Question

Compare the following pairs of methods/techniques:

Time stamping vs. optimistic methods in distributed systems

Answer 1

TIME STAMPING IN DISTRIBUTED SYSTEMS

The idea of time stamping is to come up with a mechanism that defines a partial order over events in a distributed system. It's based on the simplest model of distributed system, where a distributed system is a collection of processes. The processes only communicate by explicitly sending messages to each other. As different nodes or processes will typically not be perfectly synchronized, this algorithm is used to provide a partial ordering of events with minimal overhead, and conceptually provide a starting point for the more advanced vector clock method.Distributed algorithms such as resource synchronization often depend on some method of ordering events to function.

OPTIMISTIC METHOD IN DISTRIBUTED SYSTEMS

In these cases, the test for serializability is postponed to just before commit. Since the conflict rate is low, the probability of aborting transactions which are not serializable is also low. .

These are the phases involved in Optimistic Methods:

• Begin: Record a timestamp marking the transaction's beginning.
• Modify: Read database values, and tentatively write changes.
• Validate: Check whether other transactions have modified data that this transaction has used (read or written). This includes transactions that completed after this transaction's start time, and optionally, transactions that are still active at validation time.
• Commit/Rollback: If there is no conflict, make all changes take effect. If there is a conflict, resolve it, typically by aborting the transaction, although other resolution schemes are possible.