This question appears in Mumbai University > Parallel And Distributed System subject

Marks: 10 M

Year: Dec 2015

• Sequential consistency model is a weaker consistency model, which represents a relaxation of the rules.
• It is also must easier (possible) to implement.
• 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.

• In other words: all processes see the same interleaving set of operations, regardless of what that interleaving is.
  • A sequentially consistent data-store – the “first” write occurred after the “second” on all replicas.
  • A data-store that is not sequentially consistent – it appears the writes have occurred in a non-sequential order, and this is NOT allowed.

• The sequential consistency model as defined by Lamport is a weaker memory model than strict consistency.
• Problem with Sequential Consistency
  • With this consistency model, adjusting the protocol to favour reads over writes (or vice-versa) can have a devastating impact on performance (refer to the textbook for the gory details).
  • For this reason, other weaker consistency models have been proposed and developed.
  • Again, a relaxation of the rules allows for these weaker models to make sense.
• Sequential Consistency Requirements
  1. Each processor issues requests in the order specified by the program – Do not issue the next request unless the previous one has finished
  2. Requests to an individual memory location (storage object) are served from a single FIFO queue. – Writes occur in a single order – Once a read observes the effect of a write, it’s ordered behind that write.