COMP 655: Distributed/Operating Systems Summer 2011 Dr. Chunbo Chu Week 5: Naming 10/12/20151Distributed Systems - COMP 655

COMP 655:Distributed/Operating

SystemsSummer 2011

Dr. Chunbo ChuWeek 5: Naming

04/21/23 1Distributed Systems - COMP 655

04/21/23 Distributed Systems - Comp 655 2

What is the naming problem?

Name objects in distributed systems so that the naming system provides

1. Human-friendly names for use by people2. System-friendly names for use by systems3. Uniqueness where necessary4. Ambiguity when helpful5. A service that can be used in many

systems6. Minimal centralized administration7. Global scalability


Ambiguity when usefulwww.yahoo.akadns.net27 A 216.109.118.78

www.yahoo.akadns.net27 A 216.109.118.64








Aspects of human-friendliness

What do humans need in the names they use?

•

•

•


Examples of human-friendly names

• Albert Einstein• einstein• cs.franklin.edu• franklin.edu• edu• http://cs.franklin.edu/~chuc/655/• /home/export/cs/Htdocs/Syllabus/comp655/


Types of system-friendly names

• Addresses• Identifiers

Brainstorm• What are identifiers, addresses?


Address• Definition – a name that refers to an

access point in a communication system• Examples

– [email protected]– 205.133.198.15:1677– 00-04-E2-36-A0-5E– 201 S. Grant Avenue, Columbus, Ohio

43215– 614-797-4700– 877-341-6300


DefinitionIdentifier – a name with the following

properties:1. It refers to at most one entity2. An entity has at most one identifier3. Once used for one entity, its meaning

never changes

-- Tanenbaum, Page 181


Examples of identifiers

• {72eb61e0-8672-4303-9175-f2e4c68b2e7c}

• {b4431a67-9e32-4e2d-81e5-076a5f1b6170}

• 6a29b0a6b9ad79c8:f72617:ff5c75073e:-7ffd

• 00-04-E2-36-A0-5E• 123-45-6789 (close to being an identifier)


Globally Unique Identifiers (GUIDs)

• DCE/Microsoft:A GUID is a 128-bit integer (16 bytes) that can

be used across all computers and networks wherever a unique identifier is required. Such an identifier has a very low probability of being duplicated. GUIDs are generated using a combination of the current date and time, a counter, and the MAC address from the network card.


UIDs in Java• new java.rmi.server.UID()

– f72617:ff5c75073e:-7ffc

• Javadocs say: A UID instance contains three primitive values: – unique, an int that uniquely identifies the VM that this UID was generated in

– time, a long equal to a time (as returned by System.currentTimeMillis()) at which the VM that this UID was generated in was alive

– count, a short to distinguish UIDs generated in the same VM with the same time value

VMIDs in Java• A VMID is a identifier that is unique

across all Java virtual machines. VMIDs are used by the distributed garbage collector to identify client VMs.

• new java.rmi.dgc.VMID()– 6a29b0a6b9ad79c8:f72617:ff5c75073e:-7ffd– Adds an address to the UID.



If you remember nothing else

Distributed systems use three kinds of names:– Human-friendly names– Addresses– Identifiers

Naming services manage the associations among these kinds of names.


What about part 7 of the naming problem

(global scalability)?


Easy namingDirectory service in our projects• Only notebooks are named• Notebook names are arbitrary

character strings – no formation rules

• Light usage– A handful of names– Busy minute: a few dozen transactions

• Usually on a LAN


Hard naming – the Web• Global scalability

– Millions of entities to name– Millions of name lookups per hour– Slow and expensive communications– Many independently-administered domains

• Location-related transparency– Location– Re-location– Migration– Replication


Definition

• Name – a string of characters or bits that refers to something– Names in a computer system conform

to some set of formation rules– Names have semantics that determine

how a name maps to a thing– Some names have to be obtained from

a naming authority


Name spacedirectorytable

rootnode

path name(“absolute”)relative path name:

steen/.twmrc

global path name:nfs://flits/home/steen/.twmrc


Another name space: Java packages and classes

dino.naming.NameChooser

java.rmi.server. ObjID

dino/naming/NameChooser.class

java/rmi/server/ObjID.class


Yet another name space: DNS names

<domain> ::= <subdomain> | " "

<subdomain> ::= <label> | <subdomain> "." <label>

<label> ::= <letter> [ [ <ldh-str> ] <let-dig> ]

<ldh-str> ::= <let-dig-hyp> | <let-dig-hyp> <ldh-str>

<let-dig-hyp> ::= <let-dig> | "-"

