What are the main characteristics of NOSQL systems in the areas related to distributed systems and distributed databases?

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 CHARACTERISTICS RELATED TO DISTRIBUTED DATABASES AND DISTRIBUTED SYSTEMS



 1.  Scalability: 

  • horizontal scalability: adding more nodes for data storage and processing as the volume of data grows.
  •  Vertical scalability: expanding the storage and computing power of existing nodes. 
  • In NoSQL systems, horizontal scalability is employed while the system is operational, so techniques for distributing the existing data among new nodes without interrupting system operation are necessary.

 2. Availability, Replication, and Eventual Consistency: 

  • Data is replicated over two or more nodes in a transparent manner. 
  • An update must be applied to every copy of the replicated data items. 
  •  Eventual consistency: is a consistency model used in distributed computing to achieve high availability that informally guarantees that, if no new updates are made to a given data item, eventually all accesses to that item will return the last updated value.

 3. Replication Models: 

  • Master-slave replication: requires one copy to be the master copy; 
  • Write operations must be applied to the master copy, usually using eventual consistency 
  • For read, all reads are from the master copy, or reads at the slave copies but would not guarantee that the values are the latest writes. 
  • Master-master replication: allows reads and writes at any of the replicas. 
  • The values of an item will be temporarily inconsistent. 
  • Reconciliation method to resolve conflicting write operations of the same data item at different nodes must be implemented as part of the master-master replication scheme.

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