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Eventual consistency is a weak guarantee – most stronger models, like linearizability, are trivially eventually consistent. Eventually-consistent services are often classified as providing BASE semantics (basically-available, soft-state, eventual consistency), in contrast to traditional ACID (atomicity, consistency, isolation, durability) .
The compare function is included to illustrate a partial order on the states. The merge function is commutative, associative, and idempotent. The update function monotonically increases the internal state according to the compare function. This is thus a correctly defined state-based CRDT and will provide strong eventual consistency.
In slow consistency, [14] if a process reads a value previously written to a memory location, it cannot subsequently read any earlier value from that location. Writes performed by a process are immediately visible to that process. Slow consistency is a weaker model than PRAM and cache consistency.
Strong versus eventual consistency (storage) [ edit ] In the context of scale-out data storage , scalability is defined as the maximum storage cluster size which guarantees full data consistency, meaning there is only ever one valid version of stored data in the whole cluster, independently from the number of redundant physical data copies.
Key–value stores can use consistency models ranging from eventual consistency to serializability. Some databases support ordering of keys. Some databases support ordering of keys. There are various hardware implementations, and some users store data in memory (RAM), while others on solid-state drives (SSD) or rotating disks (aka hard disk ...
Optimistic replication, also known as lazy replication, [1] [2] is a strategy for replication, in which replicas are allowed to diverge. [3]Traditional pessimistic replication systems try to guarantee from the beginning that all of the replicas are identical to each other, as if there was only a single copy of the data all along.
I cannot imagine how ACID and BASE can possibly be considered mutually exclusive. ACID is about write operations on an individual server, BASE is about replication between at least 2 servers. You can achieve eventual consistency on a replication environment with a fully ACID-compliant DBMS. This is the most common situation.
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