A survey of Adaptive Middleware

• Middleware is connectivity software that encapsulates a set of services residing above the network operating system layer

Taxonomy of Middleware (Emmerich)

• Transactional
• Message-oriented
• Object-oriented
• Procedural
  

Types of Object-oriented programming models

1. CORBA
2. RMI
3. DCOM

Supporting paradigms for adaptation include:

• Computational Reflection:
• Component-based design
• Aspect oriented programming
• Software design patterns

Computation Reflection:

• the ability of a program to reason about and alter its own behavior

- Reflective Systems (Base-level objects): functinal aspects of the syste
- Self representation (meta-level objects): implementation aspects of the system
- MOP (meta object protocol): an interface to inspect and modify the base level objects

*Behavioral reflection in middleware: modify the behavior of a program by generating code at the self representation level and injecting it to the base level

Component-based design:

components are self-contained
large scal reuse of components for COTS
late composition and late binding are supported

Examples: DCOM, EJB, Corba Component Model (CCM)

Aspect Oriented Programming

* intervened cross-cutting concerns in complex programs
* disentangling the cross cutting concerns leads to simpler software development, maintenance, and evolution

Software Design Patterns
The goal of software design patterns is to create a common vocabulary for communicating insight and experience about recurring problems and their known refined solutions

4.Taxonomy of Adaptive Middleware

1. Schmidt's classification:

• host infra-structure: higher level API than the OS, NP, generic services
  
• distributioin: RMI, solves heterogenity of NPs and OSs
  
• common services: common functionalities such as fault tolerance, security, load balancing, event propagaion, logging, persistence, ...
  
• domain services: specific class of distributed applications
  
• application

2. Adaptation type

• Customizable -> static compile time
• Configurable -> static, before starting the system
• Tunable-> dynamic, before using the system. The middleware core remains intact
  
• Mutable-> dynamic during run-time
  

3. Application Domain

• QoS
• Dependable
• Embedded

QoS-oriented middleware is classified to the following categories:

• Real-time middleware
• Stream-oriented middleware
• Reflection-oriented middleware
• Aspect-oriented middleware

Reflection

• Structural: ability of a system to inspect and modify its internal architecture (architecture and interface)
  
• Behavioral: ability of a system to inspect and modify its computation(interception and resources)

Existing Aproaches and middleware system

Ace

• Layer: Host-infrastructure
  
• Pattern: Service configurator pattern
• Type: repeatedly tunnable middleware

ACE ORB (TAO)

• Pattern: Strategy Design Pattern
• Type: configurable middleware - repeatedly tunnable middleware
  
• Layer: distribution

Component-Integrated ACE ORB (CIAO)

• Pattern: component based design
  
• Layer: distribution
• Type: configurable middleware

DynamicTAO

• Pattern: Service configurator
• Type: repeatedly tunnable middleware

OpenORB

• component based design

Squirrel

• Layer: distribution
• Type: Tunnable and Mutable - (not repeatedly tunnable tho! tunning happens just once)

MetaSockets

• Type: repeatedly tunnable middleware
• Layer: Host infra-structure layer

OpenCorba

• Type: repeatedly tunnable middleware
• Layer: Distribution

FlexiNet

• both fine-grained (per interface) and coarse grained adaptation