<let-dig> ::= <letter> | <digit>

<letter> ::= any one of the 52 alphabetic characters A through Z in upper case and a through z in lower case

<digit> ::= any one of the ten digits 0 through 9

Source: RFC-1035

http://www.faqs.org/rfcs/rfc1035.html


Three layers of DNS names

Top-level domains

DN

S s

up

port

s to

p 2

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vels

loca

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sy

stem

http://www.icann.org/tlds/

Distributed Systems - COMP 655

Name Space Distribution (2)

04/21/23 22


Semantics - name resolution

• Given a name, look up information about the entity the name refers to

• In a hierarchical namespace, resolution works by:– Picking a starting point– Working down, across, or up the hierarchy from

there

• A closure mechanism is a way of deciding where to start name resolution– For local absolute names: where’s the root node?– For relative names: where’s the current node?– For global names: which root node?


Resolution implementation• Name resolution for common name

spaces is usually an operating system function– File system names– DNS names


Namespace collaboration• Linking• Mounting• Composite names• Abstract naming system interfaces


Linking• Aliases (aka synonyms)

– Hard links (multiple absolute paths to the same entity)

– Symbolic links (leaf nodes contains an absolute path to the entity)

• Beware of cycles• Symbolic links often integrate smoothly

into distributed systems– hard links cannot be made between files on

different file systems nor between directories


Linking

04/21/23 27


Mounting• Mounting == allowing a directory node to

store the identifier of a directory node in some other namespace

• Mount point == the local directory node• Mounting point == the remote directory

node– For UNIX file systems, mounting point is

usually a root node– For Windows file systems, mount point is

usually a root node (example – G: drive maps to \\randomserver\games)


Mounting

• Information required to mount a foreign name space in a distributed system

• The name of an access protocol.• The name of the server.• The name of the mounting point in

the foreign name space.

04/21/23 29


Linking and Mounting

04/21/23 30


Composite names

Include the three essentials in one string:

ftp: //ftp.cs.vu.nl /home/steen/mbox

protocol serverresource name (in remote name space)

http: //cs.franklin.edu /~chuc/655/


Composite names in the Java RMI Registry

//einstein:4310/SuperTalk

//:4321/Inventory

//einstein.franklin.edu/Mfg

//192.70.252.120:4300/chat

//host:port/servicenameStructure

Examples


Iterative name resolution

Resolving ftp://ftp.cs.vu.nl

NOTE: client’s name resolver may not run on the client machine. It may also be a name server for local entities


Recursive name resolution

Resolving ftp://ftp.cs.vu.nl


Compare & contrast• Advantages of recursive resolution:

– Name servers can cache addresses of lower-level nodes

– Usually, fewer long-haul communications

• Disadvantage of recursive resolution:– Higher-level name servers can become

bottlenecks


Example: The Domain Name System

04/21/23 36


Name service clients• Embedded in applications

– Web browsers– Email servers– Email clients– ftp, ssh, etc

• Command line– nslookup– dig


nslookupty

pes

on

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20

2,

cs.v

u.n

l ta

ble

on

p.

20

5$ nslookupDefault Server: ns1.franklin.eduAddress: 192.70.252.125

> set type=ANY> cs.vu.nlServer: ns1.franklin.eduAddress: 192.70.252.125

Non-authoritative answer:cs.vu.nl nameserver = top.cs.vu.nlcs.vu.nl nameserver = solo.cs.vu.nlcs.vu.nl nameserver = star.cs.vu.nlcs.vu.nl nameserver = ns.vu.nl

Authoritative answers can be found from:cs.vu.nl nameserver = solo.cs.vu.nlcs.vu.nl nameserver = star.cs.vu.nlcs.vu.nl nameserver = ns.vu.nlcs.vu.nl nameserver = top.cs.vu.nltop.cs.vu.nl internet address = 192.31.231.43solo.cs.vu.nl internet address = 130.37.24.1


digTo fi

nd

: G

oog

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ig d

ns

dow

nlo

ad


Codd on dig


JNDI

• http://java.sun.com/products/jndi/• http://java.sun.com/products/jndi/ser

viceproviders.html

http://java.sun.com/products/jndi/

http://java.sun.com/products/jndi/serviceproviders.html

http://java.sun.com/products/jndi/serviceproviders.html


Summary: naming system patterns

• Hierarchical namespace• Independent administration of sub-

trees• Symbolic links• Synonyms • Name includes the name of an

interpretative system

Documents

COMP 655: Distributed/Operating Systems Summer 2011 Dr. Chunbo Chu Week 5: Naming 10/12/20151Distributed Systems - COMP 655