Understanding Pharo’s global state IWST 2014 at ESUG, Cambridge

1. Understanding Pharo’s global state
to move programs
through time and space
Guillermo Polito, Noury Bouraqadi, Stéphane
Ducasse, Luc Fabresse
Ecole des Mines de Douai, RMoD/Inria
Stéphane Ducasse
IWST 2014, ESUG, Cambridge
Tuesday 19 August 14

2. The problem:
moving code around
environment I environment II
(another time, another machine)
MyLogger
MyCache
Tuesday 19 August 14

3. The problem:
moving code around
environment I environment II
(another time, another machine)
MyLogger
MyCache
Tuesday 19 August 14

4. The problem:
moving code around
environment I environment II
(another time, another machine)
MyLogger
MyCache
MyLogger
MyCache
Tuesday 19 August 14

5. The problem:
moving code with global state around
• should global state be reinitialized?
• should it be kept and moved as well?
• should it be rebound to some already
existing object?
Tuesday 19 August 14

6. The main challenge (by example):
non-explicitness
WeakIdentityKeyDictionary subclass: #ASTCache
classVariableNames: ’Default’.
ASTCache>>at: aCompiledMethod
^ self
at: aCompiledMethod
ifAbsentPut: [ self newASTFor: aCompiledMethod ].
ASTCache>>newASTFor: aMethod
"creation of the AST..."
ASTCache>>reset
self removeAll.
ASTCache class>>default
^ Default ifNil: [ Default := self new ].
ASTCache class>>shutDown
self default reset.
• Ad-hoc implementation of a cache
=> no clear API
• It’s incomplete (e.g., lack support for a
recycling strategy)
=> each cache implements
its own features
• We know it is a cache just because of
the class name
=> no programatic way to
identify it
Tuesday 19 August 14

7. The need:
Understanding
the global state
Tuesday 19 August 14

8. A classification
Category #
Constants 1722
Settings 236
Singletons 65
Graphical Res. 47
Caches 43
Registries 31
Session Specific 27
Process Controllers 11
Finalizables 6
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9. Some possible solutions
• Reification of implicit elements
• Moving responsibilities to the language
Tuesday 19 August 14

10. Reification provides with
• Explicitness
• Unified APIs
• Frameworks and libraries can profit from it
(particularly code-migration ones)
E.g.,
first class instance variables,
first class processes,
first class caches
Tuesday 19 August 14

11. Moving responsibilities
to the language
• So the system can introspect itself
• Modify itself
• And control itself
E.g.,
flush caches on low memory
Tuesday 19 August 14

12. Understanding Pharo’s global state
to move programs
through time and space
• A classification of global state usage
• Detected a need for reification
• Detected a need for moving some responsibilities to the
language
Guillermo Polito, Noury Bouraqadi, Stéphane
Ducasse, Luc Fabresse
Ecole des Mines de Douai, RMoD/Inria
Tuesday 19 August 14