Client ’ s Perspective : File Services

In a distributed file system (DFS), multiple clients share files provided by a shared file system. In the DFS paradigm communication between processes is done using these shared files. Although this is similar to the DSM and distributed object paradigms (in that communication is abstracted by shared resources) a major difference between these paradigms and the DFS paradigm is that the resources (files) in DFS are much longer lived. This makes it, for example, much easier to provide asynchronous and persistent communication using shared files than using DSM or distributed objects. The basic model provided by distributed file systems is that of clients accessing files and directories that are provided by one or more file servers. A file server provides a client with a file service interface and a view of the file system. Note that the view provided to different clients by the same server may be different, for example, if clients only see files that they are authorised to access. Access to files is achieved by clients performing operations from the file service interface (such as create, delete, read, write, etc.) on a file server. Depending on the implementation the operations may be executed by the servers on the actual files, or by the client on local copies of the file. We will return to this issue later. The key challenges of distributed systems apply to DFS as follows: