Eventual Consistency — Database

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ACID vs BASE

Relational databases that supports ‘strong consistency ‘ provides ACID guarantees.

• Atomicity : if transactions failed at any point, the entire operation roll back
• Consistency : the database remains structurally sound with every transaction
• Isolation : each transaction is independent of any other transaction
• Durability : all transaction results are permanently preserved

For this reason, ACID-compliant databases are generally slow, difficult to scale, and expensive to run and maintain.

Non-Relational Databases like NoSQL databases provide BASE guarantees. A BASE enable availability and relaxes the stringent consistency.

• Basic Availability : Data is available most of the time, even during partial system failure.
• Soft State : replicas are not consistent all the time.
• Eventual Consistency : data will become consistent at some point in time, with no guarantee when.

As such, NoSQL databases sacrifice a degree of consistency in order to increase availability. Rather than providing strong consistency, they provide eventual consistency. This means that a datastore that provides BASE guarantees can occasionally fail to return the result of the latest WRITE.

Resources

1. https://cloud.google.com/datastore/docs/articles/balancing-strong-and-eventual-consistency-with-google-cloud-datastore