From Traditional Fault Tolerance to Blockchain. Wenbing Zhao
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СКАЧАТЬ reconnection, message duplicate detection, and atomic message receiving and logging.

      Reconnection. A process must be able to cope with temporary connection failures and be ready to accept reconnections from other processes. This is an essential requirement for recoverable distributed system. This calls for a design in which the application logic is independent from the transport level events. This can be achieved by using a event-based [8] or document-based distributed computing architecture such as Web services [15], in conjunction with appropriate exception handling.

      Atomic message receiving and logging. In the protocol description, we implicitly assumed that the receiving of a message and the logging of the same message are carried out in a single atomic operation. Obviously the use of a reliable communication channel alone does not warrant such atomicity because the process may fail right after it receives a message but before it could successfully log the message, in which case, the message could be permanently lost. This issue is in fact a good demonstration of the end-to-end system design argument [17]. To ensure the atomicity of the message receiving and logging, additional application-level mechanism must be used. (Although the atomic receiving and logging can be achieved via special hardware [4], such solution is not practical for most modern systems.)

Schematic illustration of transport level (a) and application level (b) reliable messaging.

      To ensure application level reliable messaging, the sending process must store a copy of the message sent (in the application level) for possible retransmission until it receives an explicit acknowledgment message from the receiving process in the application level, as shown in Figure 2.12(b). Such an application level reliable messaging protocol does exist in some distributed computing paradigm, such as Web services [6]. Incidentally, the sender-based message logging protocol [13], to be introduced in a later subsection, incorporates a similar mechanism, albeit for a slightly different purpose.

      We should note that the use of such an application level reliable messaging protocol is essential not only to ensure the atomicity of message receiving and logging, but also to facilitate the distributed system to recover from process failures (for example, the failure of the process at one end point of a transport level connection, which would cause the breakage of the connection, would have no negative impact on the process at the other end of the connection, and a process is always ready to reconnect if the current connection breaks).

       ◾ Message logging and message execution can be done concurrently (illustrated in Figure 2.13(a)), hence, minimizing the latency impact due to logging.

       ◾ If a process sends out a message after receiving several incoming messages, the logging of such messages can be batched in a single I/O operation (illustrated in Figure 2.13(b)), further reducing the logging latency.

Schematic illustration of optimization of pessimistic logging: (a) concurrent message logging and execution (b) logging batched messages.

       2.3.1.3 Pessimistic Logging Cost.

      While much research efforts have been carried out to design optimistic and causal logging to avoid or minimize the number of logging operations (on disks) assuming that synchronous logging would incur significant latency overhead [1, 19, 20, 21] . In this section, we present some experimental results to show that such assumption is often unwarranted. The key reason is that it is easy to ensure sequential disk I/Os by using dedicated disks. It is common nowadays for magnetic disks to offer a maximum sustained data rate of 100MB or more per second. Such transfer rate is approaching or exceeding the effective bandwidth of Gigabit Ethernet networks. Furthermore, with the increasing availability (and reduced cost) of semiconductor solid state disks, the sequential disk I/Os can be made even faster and the latency for random disk I/Os can be dramatically reduced. By using multiple logging disks together with disk striping, Gigabytes per second I/Os have been reported [10].

Schematic illustration of probability density function of the logging latency. СКАЧАТЬ