Outline

1. Outline • Introduction (what’s it all about) • Data-centric consistency • Client-centric consistency • Replica management • Consistency protocols

2. Consistency protocol • Describes the implementation of a specific consistency model. Continuous consistence Sequential consistence Primary-based protocols Replicated-write protocols

3. Continuous Consistency (1) • Degree of consistency based on deviations of numerical values (due to operation performed)

4. Continuous consistency

5. Si propagate a write originated from Sj to Sk. And it meets the above bound \delta_i.

6. Consistency protocol • Describes the implementation of a specific consistency model. Continuous consistence Sequential consistence Primary-based protocols Replicated-write protocols

7. Consistency protocol • Primary-based protocols • sequential consistency The result of any execution is the same as if the (read and write) operations by all processes were executed in some sequential order specified by its program • Data item x has a primary to coordinate write operation. • Two types of protocols: • Remote-Write Protocols (primary-backup) • Write operations submit to a single remote server. • Read op locally, • Local-Write Protocols (primary-backup w local writes) • the primary migrates to the process wanting to perform an update.

8. Primary-based protocolsRemote-Write Protocols • Figure 7-20. The principle of a primary-backup protocol. • Long time to finish – blocking or nonblocking the initiator • Fault tolerance issue related • Primary does the ordering

9. Primary-based protocolsLocal-Write Protocols • Multiple, successive writes can be loca • Nonblocking

10. Consistency protocol • Primary-based protocols • Which protocol? • Example: Traditionally applied in distributed databases and file systems that require a high degree of fault tolerance. Replicas are often placed on same LAN. • Example: Mobile computing in disconnected mode (ship all relevant files to user before disconnecting, and update later on). Other processes can not update.

11. Consistency protocol • Describes the implementation of a specific consistency model. Continuous consistence Sequential consistence Primary-based protocols Replicated-write protocols

12. Replicated-write protocols • Write operations can be carried out at multiple replicas instead of one. • Active replication • Propagate the process (write operation) that cause the updates (in stead of the updates) to replica • Operations need to be carried in the same order everywhere. • Totally ordered Multicast or a central coordinator – sequencer. • Quorum-Based Protocols • Client need to request and acquire the permission of multiple servers before reading or writing a replicated data item

13. Quorum-Based Protocols Quorum - the smallest number of people needed to be present at a meeting before it can officially begin and before official decisions can be taken. • Ensure that each operation is carried out in such a way that a majority vote is established • more than half of the N servers (and plus 1) • So to allow determine the consistency and perform the operation • Version numbers - Newer versions

14. Quorum-Based Protocols • distinguish read quorum and write quorum • For update: When agreed, update and increase version of the data • For read: also need more than half of the N servers (and plus 1) to agree and to send data. and increase version of the data • How many data with same version? • But can be relaxed to • Nw>N/2 -- prevent write-write conflict • Nw + Nr > N -- prevent read-write conflict

15. Replicated-write protocolsQuorum-Based Protocols • Three examples of the voting algorithm. (a) A correct choice of read and write set. (b) A choice that may lead to write-write conflicts. (c) A correct choice, known as ROWA (read one, write all).