Читать книгу From Traditional Fault Tolerance to Blockchain - Wenbing Zhao - Страница 57

It is apparent that pessimistic logging has a number of very desirable characteristics:

 ◾ Processes do not need to track their dependencies. The relative ordering of the incoming messages to each process is naturally reflected in the log (i.e., during recovery, the messages in the log will be replayed in the order in which they are logged). Hence, the pessimistic logging mechanism is straightforward to implement and less error prone.

 ◾ Output commit is free with pessimistic logging. This is a great fit for distributed applications that interact with their users frequently.

 ◾ There is no need to carry out coordinated global checkpointing because by replaying the logged messages, a process can always bring itself to be consistent with other processes in the system. This further reduces the complexity of adding rollback recovery support to applications. Furthermore, a process can decide when it is the best time to take a local checkpoint, for example, when its message log is too big.

 ◾ Recovery can be done completely locally to the failed processes. The only impact to other processes is the possibility of receiving duplicate messages and discard them. Hence, the recovery is simpler and in general faster than optimistic and causal logging. The localization of failure recovery also means that pessimistic logging supports concurrent failure recovery of multiple processes.