Outline

1
Outline

• Introduction (whats it all about)
• Data-centric consistency
• Client-centric consistency
• Replica management
• Consistency protocols

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Consistency protocol

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

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Continuous Consistency (1)

• Degree of consistency based on deviations of
  numerical values (due to operation performed)

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

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

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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.

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Primary-based protocols Remote-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

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Primary-based protocols Local-Write Protocols

• Multiple, successive writes can be loca
• Nonblocking

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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.

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Consistency protocol

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

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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

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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
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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
• NwgtN/2 -- prevent write-write conflict
• Nw Nr gt N -- prevent read-write conflict

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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).