Webcal: A Domain-specific Language for Web Caching Sumit Gulvani, Asha Tarachandani, Deepak Gupta, Dheeraj Sanghi Indian Institute of Technology, Kanpur, India Luciano Porto Barreto, Gilles Muller, Charles Consel INRIA/IRISA - Compose group, Rennes, France

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Outline

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Limitations in existing web caches A DSL-based approach Language abstractions and Execution Model

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Status and on-going work

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Limited Flexibility of Web Caches

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Existing caches provide too limited customization

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Fixed policies

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Do not consider changes in the execution environment

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Reasons for Extensibility

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Why should caches be extensible ?

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Universal solutions do not exist

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Customized policies tend to provide better results

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Caches should adapt to new users and applications

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Problems in extending existing Cache Systems

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Not structured for extension purposes

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COTS, parameters...

Development time

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Code hacking Error-prone

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Safety

Can we do better ?

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A DSL-based Approach

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Why a DSL is useful for web caching ?

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(-) Development time:

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Provides powerful abstractions to the programmer improving productivity

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(+) Layer decomposition:

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Clear separation of functional components which eases code re-use

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(+) Safety/robustness:

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Improves safety via program verification Ex: message loop detection and matching, program termination

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What do we want to program ?

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Policy => Program family

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Removal

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Prefetching

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Placement

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Inter-cache protocols

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Others...

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Language Abstractions

Feature Abstraction

Cache Policy execution control Cache storage management Cache Communication Event-action model SQL-like Messages

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Event-action Model

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Events

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Temporal events

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Transition events

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Message arrival

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Actions

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Cache storehouse management: SQL-like Communication send Cache statements remove , prefetch, etc.

Event-action statements: jump , return , end

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Others: definitions, assignments, if-then-else, etc.

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Programming in WebCal

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Define cache policies as a Finite State Machine (FSM)

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Intercache protocol

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Define policy events and their predicates

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Timeouts

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Write actions to be executed when an event is triggered

Examples

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Removal Policy define MAX_CACHE_SIZE 128Mb; FSM removal [ start { entrycode { event E1 = cacheSize > 0.8*MAX_CACHE_SIZE; } E1 -> ( remove from DOC_INFO_TABLE where contentType == "gif" until cachesize < 0.5*MAX_CACHE_SIZE; ) } ]

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Prefetching Policy FSM prefetch [ start { entrycode { event E1 = at 00:00:00 hours; } E1 -> ( if cacheSize < 0.7*MAX_CACHE_SIZE { prefetch ("http://www.Mysite.Com"); ) } ]

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Webcal Execution Model

DSL Compiler

WebCal Program Caching Policy in Java

Policies

Jigsaw Library Event-action Backbone

Web Cache

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Status and on-going Work

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Webcal:

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Working prototype using Jigsaw

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Event-action backbone / Compiler

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Tested policies: prefetching, removal, placement

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Experiments

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Inter-cache protocols

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