Книга: Distributed operating systems

7.6.3.The Replication Server

7.6.3.The Replication Server

Objects managed by the directory server can be replicated automatically by using the replication server. It practices what is called lazy replication. What this means is that when a file or other object is created, initially only one copy is made. Then the replication server can be invoked to produce identical replicas, when it has time. Instead of making direct calls to it, the replication server is kept running in the background all the time, scanning specified parts of the directory system periodically. Whenever it finds a directory entry that is supposed to contain n capabilities but contains fewer, it contacts the relevant servers and arranges for additional copies to be made. Although the replication server can be used to replicate any kind of object, it works best for immutable objects, such as bullet files. The advantage is that immutable objects cannot change during the replication process, so it can safely go in the background, even if it takes a substantial time. Mutable objects might change during the replication process, adding additional complexity to avoid inconsistency.

In addition, the replication server runs the aging and garbage collection mechanism used by the bullet server and other servers. Periodically, it touches every object under the directory server's control, to prevent them from timing out. It also sends the age messages to the servers to cause them to decrement all the object counters and garbage collect any that have reached zero.

Оглавление книги