Tecnología / Sistemas Distribuidos
Sistemas DistribuidosEnsayos para estudiantes: Sistemas Distribuidos
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Enviado por: laboratoip 17 agosto 2013
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may crash, and they are then deemed to have left the system (although they may be
replaced). In a static system, replica managers do not crash (crashing implies never
executing another step), but they may cease operating for an indefinite period. We return
to the issue of failure in Section 18.4.2.
The general model of replica management is shown in Figure 18.1. A collection
of replica managers provides a service to clients. The clients see a service that gives
them access to objects (for example, diaries or bank accounts), which in fact are
replicated at the managers. Each client requests a series of operations – invocations upon
one or more of the objects. An operation may involve a combination of reads of objects
and updates to objects. Requested operations that involve no updates are called readonly
requests; requested operations that update an object are called update requests
(these may also involve reads).
Each client’s requests are first handled by a component called a front end. The role
of the front end is to communicate by message passing with one or more of the replica
managers, rather than forcing the client to do this itself explicitly. It is the vehicle for
making replication transparent. A front end may be implemented in the client’s address
space, or it may be a separate process.
In general, five phases are involved in the performance of a single request upon
the replicated objects [Wiesmann et al. 2000]. The actions in each phase vary according
to the type of system, as will become clear in the next two sections. For example, a
service that supports disconnected operation behaves differently from one that provides
a fault-tolerant service. The phases are as follows:
Request: The front end issues the request to one or more replica managers:
– either the front end communicates with a single replica manager, which in turn
communicates with other replica managers;
– or the front end multicasts the request to the replica manager
Coordination: The replica managers coordinate in preparation for executing the
request consistently. They agree, if necessary at this stage, on whether the request is
to be applied (it might not be applied at all if failures occur at this stage). They also
decide on the ordering of this request relative to others. All of the types of ordering
defined for multicast in Section 15.4.3 also apply to request handling and we define
those orders again for this context:
FIFO ordering: If a front end issues request r and then request r ...
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