Editing Safe semantics (section)

=== Write ===
Client (''cw'') sends an inquiry into the system in different rounds and waits until it receives two acknowledgment. (''sn'' =sequence number)


The reason for receiving two acknowledgments is to avoid danger in a system. When a process sends an acknowledgement (''ack''), it may die after one millisecond. Therefore, no  confirmation is received by the client.

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.<ref name=BBN/> Given two quorum systems (Qw, Qr) Qw indicates the servers that know about the latest value, and Qr indicates values of read responses. The size of each quorum is equal to n-f-j.<ref name=BBN/> Proving the safe register's validity requires proving

<math>(Qw \cup Qr)\backslash B >(Qr \cup B)</math>

were ''B'' is the number of Byzantine failures.

Proof: Red region indicates (Qw∩Qr)\B and the blue region indicates Qr∩B. From the assumption, the size of each quorum is n-f-j, so the red region has n-3f-2j active servers. Therefore,

<math>n-3f-2J > f \implies n > 4f+2J \implies n </math> is strictly greater than f.  
[[File:Validity.jpg|center|validity]]