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

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