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
1
Outline
• Limitations in existing web caches
• A DSL-based approach
• Language abstractions and
Execution Model
• Status and on-going work
2
Limited Flexibility
of Web Caches
• Existing caches provide too limited
customization
– Fixed policies
– Do not consider changes in the execution
environment
3
Reasons for Extensibility
• Why should caches be extensible ?
– Universal solutions do not exist
– Customized policies tend to provide better
results
– Caches should adapt to new users and
applications
4
Problems in extending
existing Cache Systems
• Not structured for extension purposes
– COTS, parameters...
• Development time
– Code hacking
• Error-prone
– Safety
Can we do better ?
5
A DSL-based Approach
• Why a DSL is useful for web caching ?
– (-) Development time:
• Provides powerful abstractions to the programmer
improving productivity
– (+) Layer decomposition:
• Clear separation of functional components which
eases code re-use
– (+) Safety/robustness:
• Improves safety via program verification
• Ex: message loop detection and matching, program
termination
6
What do we want to program ?
• Policy => Program family
– Removal
– Prefetching
– Placement
– Inter-cache protocols
– Others...
7
Language Abstractions
Feature
Abstraction
Cache Policy
execution control
Cache storage
management
Cache
Communication
Event-action model
SQL-like
Messages
8
Event-action Model
• Events
– Temporal events
– Transition events
– Message arrival
• Actions
•
•
•
•
Cache storehouse management: SQL-like
Communication - send
Cache statements - remove, prefetch, etc.
Event-action statements: jump, return, end
• Others: definitions, assignments, if-then-else, etc.
9
Programming in WebCal
• Define cache policies as a Finite State
Machine (FSM)
– Intercache protocol
• Define policy events and their predicates
– Timeouts
• Write actions to be executed when an
event is triggered
Examples
10
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;
)
}
11
]
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");
)
}
]
12
Webcal Execution Model
DSL Compiler
WebCal
Program
Caching
Policy in Java
Event-action
Backbone
Policies
Jigsaw
Library
Web Cache
13
Status and on-going Work
• Webcal:
– Working prototype using Jigsaw
– Event-action backbone / Compiler
– Tested policies: prefetching, removal,
placement
• Experiments
– Inter-cache protocols
14