Scalable Human Blog

Eventual Consistency

What is it?

• Theoretical guarantee
• If no new updates to an entity are made
  • All reads of the entity will eventually return the last updated value

Example of an eventual consistency model?

• Internet Domain Name System (DNS)!
• DNS servers are cached and replicated across directories over the internet
  • Time is needed to replicated modified changes to all DNS clients and servers
• DNS is the foundations of the internet so it been a very successful system
• It is has high availability and scalable
  • This has enabled name lookups for any device connected to the internet

Warning

• Whilst availability is high on reads… replicas may be inconsistent with the latest write on the originating node

Strong Consistency

What is it?

• Traditional relational databases have been designed based on this concept of strong consitency
• Also called immediate consistency
• Means that data is viewed immediately after an update
  • Therefore, all observers of the entity will be consistent

Trade offs

• To have strong consistency… scalability and performance are compromised!
• Data would required locking during the period of updates or replication process (replication lag)
  • To ensure that no other processes are updating on the same data