Designing a Global Name Service written by Butler W. Lampson, - - PowerPoint PPT Presentation

Faculty of Computer Science Institute for System Architecture, Operating Systems Group

Designing a Global Name Service

pizza-reading-group, summer term 2007 written by Butler W. Lampson, presented by Thomas Knauth

TU Dresden, 2007-05-23 Designing a Global Name Service Slide 2 of 11

Introduction

• What is a name service after all?

– maps names to values e.g. domain names to IP addresses

• What are the design challenges?

– size – lifetime – high availability – fault-tolerance – mistrust

• Where are name services used today?

– most prominent is probably DNS – in fact, this design uses the name service to look up server names

TU Dresden, 2007-05-23 Designing a Global Name Service Slide 3 of 11

Directory tree

• basic element is the

directory

• directory identifier (DI)
• directory reference is

value of the name i.e. the DI

• directory maps

directory names to directory identifiers

TU Dresden, 2007-05-23 Designing a Global Name Service Slide 4 of 11

Values in a directory

• directory maps names

to trees

• labels
• timestamps for

synchronization

• mark indicates

whether node is present or absent

• value of a path can be

– single leaf – set of leaves – sub-tree

TU Dresden, 2007-05-23 Designing a Global Name Service Slide 5 of 11

Behind the scenes

• multiple directory

copies

• copies loosely

synchronized

– sync by sweeps, or – by messages

• copies linked into a

ring

TU Dresden, 2007-05-23 Designing a Global Name Service Slide 6 of 11

Growing the name space

• adding is straightforward
• combining by creating a new root

– keep set of well-known directory identifiers

TU Dresden, 2007-05-23 Designing a Global Name Service Slide 7 of 11

Restructuring

• making IBM subtree of DEC

– create a link if old name should still work

TU Dresden, 2007-05-23 Designing a Global Name Service Slide 8 of 11

Caching

• directory reference upper bound on validity

called expiration time

• directory lookup can be safely cached for

t=minimum over all expiration times

TU Dresden, 2007-05-23 Designing a Global Name Service Slide 9 of 11

What I left out

• intricacies of update/sweeping mechanism
• name service interface

– client/administrators way to interact with the service

• specification

– collection of predicates that must hold during execution

TU Dresden, 2007-05-23 Designing a Global Name Service Slide 10 of 11

Rather closed questions

• “I expect that most updates will be

distributed in messages, but it is extremely difficult to make this method fully reliable. The sweep, [...], is quite easy to implement reliably.” Why?

• What is SRC/Birrell?

– l*/l where l* is empty? But it's not an only child ... – l*/li where li is a leaf node? But not all are leaf nodes ... – l* were the node where it ends is the value of l*; is there a node for l* ?!

TU Dresden, 2007-05-23 Designing a Global Name Service Slide 11 of 11

Open questions

• Would you expect anything else from a name

service? Does technological advance allow for some other design?

• What do current name services (e.g. DNS)

look like?

• Has this design proven practical?
• What about multiple roots and links from one

tree into another?