Naming in Distributed System

Naming in Distributed SystemG.Ramesh Babu

Contents

• Naming Entities– Names, Identifiers and Address– Name Spaces

• Name Resolution– Closure Mechanism– Linking and Mounting

• Implementation of Name Space• Implementation of Resolution• Conclusion

Why naming is important?

• Names are used to – Share resources– Uniquely identify entities– To refer locations, and so on…

• Name resolution allows a process to access the named entity

Naming Entities

• Name string of characters used to refer to an entity– Entity in DS can be anything, e.g., hosts,

printers, disks, files, mailboxes, web pages, etc

• Access Point To access an entity

• Address name of access point

• Access points of an entity may change

Identifier and True Identifiers

• We need – single name of entity independent from the address of

that entity location independent

• Identifiers name that uniquely identifies an entity

• True Identifier has three properties– Refers to at most one entity– Each entity is referred to by at most one identifier– Never reused

• Differentiating point for Address and Identifier

Name Space

• Names in DS are organized into Name Spaces• Name Space represented as labeled, directed

graph• Leaf node no outgoing edges• Directory node number of labeled outgoing

edges– Stores directory table containing entries for each

outgoing edge as a pair (edge label, node identifier)• Root Node only outgoing edges• Path Name sequence of labels

– Absolute Path first node in path name is root– Relative Path the opposite case

General Naming Graph

Name Resolution• The process of looking up a name• Closure Mechanism Knowing how and where to start

name resolution• Mounting transparent way for name resolution with

different name spaces• Mounted File System letting a directory node store the

identifier of a directory node from a different name space (foreign name space)– Mount point directory node storing the node identifier– Mounting point directory node in the foreign name

space• Normally the mounting point is root

Mounted File System

• During resolution, mounting point is looked up & resolution proceeds by accessing its directory table

• Mounting requires at least– Name of an access protocol (for communication)– Name of the server (resolved to address)– Name of mounting point in foreign name space

(resolved to node identifier in foreign NS)• Each of these names needs to be resolved• Three names can be represented as URL nfs://oslab.khu.ac.kr/home/faraz

Mounted File System

Global Name Service (GNS)

• Another way to merge different name spaces• Mechanism add a new root node and make

the exiting root node its children• Problem

– Existing names need to be changed. E.g.,

home/faraz people/home/faraz

• Expansion is generally hidden from user• Has a significant performance overhead when

merging 100s or 1000s of name spaces

Global Name Service (GNS)

Implementation of Name Space

• For large scale DS, name spaces are organized hierarchically

• Name Spaces are partitioned into three logical layers– Global Layer formed by highest-level

nodes– Administration Layer formed by directory

nodes managed within a single organization– Managerial Layer formed by nodes that

may typically change regularly



Item Global Administrational Managerial

Geographical scale of network Worldwide Organization Department

Total number of nodes Few Many Vast numbers

Responsiveness to lookups Seconds Milliseconds Immediate

Update propagation Lazy Immediate Immediate

Number of replicas Many None or few None

Is client-side caching applied? Yes Yes Sometimes

Implementation of Name Resolution• Assumptions

– No replication of name servers– No client side caching– Each client has access to a local name server

• Two possible implementations – Iterative Name Resolution

• Server will resolve the path name as far as it can, and return each intermediate result to the client

– Recursive Name Resolution• A name server passes the result to the next name server

found by it

Iterative Name Resolution

• Advantage– Less burden on name sever

• Disadvantage– More communication cost

Recursive Name Resolution

• Advantages– Caching result is more effective– Reduced communication cost

• Disadvantage– Demands high performance on each name server

Domain Name System (DNS)

• An example implementation of name resolution• Primarily used for looking up host address and

mail servers• DNS name space is hierarchically organized as

a rooted tree• A label is a case sensitive string with max.

length of 63 characters• Max. length of complete path name is 255

characters• The root is represented by a dot

– We generally omit this dot for readability

Locating Mobile Entities

Naming versus Locating Entities

• Entities are named for lookup and subsequent access– Human-friendly Names– Identifiers– Addresses

