2. Introduction
• In this section, examine how to provide a service that is
correct despite up to process failures, by replicating data
and functionality at replica managers.
• Intuitively, a service based on replication is correct if it
keeps responding despite failures and if clients cannot tell
the difference between the service they obtain from an
implementation with replicated data and one provided by
a single correct replica manager.
5. A replicated shared object service is said to be linearizable
if for any execution there is some interleaving of the series
of operations issued by all the clients that satisfies the
following two criteria:
• The interleaved sequence of operations meets the
specification of a (single) correct copy of the objects.
• The order of operations in the interleaving is consistent
with the real times at which the operations occurred in the
actual execution.
Linearizability
6. Sequential consistency
A replicated shared object service is said to be sequentially
consistent if for any execution there is some interleaving of
the series of operations issued by all the clients that satisfies
the following two criteria:
• The interleaved sequence of operations meets the
specification of a (single) correct copy of the objects.
• The order of operations in the interleaving is consistent
with the program order in which each individual client
executed them.
7.
8. Replication Techniques
Replication present in this section two fundamental
classes of techniques that ensure linearizability:
• Passive (primary - backup) replication technique
• The active replication technique
9. Passive (primary-backup) replication
• In the passive or primary-backup model of replication for
fault tolerance there is at any one time a single primary
replica manager and one or more secondary replica
managers – ‘backups’ or ‘slaves’.
• This system obviously implements linearizability if the
primary is correct, since the primary sequences all the
operations upon the shared objects.
• When the primary crashes, the communication system
eventually delivers a new view to the surviving backups,
one that excludes the old primary.
10. The sequence of events
when a client requests
an operation to be
performed is as follows:
• Request
• Coordination
• Execution
• Agreement
• Response
The passive (primary-backup) model
11. Active replication
• In the active model of replication for fault tolerance the
replica managers are state machines that play equivalent
roles and are organized as a group.
• The active replication system does not achieve
linearizability. This is because the total order in which the
replica managers process requests is not necessarily the
same as the real-time order in which the clients made
their requests.
• The reliability of the multicast ensures that every correct
replica manager processes the same set of requests and
the total order ensures that they process them in the same
order.
12. the sequence of events
when a client requests
an operation to be
performed is as follows:
• Request
• Coordination
• Execution
• Agreement
• Response
Active replication
13. Conclusion
• Linearizability has been introduced as the abstract
correctness criterion, and “active replication/primary-
backup" have been presented as the two main classes of
replication techniques.
• The real issue in achieving fault-tolerance by replication
is thus related to the implementation of the group
multicast primitives.
