======  Transactions ======

===== ACID =====

  * Atomicity: All ops happen, or none
  * Consistency: Data will be consistent, no constraints violated
  * Isolation: One transaction cannot read from another non-completed one. If t2 reads write from t1, needs to wait.
  * Durability: Changes have been recorded

===== CAP =====

"In presence of a network partition, one has to choose between **consistency** and **availability**".

==== Consistency ====
  * Strong consistency
  * Timeline consistency: Typically async master-slave replication.
  * Eventual consistency: Temporarily inconsistent

====== Scenario ======

Scaling out NOSQL across regions/public clouds. Partitioned db across clusters. How to ensure that transaction is commited on all nodes.

Ensuring atomicity: Multi-phase commit, pessimistic read-write locking
But: Network partitions