• Virtually all naming systems maintain mapping from Human-friendly names to addresses

• Partitioning of Name space– Global Level– Administrator Level– Managerial Level


ftp.cs.vu.nl

cs.vu.nl cs.vu.nl

ftp.cs.vu.nl ftp.abc.cs.vu.nl

abc

cs.vu.nl

ftp.cs.vu.nl

ftp.khu.ac.kr


• Possible Solutions– Record the address of new machine

• Lookup operation shall work• Another update shall be required to database in case it

changes again

– Record the name of the new machine• Less efficient

– Find the name of new machine

– Lookup the address associated with the name

• Addition of step to lookup operation

• For highly mobile entities, it becomes only worse


• Direct, single level mapping between names and addresses.• T-level mapping using identities.

Simple solutions: Broadcasting and multicasting

• A location service accepts an identifier as input and returns the current address of the identified entity.

• Simple solutions exist to work in local area network.• Address Resolution Protocol (ARP) to map the IP

address of a machine to its data-link address, which uses broadcasting.

• Multicasting can be used to locate entities in point-to-point networks (such as the Internet).

• Each multicasting address can be associated with multiple replicated entities.

Forwarding Pointers (1)

• The principle of forwarding pointers using (proxy, skeleton) pairs.

Forwarding Pointers (1)

• Redirecting a forwarding pointer, by storing a shortcut in a proxy.

Home-Based Approaches

• Example: The principle of Mobile IP. (Perkins, 1997)

Hierarchical Approaches (1)

• Hierarchical organization of a location service into domains, each having an associated directory node.


• An example of storing information of an entity having two addresses in different leaf domains.


• Looking up a location in a hierarchically organized location service.


a) An insert request is forwarded to the first node that knows about entity E.

b) A chain of forwarding pointers to the leaf node is created.

Pointer Caches (1)

• Caching a reference to a directory node of the lowest-level domain in which an entity will reside most of the time.

Pointer Caches (2)

• A cache entry that needs to be invalidated because it returns a nonlocal address, while such an address is available.

Scalability Issues

• The scalability issues related to uniformly placing subnodes of a partitioned root node across the network covered by a location service.

The Problem of Unreferenced Objects• An example of a graph representing objects

containing references to each other.

Reference Counting (1)

• The problem of maintaining a proper reference count in the presence of unreliable communication.

Reference Counting (2)

a) Copying a reference to another process and incrementing the counter too late

b) A solution.

Advanced Referencing Counting (1)

a) The initial assignment of weights in weighted reference counting

b) Weight assignment when creating a new reference.


c) Weight assignment when copying a reference.


• Creating an indirection when the partial weight of a reference has reached 1.


• Creating and copying a remote reference in generation reference counting.

Reference Listing (1)• Skeleton Keeps track of Proxies

– Instead of counting them maintain an explicit list of references

• Adding/removing references to the list have no effect on the fact the proxy is already exists/removed

• Idempotent Operations– Repeatable without affecting the end result

• Increment/decrement operation are clearly not idempotent

Reference Listing (2)• Advantages

– Don’t require reliable communication– Duplicate messages need not to be detected– Only insertion/deletion should be acknowledged– Easier to keep system consistent in case of process

failures

• Drawback– Scale badly

• Solution– Leasing

Identifying Unreachable Entities• Trace based garbage collection

– Scalability problems

• Naïve tracing– Mark and sweep collectors

• White, Grey, Black marks

• Drawbacks– Reachability graphs need to remain same during

both phases– No process can run when GC is running

Tracing in Groups (1)

• Initial marking of skeletons.


• After local propagation in each process.


• Final marking.

Conclusion

• Naming, organization of names and name resolution are key issue in any distributed systems

• Locating entities is an open research issues. There are few methods like Forwarding pointers, hierarchical approaches, home based approaches and pointer caches but each has its own short comings

• Reference counting, advanced reference counting and Reference listing are few methods that can be used for unreferenced objects

- All is well that ends well !

Thank you all

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Naming in Distributed System

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