# Conflict-free Replicated Data Type

* data structure that simplifies distributed data storage systems and multi-user applications.
* copies of some data need to be stored on multiple computers
  * the data may be concurrently modified on different replicas.
  * Can be handled:
    * Strongly consistent replication
      * the replicas coordinate with each other to decide when and how to apply the modifications
      * enables strong consistency models such as serializable transactions and linearizability
      * waiting for this coordination reduces the performance of these systems
    * Optimistic replication
      * users may modify the data on any replica independently of any other replica, even if the replica is offline or disconnected from the others.
      * enables maximum performance and availability, but it can lead to conflicts when multiple clients or users concurrently modify the same piece of data.
      * conflicts then need to be resolved when the replicas communicate with each other.
* Conflict-free Replicated Data Types (CRDTs) are used in systems with optimistic replication, where they take care of conflict resolution
  * ensure that, no matter what data modifications are made on different replicas, the data can always be merged into a consistent state.
  * merge is performed automatically by the CRDT
  * support decentralised operation: they do not assume the use of a single server, so they can be used in peer-to-peer networks and other decentralised settings

## Links

* <https://crdt.tech/>
* <https://arxiv.org/pdf/1805.06358.pdf>
* <https://inria.hal.science/inria-00555588/document>