OSF DCE Application Development Guide--Core Components

OSF DCE Application Development Guide—Core Components
12.5 Execution Semantics
Execution semantics identify the ability of a procedure to execute more than once
during a given remote procedure call. The communications environment that
underlies remote procedure calls affects their reliability. A communications link
can break for a variety of reasons such as a server termination, a remote system
crash, a network failure, and so forth; all invocations of remote procedures risk
disruption due to communications failures. However, some procedures are more
sensitive to such failures, and their impact depends partly on how reinvoking an
operation affects its results.
To maximize valid outcomes for its operations, the operation declarations of an
RPC interface definition indicate the effect of multiple invocations on the
outcome of the operations.
Table 12-1 summarizes the execution semantics for DCE RPC calls.
TABLE 12-1. Execution Semantics for DCE RPC Calls
__________________________________________________________________________
Semantics Meaning
__________________________________________________________________________
__________________________________________________________________________
The operation must execute either once, partially, or not at all; for
example, adding or deleting an appointment from a calendar can use
at-most-once semantics. This is the default execution semantics for
remote procedure calls.
at-most-once
__________________________________________________________________________
The operation can execute more than once; executing more than once
using the same input arguments produces identical outcomes without
undesirable side effects; for example, an operation that reads a block of
an immutable file is idempotent. DCE RPC supports maybe semantics
and broadcast semantics as special forms of idempotent operations.
idempotent
____________________________________________________________
Semantics Meaning
____________________________________________________________
The caller neither requires nor receives any response
or fault indication for an operation, even though
there is no guarantee that the operation completed.
An operation with maybe semantics is implicitly
idempotent and must lack output parameters.
maybe
____________________________________________________________
The operation is always broadcast to one server on
each host system on the local network, rather than
delivered to a specific server, and one reply is
returned to the client. An operation with broadcast
semantics is implicitly idempotent.
broadcast
__________________________________________________________________________
12 − 12 Tandem Computers Incorporated 124245