consistency protocols

Consistency protocols

Speaker：呂宗螢Adviser：梁文耀　老師Date ： 2006/11/29

Embedded and Parallel Systems Lab

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Consistency Protocols

Implementation of a consistency modelHow do we order operations according to a

consistency modelHow are multiple writes applied and

propagated to different replicas? Data-consistency protocols

Implement data-consistency modelNeed some sort of globally serialized

operations


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Consistency Protocols

Primary-based protocolsEach data item has a primary replica

Replication-based protocolsOperations can be carried out at multiple

replicas


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Primary-based Protocol

Each data item has a primary replica All writes are applied to and coordinated

by the primary Two types：

Remote-Write ： The primary is fixed and remote

Local-Write ： The primary is copied locally before applying writes


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Remote-Write Protocols

All read and Write operations are carried out at a remote server.

Client-server systems

Primary-backup protocols


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Remote-Write Protocols


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Primary-Backup Protocols

Budhiraja et al., 1993 Read operations can happen locally Write operations also carried out on the

primary A write is complete only when all backups

have updated Update is Blocking


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Primary can order all incoming writes All processes are see all write operations in the

same order Sequential consistency Can use write-invalidate Update with Non-blocking(Budhiraja and

Marzullo, 1992) No assurance that the backups will actually receive

the write Not guaranteed that the effect of a write will be seen

in all subsequent reads


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Local-Write Protocols

Primary-base local-write protocolsThere is only a single copy of data item x.

Where is the data? How to find?Forwarding pointersHome-based

Primary-backup local-write protocolsMultiple copies of each data item x exist, but

one of them are primaryCan disconnected mode


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Primary-base local-write protocols


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Primary-backup local-write protocols


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Replicated-Write Protocols

Write operations can be carried out at multiple replicas instead of only one

Active ReplicationWrite operation is forwarded to all replicasNeed a total ordering

Use Lamport timestampsCentral sequencer

One problem to deal with replicated invocations


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Replicated Invocations


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Replicated Invocations (one solution)

Mazouni et al., 1995


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Replicated-Write Protocols

Quorum-Based ProtocolsVoting protocolsGifford ,1979In a system where N replicas exist, a client

needs to assemble a read quorum of NR servers to read, and a write quorum of NW servers to write, where

NR+NW > N (prevents read-write conflicts)

NW > N/2 (prevents write-write conflicts)


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Quorum-Based Protocols


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Home-Based

The home location is often chosen to be the place where an entity was created

Use Home agent


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Home-Based

Client’s location

Host’s home agent

3.Tunnel packet to current location

4.Send successive packets to current location

1.Send packet to host at its home

2.Return address current location

consistency protocols

Software