Книга: Distributed operating systems

7.2. OBJECTS AND CAPABILITIES IN AMOEBA

The basic unifying concept underlying all the Amoeba servers and the services they provide is the object. An object is an encapsulated piece of data upon which certain well-defined operations may be performed. It is, in essence, an abstract data type. Objects are passive. They do not contain processes or methods or other active entities that "do" things. Instead, each object is managed by a server process.

To perform an operation on an object, a client does an RPC with the server, specifying the object, the operation to be performed, and optionally, any parameters needed. The server does the work and returns the answer. Operations are performed synchronously, that is, after initiating an RPC with a server to get some work done, the client thread is blocked until the server replies. Other threads in the same process are still runnable, however.

Clients are unaware of the locations of the objects they use and the servers that manage these objects. A server might be running on the same machine as the client, on a different machine on the same LAN, or even on a machine thousands of kilometers away. Furthermore, although most servers run as user processes, a few low-level ones, such as the segment (i.e., memory) server and process server, run as threads in the kernel. This distinction, too, is invisible to clients. The RPC protocol for talking to user servers or kernel servers, whether local or remote, is identical in all cases. Thus a client is concerned entirely with what it wants to do, not where objects are stored and where servers run. A certain directory contains the capabilities for all the accessible file servers along with a specification of the default choice, so a user can override the default in cases where it matters. Usually, the system administrator sets up the default to be the local one.

Оглавление книги