Mumbai University > Information Technology > Sem 6 > Distributed System

Marks: 5 Marks

Year: May 2016

Definition of Sequential Consistency:

The result of any execution is the same as if the (read and write) operations by all processes on the data-store were executed in the same sequential order and the operations of each individual process appear in this sequence in the order specified by its program.

Different Strategies are:

• Non-Replicated, Non-Migrating Blocks(NRNMBs)
• Non-Replicated ,Migrating blocks (NRMBs)
• Replicated, Migrating blocks(RMBs)
• Replicated ,Non-Migrating blocks(RNMBs)

Non-Replicated ,Migrating blocks (NRMBs):

1. In this strategy each block of the shared memory has a single copy in the entire system.However,each access to a block causes the block to migrate from its current node to the node from where it is accessed.
2. When a block is migrated away,it is removed from any local address space it has been mapped into.Notice that in this strategy only the processes executing on one node can read or write a given data item at any one time.Therefore the method ensures sequential consistency.

Fig : Nonreplicated,migrating blocks(NRNB) strategy

The method has the following advantages:

1. No communication cost are incurred when a process accesses the data currently held locally.
2. It allows the applications to take advantage of data access locality.If the application exhibits high locality of reference, the cost of data migration is amortized over multiple accesses