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OSF DCE Application Development Guide—Introduction and Style Guide
The most important issues concern the role of UUIDs in the binding model. Interface
identifiers, which consist of a UUID and version number, have a well-defined and
unambiguous role. But object UUIDs are somewhat overloaded by the binding model.
An object UUID may be used to select bindings from the name service, to select
endpoints from the endpoint mapper, and to map a call to the correct manager type
within the server. Furthermore, a server may use object UUIDs in some application-
specific way to identify and manipulate the objects it manages.
There is great potential for conflict between the use of object UUIDs to select bindings
and endpoints and their use to identify objects and routes to manager types. This conflict
is particularly evident in the case of servers that provide so-called ubiquitous interfaces,
such as the rdacl interface. Because many servers on a host are likely to export such an
interface, it is essential to have an object UUID to identify the correct endpoint in the
endpoint map. Without an object UUID, the endpoint mapper can only return the
endpoint of some server that exports the requested interface, very likely the wrong one.
An alternative strategy does exist: a client can call rpc_ep_resolve_binding() using a
nonubiquitous interface that it knows the server of interest does export. The call to the
ubiquitous interface can then be made with the resolved binding. Clients often use this
technique to call the remote pc_mgmt_* routines. Nevertheless, the objection remains
that it is still impossible to select among endpoints of servers or server instances that
export the same nonubiquitous interface.
The most straightforward solution is for a server to export a UUID to the namespace
where it functions as an unambiguous tag for the servers’ endpoints. Clients can find this
UUID either by importing it from a named entry or it may be made well-known,
effectively becoming a stable, well-known tag for the server’s volatile endpoints. When
endpoint UUIDs are well-known, they become useful for finding servers even when the
client is interested in a nonubiquitous interface. Exactly how servers export and clients
find these UUIDs depends on the resource model adopted, as discussed in Chapter 5.
This obvious use of UUIDs as endpoint identifiers, however, potentially conflicts with
their use as object identifiers. According to the RPC binding model, when clients import
bindings based on object UUIDs, these UUIDs are incorporated into call bindings where
they may used for endpoint selection, for manager selection, and possibly for some
application-specific purpose.
If an application
exports its object UUIDs to the
namespace, then they are used both to identify objects and to identify endpoints. This
means that, at a minimum, a server would need to maintain a potentially large number of
mappings to the same endpoints.
Moreover, especially when servers manage many objects or create them dynamically,
clients will typically know objects by names rather than by UUIDs. Servers can provide
such mappings via the namespace itself by exporting each object UUID to a different
namespace entry, but this even further complicates the server’s job of maintaining its
exports and mappings.
The obvious solution to these problems is to have servers maintain their object UUIDs
and name-to-object UUID mappings internally. The basic RPC binding mechanism does
not provide much support for this approach: there is no generic way for servers to make
objects or names available to clients except through the name service. Also, a UUID
used to identify a server endpoint is probably useless for call routing to a manager type
within a server. However, the higher-level object management interfaces discussed in
Chapter 5 provide this functionality.
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Tandem Computers Incorporated
124246
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