Sistemas Distribuidos

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 managers.

Coordination: The

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