Consistency protocols

1. Consistency protocols
Speaker ：呂宗螢
Adviser ：梁文耀 老師
Date ： 2006/11/29

2. Embedded and Parallel Systems Lab2
Consistency Protocols
 Implementation of a consistency model
How do we order operations according to a
consistency model
How are multiple writes applied and
propagated to different replicas?
 Data-consistency protocols
Implement data-consistency model
Need some sort of globally serialized
operations

3. Embedded and Parallel Systems Lab3
Consistency Protocols
 Primary-based protocols
Each data item has a primary replica
 Replication-based protocols
Operations can be carried out at multiple
replicas

4. Embedded and Parallel Systems Lab4
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

5. Embedded and Parallel Systems Lab5
Remote-Write Protocols
 All read and Write operations are carried out at a
remote server.
 Client-server systems
 Primary-backup protocols

6. Embedded and Parallel Systems Lab6
Remote-Write Protocols

7. Embedded and Parallel Systems Lab7
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

8. Embedded and Parallel Systems Lab8
Primary-Backup Protocols

9. Embedded and Parallel Systems Lab9
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

10. Embedded and Parallel Systems Lab10
Local-Write Protocols
 Primary-base local-write protocols
There is only a single copy of data item x.
 Where is the data? How to find?
Forwarding pointers
Home-based
 Primary-backup local-write protocols
Multiple copies of each data item x exist, but
one of them are primary
Can disconnected mode

11. Embedded and Parallel Systems Lab11
Primary-base local-write protocols

12. Embedded and Parallel Systems Lab12
Primary-backup local-write
protocols

13. Embedded and Parallel Systems Lab13
Replicated-Write Protocols
 Write operations can be carried out at
multiple replicas instead of only one
 Active Replication
Write operation is forwarded to all replicas
Need a total ordering
Use Lamport timestamps
Central sequencer
One problem to deal with replicated
invocations

14. Embedded and Parallel Systems Lab14
Replicated Invocations

15. Embedded and Parallel Systems Lab15
Replicated Invocations
(one solution)
 Mazouni et al., 1995

16. Embedded and Parallel Systems Lab16
Replicated-Write Protocols
 Quorum-Based Protocols
Voting protocols
Gifford ,1979
In 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)

17. Embedded and Parallel Systems Lab17
Quorum-Based Protocols

18. Embedded and Parallel Systems Lab18
Home-Based
 The home location is often chosen to be
the place where an entity was created
 Use Home agent

19. Embedded and Parallel Systems Lab19
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