Separating Adaptable Persistence Attributes through Computational Reflection

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Separating Adaptable Persistence Attributes
through Computational Reflection

• Authors Francisco Ortin, Benjamin Lopez, J.
  Baltazar Garcia Perez-Schofield.
• IEEE Software, Nov Dec 2004

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Dominant Persistent-Application Development

• Directly Interface with SQL
• Example
• List coffees new Vector()
• String query "SELECT COF_NAME, PRICE FROM
  COFFEES"
• ResultSet rs stmt.executeQuery(query)
• while (rs.next())
• Coffee coffee new Coffee()
• coffee.setName(rs.getString("COF_NAME"))
• coffee.setPrice(rs.getFloat("PRICE"))
• coffees.add(coffee)

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Dominant Persistent-Application Development (2)

• Use Object-Relational Mapping Software
  (Hibernate, OJB, etc)
• Example
• ltclass-descriptor class"jdbctutorial.Coffee"
  table"Coffee"gt
• ltfield-descriptor name"name" column"Name"
  jdbc-type"VARCHAR" length"100" /gt
• ltfield-descriptor name"price" column"Price"
  jdbc-type"FLOAT" length"100" /gt
• lt/class-descriptorgt

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Dominant Persistent-Application Development (3)

• Object Serialization
• Example
• ltcoffeegt
• ltnamegt Colombian lt/namegt
• ltpricegt 11.22 lt/pricegt
• lt/coffeegt

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Dominant Persistent-Application Development (4)
Problems

• Runtime overhead
• Impedance mismatch
• http//www.agiledata.org/essays/impedanceMismatch.
  html
• Explicitly write code to make objects persist

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Orthogonal Persistence

• Purpose to provide a single uniform
  computational model for all aspects of an
  application handling long lived data
• Principles
• Type Orthogonality all data objects should have
  a full range of persistence
• Persistence Independence A program's form
  should be independent of the longevity of the
  data it manipulates
• Persistence By Reachability reachability from a
  set of root objects should determine each
  object's lifetime.

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Orthogonal Persistence (2)

• Examples Orthogonal Persistence for Java (OPJ),
  Persistent Java (Pjama), PEVM
• Drawbacks
• don't treat persistence as a completely separate
  concern programmers have to follow certain steps
  in the application to retrieve existing or create
  new persistent objects.
• no adaptation of existing features such as
  security and concurrency)

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Persistence in Aspect-Oriented Programming

• AOP Approach Identify a set of cross-cutting
  concerns and allow to program them separately
  (outside of the application main logic)
• AOP Persistence Goals
• Modularize Persistence as an effective aspect,
  employing AOP techniques
• Reuse persistence aspects regardless of the
  application
• Develop programs that are unaware of their data's
  persistence nature.

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Persistence in Aspect-Oriented Programming (2)

• Examples Shortcomings
• AspectJ example persistence and application
  logic could be separate retrieval and deletion
  have to be explicit, storing and updating don't.
• PersAJ provides a model for persistence,
  doesn't separate application data and persistence
  code
• Jorg Kienzle Rachid Guerraoui case study on
  separating concurrency control and failure
  handling code in a distributed system only
  reviewed translations one facet of persistence

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Reflective Systems

• Definition a computational system's capability
  to reason about and act upon itself.
• Levels of Reflection
• Introspection access to system structure but not
  modification (Java, C, C)
• Structural Reflection dynamic altering of a
  system's structure (Python, Smalltalk)
• Computational Reflection dynamic customization
  of system structure and semantics

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The nitr0 reflective system

• Nonrestrictive computationally reflective system
• Language Neutral currently Python, ECMAScript,
  Java (subset)
• Developed in Python 2.2
• Generic Interpreter capable of interpreting any
  language based on a specification
• Language Specification Files express lexical
  and syntactic features using context free
  grammar.

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Nitr0 Sample Application

• Sample Program

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Nitr0 Sample Application (2)

• Things to note
• Java app statements to identify the application
  name and language
• The reflective app can access the author
  program's internal structure, and language
  semantics, dynamically adapting the application
  through computational reflection.
• Python app modifies 'firstName' and 'surname'
  attributes, adds a 'middleName' attribute.
• Could also modify the show method to display
  middle name

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The Nitr0 Persistence System

• Subsystems
• Interpreter contextual analysis and application
  execution. A subset of Java, eliminating
  primitives.
• Application offers representation of every
  running program (classes, methods, objects, etc).
  Independent of language (must be OO).
• Persistence allows dynamic and transparent
  customization of the app's reflective features
  (e.g. storage, indexing, and update policy)

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The Nitr0 Persistence System Interpreter

• Generates the program's Abstract Syntax Tree
  (AST) based on Java specification
• Interprets AST
• Performs decoration of AST, following the Visitor
  design pattern http//exciton.cs.oberlin.edu/javar
  esources/DesignPatterns/VisitorPattern.htm
• The programmer can obtain computational
  reflection my modifying the execution visitor's
  visit methods.

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The Nitr0 Persistence System Application

• Java runtime classes Classes (JClass), Fields
  (JField), constructors (JConstructor), methods
  constructors (JMethodGroup), references (JRef)
• Instance interface (methods to make an element
  persistent) makePersistent, makeTransient,
  store, restore, getID
• Unique ID Considerations JInstance returns IP
  address, process ID, user ID, active thread ID,
  millis since 01/01/1970

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The Nitr0 Persistence System Persistence

• Class Diagram

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The Nitr0 Persistence System Persistence (2)

• Notes
• Manager the module's Facade, implemented as a
  Singleton
• Storage and StoragePolicy abstract classes,
  partial implementation.

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The Nitr0 Persistence System Persistence -
Storage

• Storages a way of keeping persistent
  information
• 3 Types of storage
• SimpleStorage dictionary
• BSDStorage linear hash, B tree, variable
  length record storage
• DBMStorage a unix based DBMS library.

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The Nitr0 Persistence System Persistence -
Policies

• Policies how to update the objects into the
  selected storage
• 2 policies
• SimplePolicy whenever a persistence object is
  modified.
• TimedPolicy whenever a timer reaches a
  configurable number of seconds

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The Nitr0 Persistence System Persistence
Serialization

• Object serialization Python pickle module
  (Object -gtbyte array and vice versa)
• Swizzling Unswizzling converting persistence
  object Ids (PIDs) to references.
• Manager translates all persistent objects that
  the current one has references to.
• InstanceTable weak dictionary mapping PIDs to
  memory references, also a cache.
• InstanceTable uses weak references, garbage
  collected.

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Sample Bibliography application
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Conclusion

• Runtime Performance
• To be addressed in the future using JIT
  (Just-In-Time) compilation or adaptable native
  code generation for the generic interpreter
• Can be improved via a Python JIT compiler as all
  objects are converted to Python.
• Available at http//www.di.uniovi.es/reflection/la
  b