and performance, depending on the scenario

2.2 Eventual consistency

• All nodes are always available to be read but some nodes may have stale data at a particular point of time
• Potential for data loss if the node fails

2.2.1 Usages

Some scenarios may not require strict consistency

• Photo sharing system like Flicker
• Message timeline for a social app like Facebook or Twitter
• Domain Name System(DNS)
• Adding items to a shopping cart

2.2.2 Support for Eventual Consistency in Modern Databases

3. Reference

https://en.wikipedia.org/wiki/Eventual_consistency#targetText=Eventual%20consistency%20is%20a%20consistency,return%20the%20last%20updated%20value.

https://www.cohesity.com/blog/strict-vs-eventual-consistency/

https://www.acodersjourney.com/eventual-consistency/