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Distributed Systncy model
A contract between a (distributed) data store and processes, in which the
data store specifies precisely what the results of read and write operations
are in the presence of concurrency.
Essential
A data store is a distributed collection of storages:
Process Process Process
Local copy
Distributed data store
5 / 42Consistency & Replication Data-Centric Consistency Models
Continuous Consistency
Observation
We can actually talk a about a degree of consistency:
replicas may differ in their numerical value
replicas may differ in their relative staleness
there may be differences with respect to (number and order) of
performed update operations
Conit
Consistency unit ⇒ specifies the data unit over which consistency is to
be measured.
6 / 42Consistency & Replication Data-Centric Consistency Models
Example: Conit
Replica A Replica B
Conit Conit
x = 6; y = 3 x = 2; y = 5
Operation Result Operation Result
< 5, B>
x := x + 2 [ x = 2 ] < 5, B>
x := x + 2 [ x = 2 ]
< 8, A> y := y + 2 [ y = 2 ] <10, B> y := y + 5 [ y = 5 ]
<12, A> y := y + 1 [ y = 3 ]
<14, A> x := y * 2 [ x = 6 ]
Vector clock A
= (15, 5)
Vector clock