Editing Safe semantics (section)

== Description ==
Safe semantics are defined for a variable with a single writer but multiple readers (SWMR). A SWMR register is safe if each read operation satisfies these properties:[[File:Safe register-no overalapping.jpg|thumb|Safe register-no overlapping]]

# A read operation not concurrent with any write operation returns the value written by the latest write operation.
# A read operation that is concurrent with a write operation may return any value within the register's allowed range of values (for example, 0,1,2,...). [[File:Safe register-overallping.jpg|thumb|safe register-overlapping]]

In particular, given concurrency of a read and a write operation, the read can return a value that has not been written by a write. The return value need only belong to the register domain.

A binary safe register can be seen as modeling a bit flickering. Whatever the previous value of the register is, its value could flicker until the write finishes. Therefore, the read that overlaps with a write could return 0 or 1.

''Churn'' refers to the entry and exit of servers to/from a distributed system. Baldoni et al. show that no register can have the stronger property of [[regular semantics]] in a [[Synchronous circuit|synchronous system]] under continuous churn.<ref>{{Cite journal|last1=Baldoni|first1=Roberto|last2=Bonomi|first2=Silvia|last3=Raynal|first3=Michel|date=January 2012|title=Implementing a Regular Register in an Eventually Synchronous Distributed System Prone to Continuous Churn|journal=IEEE Transactions on Parallel and Distributed Systems|volume=23|issue=1|pages=102–109|doi=10.1109/TPDS.2011.97|s2cid=12717004 |issn=1045-9219}}</ref> However, a safe register can be implemented under continuous churn in a non-synchronous system.<ref name="BBN" /> Modeling and implementing a type of storage memory (Safe Register) under non-quiescent churn requires some system models such as client and server systems.<ref name="BBN">{{Cite journal|last1=Baldoni|first1=Roberto|last2=Bonomi|first2=Silvia|last3=Nezhad|first3=Amir Soltani|date=November 2013|title=A protocol for implementing byzantine storage in churn-prone distributed systems|journal=Theoretical Computer Science|volume=512|pages=28–40|doi=10.1016/j.tcs.2013.04.005|doi-access=free}}</ref> Client systems contains a finite, arbitrary number of processes that are responsible for reading and writing the server system. However, the server system must ensure that read and write operations happen properly.