The GlobeDoc Proxy: A Web Gateway for GlobeDoc objects - PowerPoint PPT Presentation

Title: The GlobeDoc Proxy: A Web Gateway for GlobeDoc objects


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The GlobeDoc Proxy A Web Gateway for GlobeDoc
objects

• Achmad Imam Kistijantoro
• TU Delft - KPN Research 1999

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Content

• Why Globe?
• Globe background
• Basic Design of Globe
• Distributed Shared Object
• Globe Object Local Representative
• GlobeDoc and stores concept
• GlobeDoc Proxy
• Binding mechanism in the Globe
• Object management in the Proxy
• Software Design Implementation
• Summary

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Globe Background

• Internet Explosion -gt numerous distributed
  applications
• Common required features Scalability,
  transparency, consistent naming schemes,
  security, reliability and migration
• Incompatibility problems and the development of
  new applications often requires to redesigning
  and re-implementing these features
• Needs of uniform framework, that can be reused,
  flexible enough to support different kinds of
  applications

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The Globe approach

• Uniform framework for distributed computing
• provides basic services needed by distributed
  applications
• Flexible implementation
• to support heterogeneous nature of wide area
  systems like the Internet
• Object-based approach, separating object
  interfaces from their implementations
• Ensure world-wide scalability
• Extensive support for partitioning and
  replicating objects
• Distributed Shared Object concept

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Distributed Shared Object Concept

• The state of the object may be distributed across
  multiple address spaces

Globe Object
Network
Globe Objects LR
Clients machine
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Sub objects of the Globe Object Local
Representative

• picture of the Globe Object Local Representative

Control object
Interface
Replication object
Semantic object
Communication object
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GlobeDoc A Scalable Web document

• A GlobeDoc object a web document, a collection
  of related web pages
• Root, Elements (HTML pages, images, etc),
  Properties
• Accessible from clients as a Globe object
• Choices for different replication policy for each
  GlobeDoc object
• Stores location where the GlobeDoc objects
  state / web document are kept

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GlobeDoc Replication

• Stores, mirror servers and proxies

Permanent stores (Primary server)

• Document-initiated stores
• (Mirror)

Contact point
Client-initiated stores (Proxy)
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The Design and Implementation of the GlobeDoc
Proxy

• GlobeDoc Proxy provides access to the GlobeDoc
  object from a web browser
• Receives queries from the Web, bind to the
  correspondent object, call the object methods to
  retrieve the document and return the result to
  the browser
• Focus on two aspects binding mechanism and
  object management (preliminary study)

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Binding Mechanism for Globe objects

• Binding make globe object interfaces available
  in the local address space - create globe object
  local representative in the local address space
• How to get the object implementation in the
  clients address space
• Binding steps

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Binding Mechanism
Client process
Object Name
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Name server
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Object Handle

• Globe Run-time service
• Name service
• Location service
• Selector
• (Local) Implementation Repository
• Class Loader

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Location server
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GlobeDoc LR
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Contact Address(es)
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Repository
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Object Loading

• Contact Address
• Contact points location
• Implementation and its configuration
• Additional information, such as replication
  policy, coherence model
• Class Archive
• Provides implementation binaries for GlobeDoc LR
• Class Loader
• Specific for each implementation binary type
• Implementation Repository
• Provides class archives and implementation
  binaries for GlobeDoc LR

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Binding - Loading Mechanism
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Object Management in the GlobeDoc Proxy

• Each object manage its own replication strategy
• Complicates the cache management in the proxy
• Possibility of active replication, that waste
  CPU, Network and Memory resources
• Proposed solution

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GlobeDoc Proxy location

• Document-initiated stores
• Object binding/unbinding is controlled by the
  GlobeDoc owner
• Mechanism for controlling the proxy from remote
  machine
• Client-initiated stores
• Either in the clients machine/as part of the
  clients browser or in a special machine for
  proxy
• Either available for a specific client or for
  many clients
• There is no specific instruction for object
  unloading, thus the proxy must develop its own
  mechanism for removing objects

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Object Management in the Proxy
proxy
Client
Active replication
Client
Client
Client
Clients Browsers
Passive replication
Passive replication

• Space
• Bandwidth
• CPU resources

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Cache Replacement Algorithms

• General model uniform/non uniform objects,
  uniform/non uniform cost model
• Access behavior only (fixed size and fixed cost)
• LRU, LFU
• Object size and Objects cost
• Size, Hyper-G, Greedy-Dual-Size
• Weighted function/parameter based algorithm
• Bolot-Hoschka, Wooster-Abrams, NNC
• Practical CR algorithms should be based on simple
  computations

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GlobeDoc Object

• Non uniform object, non uniform cost
• each object may have different replication
  policy, but this should be transparent to outside
• objects size might not be fixed
• active replication may lead to waste of
  resources, if accessed less frequently, but may
  lead to a reduced response time as perceived by
  clients
• initialization of an object is bigger than other
  operation
• proposal heuristic based algorithm (Hosseini97)

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Heuristic based algorithm

• Based on LRU
• If an item is removed, it could not have been
  retained in cache without exceeding cache
  capacity
• frequently used object should not be removed from
  the cache
• object that is not frequently used must be
  removed from the cache

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Cache Replacement Algorithm - Cost Density

• Cost density the cost or value of the object for
  each unit cache resources usage
• cost the time differences between cached object
  and uncached object (e.g. binding timepredictive
  document fetching)
• cache resources space, network bandwidth, CPU
  (?) (e.g. space usage, updates count
• Cost density Cost(object i)/Resource(object i)

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Cache Replacement Algorithm

• Cache list sorted by activeness (for example,
  based on LRU)
• candidate for removal (the most inactive objects)
  are sorted by cost density, and remove objects
  with least value, until certain resources
  threshold available

Most active
Highest Density Cost
removed
Least active
Lowest Density Cost
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Open issues

• Drawbacks
• Rather complicated computation
• Maintain separate list for each resources
• Computation for each object update and access
• Cost measurements probabilistic, depends on
  network condition
• Network resource usage measurement

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GlobeDoc Proxy Design Implementation
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Summary

• In this project, we have
• investigated the structure of Globe Run-time
  system, especially for object loading mechanism
  in binding
• designed and implemented GlobeDoc proxy
• investigated preliminary cache management for
  GlobeDoc proxy
• The preliminary implementation of the Globe
  object and proxy proves the practicability of the
  Globe concepts
• Further investigation is needed to deliver Globe
  capability in the real application, such as the
  behavior of Globe object in wide-scale
  application, mechanism for controlling proxy and
  mirror servers, unbinding mechanism for Globe
  objects