Protocol 3: Asynchronous Client Duplication

Figure 4: The steps of duplication process for Protocols 3 and 4.

In this protocol, the duplication task is assigned to the client, as shown in Figure 4. After fetching, the client can write to the primary and backup servers simultaneously. The duplication process is regarded as successful after receiving at least one acknowledgment among each pair of mirrored servers. Obviously, there is a potential problem if the slower server in the pair fails before acknowledgment. This problem is similar, but not identical to that in Protocol 1.