# Performance and Scalability To **keep consistency between replicates**, it's usually necessary to **guarantee that conflicting operations are done in the same order**. Conflict-generating operations: * **Read-write:** reading and writing concurrent operations. * **Write-write:** two write operations. Ensuring a global order in conflicting operations can be a time-consuming operation, which lowers the scalability potential. * **Solution:** lower the consistency requirements so that the necessity for global synchronism can be avoided. ## Data-centric models {% hint style="info" %} ### Consistency Model A contract between a distributed data store and processes where the **data store specifies precisely which are the results for a concurrent read and write operation**. {% endhint %}

### Continuous consistency #### Degree of consistency: * Replicates can differ in their **numeric value**. * Replicates can differ in their **relative immutability**. * There can be differences in the **number and order of update operations.** #### **Conit** Unity of consistency -> specifies the **unit of data** over which there **should be a consistency measure**. #### Example: Conit

Each replicate has a vector clock. *B* sends to *A* the operation in grey. * *A* permanently executed that operation. Conit: * *A* has 3 pendant operations: * Deviation of **order = 3**. * *A* loses two of *B's* operations: * Max diff is *70 + 412 = (2, 482)*. * **Numerical** deviation of 482. ### Sequential consistency The **result of any execution is the same if all operations of all processes are executed in the same sequential order**, and the operations of each individual process appear in the program's specified order.

### Causal Consistency **Writes** that are related casuistically **have to be seen by all processes in the same order**. Concurrent writes can be seen in different orders by different processes.

## Operation grouping ### Definition * Accesses to **locks** are consistent sequentially. * Accesses to locks are **not permitted** **before** all the **previous writes** have been **completed.** * Access to the data is **not allowed** **until** all previous **accesses** to **locks** have been **completed**. ### Base idea It does not matter if the reads and writes of a **set of operations** are immediately known to all other processes. Only the **effect** of the set **needs** to be known.