tailieunhanh - DISTRIBUTED SYSTEMS principles and paradigms Second Edition phần 10

Phối hợp-hệ thống phân phối đóng một vai trò quan trọng trong việc xây dựng các ứng dụng phân tán. Hầu hết trong số này tập trung hệ thống tách cặp tham chiếu của các quá trình, có nghĩa là quá trình không rõ ràng tham khảo với nhau để cho phép giao tiếp. | SEC. SUMMARY 621 SUMMARY Coordination-based distributed systems play an important role in building distributed applications. Most of these systems focus on referential uncoupling of processes meaning that processes need not explicitly refer to each other to enable communication. In addition it is also possible to provide temporal decoupling by which processes do not have to coexist in order to communicate. An important group of coordination-based systems is formed by those systems that follow the publish subscribe paradigm as is done in TIBlRendezvous. In this model messages do not carry the address of their receiver s but instead are addressed by a subject. Processes that wish to receive messages should subscribe to a specific subject the middleware will take care that messages are routed from publishers to subscribers. More sophisticated are the systems in which subscribers can formulate predicates over the attributes of published data items. In such cases we are dealing with content-based publish subscribe systems. For efficiency it is important that routers can install filters such that published data is forwarded only across those outgoing links for which it is known that there are subscribers. Another group of coordination-based systems uses generative communication which takes place by means of a shared dataspace of tuples. A tuple is a typed data structure similar to a record. To read a tuple from a tuple space a process specifies what it is looking for by providing a template tuple. A tuple that matches thar ternplate is then selected and returned to the requesting process. If no match could be found the process blocks. Coordination-based systems are different from many other distributed systems in that they concentrate fully on providing a convenient way for processes to communicate without knowing each other in advance. Also communication may continue in an anonymous way. The main advantage of this approach is flexibility as it becomes easier