Книга: Distributed operating systems

10.3.4. Performing an RPC

10.3.4. Performing an RPC

The actual RPC is carried out transparently and in the usual way. The client stub marshals the parameters and passes the resulting buffer (possibly in chunks) to the runtime library for transmission using the protocol chosen at binding time. When a message arrives at the server side, it is routed to the correct server based on the endpoint contained in the incoming message. The runtime library passes the message to the server stub, which unmarshals the parameters and calls the server. The reply goes back by the reverse route.

DCE provides several semantic options. The default is at-most-once operation, in which case no call is ever carried out more than once, even in the face of system crashes. In practice, what this means is that if a server crashes during an RPC and then recovers quickly, the client does not repeat the operation, for fear that it might already have been carried out once.

Alternatively, it is possible to mark a remote procedure as idempotent (in the IDL file), in which case it can be repeated multiple times without harm. For example, reading a specified block from a file can be tried over and over until it succeeds. When an idempotent RPC fails due to a server crash, the client can wait until the server reboots and then try again. Other semantics are also theoretically available (but rarely used), including broadcasting the RPC to all the machines on the local network.

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