Читать книгу From Traditional Fault Tolerance to Blockchain - Wenbing Zhao - Страница 63

In the example shown in Figure 2.18, the distributed system consists of three processes. Both the seq counter and rsn counter are initialized to be 0, and the message log is empty at each process. Process P₀ first sends a regular message, <REGULAR,0,?,m₀>, to P₁. Upon sending the message, P₀ increments its seq counter to 1 and log the message in its volatile buffer. At this point, the rsn value for the message is unknown, hence it is denoted as a question mark.

On receiving the regular message <REGULAR,0,?,m₀>, P₁ assigns the current rsn counter value, which is 0, to this message indicating its receiving order, increments its rsn counter to 1, and sends P₀ an ORDER message <ORDER,[m₀],0>. When P₀ receives this ORDER message, it updates the entry in its message log to reflect the ordering number for message m₀, and sends an sc ack message, <ACK,[m₀]>, to P₁.

Once receiving the ACK message, P₁ is permitted to send a regular message, <REGULAR,0,?,m₁>, to P₂. The handling of the message and the corresponding ORDER and ACK messages are similar to the previous ones.

Figure 2.18 An example normal operation of the sender-based logging protocol.

Subsequently, P₀ and P₂ send three regular messages m₂, m₃, m₄, nearly concurrently to P₀. P₁ assigns 1 as the rsn value for the first of the three messages (for m₂) and sends an ordering message to P₀, and assigns 2 and 3 for the two back-to-back regular messages (for m₃ and m₄) from P₂. For the two messages from P₂, P₁ can batch the ORDER messages and sends them together to P₂, and P₂ can batch the corresponding the ACK messages to P₁ too. Upon receiving the ACK messages for all three ORDER messages, P₁ sends another regular message containing m₅ with sequence number 1, updates the seq counter to 2, and log the message.