Safe semantics

Safe semantics are defined for a variable with a single writer but multiple readers (SWMR).

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 system under continuous churn.

[3] However, a safe register can be implemented under continuous churn in a non-synchronous system.

However, the server system must ensure that read and write operations happen properly.

Once other server receive this inquiry they send reply messages to si.

Si waits until it gets enough replies (n-f-j) from other servers then it picks the most frequently received value.

Once the client receives enough replies (n-f-j) it stops sending an inquiry.

Client (cw) sends an inquiry into the system in different rounds and waits until it receives two acknowledgment.

The validity of the safe register (If a read is not concurrent with any write, return the last value written) was proved based on the quorum system.

From the assumption, the size of each quorum is n-f-j, so the red region has n-3f-2j active servers.