Foundamentals

• Replication : Replication refers to frequently copying the data across multiple machines. Post replication, multiple copies of the data exists across machines. This might help in case one or more of the machines die due to some failure.

• Consistency: Assuming you have a storage system which has more than one machine, consistency implies that the data is same across the cluster, so you can read or write to/from any node and get the same data.

• Eventual consistency : Exactly what the name suggests. In a cluster, if multiple machines store the same data, an eventual consistent model implies that all machines will have the same data eventually. Its possible that at a given instance, those machines have different versions of the same data ( temporarily inconsistent ) but they will eventually reach a state where they have the same data.

• Availability: In the context of a database cluster, Availability refers to the ability to always respond to queries ( read or write ) irrespective of nodes going down.

• Partition Tolerance: In the context of a database cluster, cluster continues to function even if there is a “partition” (communications break) between two nodes (both nodes are up, but can’t communicate).