tailieunhanh - DISTRIBUTED SYSTEMS principles and paradigms Second Edition phần 4

Chèn một địa chỉ như vừa mô tả dẫn cài đặt dây chuyền của con trỏ trong một thời trang hàng đầu xuống bắt đầu từ nút thư mục cấp thấp nhất có một hồ sơ vị trí cho thực thể E. Một cách khác là tạo ra một hồ sơ vị trí trước khi đi qua các yêu cầu chèn vào nút cha. Nói cách khác, chuỗi các con trỏ được xây dựng từ dưới lên. | SEC. FLAT NAMING 195 Inserting an address as just described leads to installing the chain of pointers in a top-down fashion starting at the lowest-level directory node that has a location record for entity E. An alternative is to create a location record before passing the insert request to the parent node. In other words the chain of pointers is constructed from the bottom up. The advantage of the latter is that an address becomes available for lookups as soon as possible. Consequently if a parent node is temporarily unreachable the address can still be looked up within the domain represented by the current node. A delete operation is analogous to an insert operation. When an address for entity E in leaf domain D needs to be removed directory node dir D is requested to remove that address from its location record for E. If that location record becomes empty that is it contains no other addresses for E in D the record can be removed. In that case the parent node of direD wants to remove its pointer to dir D . If the location record for E at the parent now also becomes empty that record should be removed as well and the next higher-level directory node should be informed. Again this process continues until a pointer is removed from a location record that remains nonempty afterward or until the root is reached. STRUCTURED NAMING Flat names are good for machines but are generally not very convenient for humans to use. As an alternative naming systems generally support structured names that are composed from simple human-readable names. Not only file naming but also host naming on the Internet follow this approach. In this section we concentrate on structured names and the way that these names are resolved to addresses. Name Spaces Names are commonly organized into what is called a name space. Name spaces for structured names can be represented as a labeled directed graph with two types of nodes. A leaf node represents a named entity and has the property