Distributed Systems Naming

Transcript

1. 5.3 Structured Naming
   Distributed Systems
   Chapter 5
   Naming
   Prepared by: Ahmed Magdy Ezzeldin
   Reference: Distributed Systems Principles and Paradigms 2nd Edition
   

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2. 5.3.1 Name Spaces
   ●
   A Name space is a labeled, directed graph with two types of
   nodes.
   ●
   A leaf node represents a named entity and has the property that it
   has no outgoing edges.
   ●
   And a directory node that has a number of outgoing edges, each
   labeled with a name, and has an associated identifier.
   ●
   A directory table holds (edge label, node identifier) for the
   directory contents.
   ●
   n0 is the root node and name : N label_1 , label_2 , ..... label_n is
   called a path. If N is the root then the path is called an absolute
   path, otherwise it is called a relative path. We use "/" as a
   separator between labels and as a representation for n0.
   

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3. 5.3.1 Name Spaces [continued]
   A name space can be strictly hierarchical so that the naming graph
   is organized as a tree (one absolute path for each node). In other
   cases it can be organized in a directed acyclic graph (not permitted
   to have a cycle).
   

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4. 5.3.2 Name Resolution
   - Resolution starts by reading the directory table
   at the first node of the path and then proceeding
   to the next identified node and continue the look-
   up until reaching the last node where it returns the
   node identifier for the last node in the path.
   - Closure Mechanism
   Knowing how and where to start name resolution
   is generally referred to as a closure mechanism.
   Essentially, a closure mechanism deals with
   selecting the initial node in a name space from
   which name resolution is to start.
   

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5. 5.3.2 Name Resolution [continued]
   

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6. 5.3.2 Name Resolution [continued]
   - Linking and Mounting
   There are 2 ways of aliasing naming
   1- In a directed graph where there are 2 absolute paths to
   a certain node. (Like Hard Links in UNIX file system)
   2- In a tree structure, we can represent an entity by a leaf
   node that stores an absolute path name of another node. (like
   symbolic links in UNIX file system) [Figure 5-11]
   A mounted file system corresponds to letting a directory node
   store the identifier of a directory node from a different name
   space. To mount a foreign name space in a distributed
   system requires at least the following information:
   1. The name of an access protocol.
   2. The name of the server.
   3. The name of the mounting point in the foreign name
   space. [Figure 5-12]
   

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7. 5.3.2 Name Resolution [continued]
   

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8. 5.3.3 Implementation of a Name Space
   - Name Space Distribution
   Name spaces are organized hierarchically and also
   partitioned into logical layers:
   1- Global layer (Root and its direct children) which is
   stable (rarely changed). In this layer availability is more
   important than performance as caching can be used.
   2- The administrational layer (they represent groups of
   entities that belong to the same organization or
   administrational unit) stable but not like the global layer.
   3- The managerial layer consists of nodes that may
   typically change regularly. For example, nodes
   representing hosts in the local network belong to this
   layer. [Figure 5-13] & [Figure 5-14]
   

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9. 5.3.3 Implementation of a Name Space [continued]
   

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10. 5.3.3 Implementation of a Name Space [continued]
    

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11. ●
    Implementation of Name Resolution
    - Iterative name resolution:
    Where the client name resolver requests from the
    root name server to respond with the URL parts
    identifiers. The root server replies with what it can
    resolve then the response is sent back to the
    client that sends a new request to the next name
    server that has been resolved by the root server
    and so on until the whole URL has been resolved.
    [Figure 5-15]
    5.3.3 Implementation of a Name Space [continued]
    

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12. 5.3.3 Implementation of a Name Space [continued]
    

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13. - Recursive Name resolution:
    Where it is the responsibility of the name server
    not the client to resolve the name by requesting
    the rest of the name to be resolved from the other
    servers recursively which adds a performance
    demand on each server as it has to resolve the
    whole URL, but caching can be used more
    effectively than iterative name resolution.
    [Figure 5-16] & [Figure 5-17] & [Figure 5-18]
    5.3.3 Implementation of a Name Space [continued]
    

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14. 5.3.3 Implementation of a Name Space [continued]
    

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15. 5.3.3 Implementation of a Name Space [continued]
    

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16. 5.3.3 Implementation of a Name Space [continued]
    

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17. 5.3.4 The DNS Name Space [continued]
    ●
    The DNS name space is hierarchically organized as a
    rooted tree.
    ●
    A label is a case-insensitive string made up of
    alphanumeric characters.
    ●
    A label has a maximum length of 63 characters.
    The length of a complete path name is restricted to 255
    characters.
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    The string of a path name consists of its labels, starting
    with the rightmost one, and separating the labels by a dot.
    ●
    The root is represented by a dot. e.g. the path name root:
    , is represented by the string
    "s1.web.google.com.", which includes the rightmost dot to
    indicate the root node (the dot is removed for readability).
    ●
    A subtree is called a domain; a path name to its root node
    is called a domain name. [Figure 5-19]
    

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18. 5.3.4 The DNS Name Space [continued]
    

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19. - DNS Implementation
    ●
    The DNS name space is divided into a global
    layer and an administrational layer.
    ●
    Each zone is implemented by a name server,
    which is virtually always replicated for availability.
    ●
    A DNS database is implemented as a (small)
    collection of files, a file for each zone.
    5.3.4 The DNS Name Space [continued]
    

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20. Thank you
    Questions
    ???
    

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