Chapter 5 Naming. Names, Identifiers, and Addresses

Transcription

1 Chapter 5 Naming 1 Names, Identifiers, and Addresses In a distributed system, a name is used to refer to an entity (e.g., computer, service, remote object, file, user) An address is a name that refers to an access point of an entity E.g. a server s address consists of an IP address and a port number An entity may have multiple access points and addresses E.g., a person has several phone numbers (e.g. work, home, mobile) An entity may change its access points E.g., a service is moved to a different host, a person changes its address after changing his job Identifiers are used to uniquely identify an entity; an identifier is a name with the following properties An identifier refers to at most one entity Each entity is referred to by at most one identifier An identifier always refers to the same entity (i.e., it is never reused) 2 1

2 Naming Systems A naming system maintains a name toaddress binding to resolve names to addresses In a distributed system, the implementation of a naming system is often distributed across multiple machines We will study two classes of naming systems Flat Naming: resolving identifiers to addresses Structured Naming: resolving human friendly names to addresses 3 Flat Naming Each entity is assigned an identifier that uniquely identifies the entity Identifiers are fixed length bit strings, which we refer to as flat names (or unstructured names) Flat names can be efficiently handled by machines E.g., IP addresses, Ethernet addresses How to resolve an identifier to the address of the associated entity? We will study how identifiers are resolved in Chord 4 2

3 Name Resolution in Chord Every node is assigned a random m bit identifier Every data item is assigned a unique m bit key Data item with key k is managed by the node with the smallest identifier id k, denoted as succ(k) Each node p maintains a finger table with m entries, value of the i th entry is FT p [i]=succ(p+2 i 1 ) To look up a key k, a node p performs the following If p is responsible for k, then return the address of node p If p < k < FT p [1], then forward the request to FT p [1] If FT p [j] k < FT p [j+1], then forward the request to node q = FT p [j] A lookup requires O(logN) steps, where N is the number of nodes in the system 5 m=5 Resolving key 26 from node 1 and key 12 from node 28 in a Chord system. 6 3

4 Structured Naming Flat names are difficult for humans to remember Structured names are composed of several parts; these names are human friendly Example: file names, Internet host names How to locate an entity given its structured name? We will study two examples: file name resolution and host name resolution in the Internet (DNS) 7 Name Spaces for File Names File names are organized into a name space, which is represented by a labeled directed graph with two types of nodes A leaf node represents a file and stores information about the file (e.g., owner, last modification time, location of data blocks, etc.) A directory node represents a file directory and stores a directory table of (edge label, node ID) pairs A naming graph with a single root node. 8 4

5 Path Names A path name is used to refer to a node in the naming graph A path name is represented by a sequence of edge labels separated by a slash An absolute path name starts from the root node and begins with a slash (e.g., /home/steen/mbox) A relative path name starts from the current working directory and does not begin with a slash (e.g., steen/mbox) 9 Name Resolution Name resolution is the process of locating the node referred to by a given path name Name resolution is performed by traversing the naming graph Resolution of absolute path names starts from the root node Resolution of relative path names starts from the current directory node (e.g., resolution of steen/mbox starts from /home) 10 5

6 Hard Links Some file systems allow multiple absolute path names to refer to the same leaf node, these absolute path names are called hard links to the node E.g., the path names /keys and /home/steen/keys are hard links to node n5 This has the effect of creating multiple names for the same file If the file is opened by one of its names, and changes are made to its content, then the changes will be visible when the file is opened by an alternative name. 11 Symbolic Links A symbolic link is a special type of file that contains a reference to another file or directory in the form of an absolute path name E.g., /home/steen/keys is a symbolic link to node n5 When resolving /home/steen/keys, the path name stored in n6 (i.e., /keys) will be returned, and name resolution continues with resolving that path name 12 6

7 Merging Name Spaces (1) Two names spaces can be merged by mounting one name space into another name space To mount a foreign name space NS2 into a name space NS1, we let a directory node in NS1 (called a mount point) store the identifier of a directory node in NS2 (called a mounting point) During name resolution, the mounting point is looked up and resolution proceeds by accessing its directory table mount point mounting point 13 Merging Name Spaces (2) If the foreign name space is located on a different machine, the mount point must store the following information The name of the access protocol used to communicate with the server of the foreign name space The name of the server of the foreign name space The name of the mounting point in the foreign name space (to be resolved by the server of the foreign name space) 14 7

8 Mounting Remote Name Space in NFS Sun s Network File System (NFS) is a distributed file system that enables clients to access files stored on remote file servers The client can use the name /remote/vu/mbox to access the file /home/steen/mbox on the remote file server 15 8