consistency protocols

Consistency protocols

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

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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-Backup Protocols

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Primary-Backup Protocols

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