Wednesday, September 23, 2009

First Shot at Service Component Architecture

First Shot at Service Component Architecture

Looking at the recent buzzword or key technologies in IT Industry - web services, Services Oriented Architecture, Software as a Service , Platform as a Service etc.The one thing is the Services is one of the common factor in those buzzwords and going forward IT industry can be renamed as Services Industry.
I am trying to write on Service Component Architecture specifications. heard about this specification from my SOA Knowledge Bank colleague namedd Ravi Venkidapathy. The first impression of this specification - SCA attempts to simplify building of Service Oriented Architectures by focusing on: Composition, Assembly and Securitized deployment aspects of SOA. But whether i agree or disagree to the definition defined by my colleague will be presently in latter half of this blog.

High Level View of Specifications

The SCA specifications define how to create components and how to combine those components into complete applications. Well these specification works on primary concept of breaking down of application/process in components and how these components work together. SCA brought using terms like Service, Component, Composite, Domain, Contribute and new configuration XML file called SCDL(Service Component Definition Language). Diagram 1 tells about this relationship

But within SCA application, components can be written in language independent way and even application can be accessed by non-SCA world. Although SCA application is defining the components and is not primarily defined for UI and Data services. They provide good integration APIS to integrate with UI and data tiers.
Defining Components as Remotable and local are reminding the feature of remote and local EJBs to avoid network latency.This information goes in Component but not in SCDL. THIS CAN BE CONSIDERED AS ENHANCEMENT TO SPECIFICATIONS as Remotable/Local component configurations should be set in SCDL where composites are being configured.
To allow cross-vendor interoperability, all remotable interfaces must be expressible in WSDL, which means they can potentially be accessed via SOAP.

A composite can also expose one or more services, where these services are implemented by components within composite - Known as Promotion of Services.

Shot at SCDL

SCDL takes its name analogy from WSDL and contents from Spring as Bean Configuration.SCDL defines references for other services and properties defined for component. Springs' dependency injection is used to set the values into component using constructore-level dependency injection, setter-method injection & property-level injection.SCDL alos defines the definition of composites, components(their names,implementation classes), bindings for services defined.Bindings can be assigned to services and to references, and each one specifies a particular protocol.
Bindings separate how a component communicates from what it does, they let the component’s business logic be largely divorced from the details of communication.A single service or reference can have multiple bindings, allowing different remote software to communicate with it in different ways, so separating these independent concerns can make life simpler for application designers and developers.
Instead, the bindings a service or reference relies on are either chosen by the runtime, for intra-domain communication, or set explicitly in a component’s SCDL configuration file[with URI information].

Domain-Runtime depenency

Runtime is presently used for wiring of the components(when have references defined) by creating the Proxy object for the component's references.It’s up to the SCA runtime to generate WSDL interfaces from the Java interfaces[for servcies defined with WS binding], fix up the service to be callable via SOAP, and do everything else required to let this component communicate via Web services.
Concept of wiring is similar to the one brought by Spring - "A wire is an abstract representation of the relationship between a reference and some service that meets the needs of that reference. Exactly what kind of communication a wire provides can vary:it depends on the specific runtime that’s used, what bindings are specified (if any), and other things."

Even though an SCA composite runs in a single-vendor environment, it can still communicate with applications outside its own domain.All of the communication between components and composites within each domain is done in a vendor-specific way.An SCA application communicating with another SCA application in a different domain sees that application just like a non-SCA application; its use of SCA isn’t visible outside its domain.

Every SCA runtime also provides an SCA binding.Instead, the SCA binding is only used when a service and its client are both running in the same domain.

SCA policy association framework that allows policies and policy subjects specified using WS-Policy and WS-PolicyAttachment, as well as with other policy languages(like WS-ReliableMessaging), to be associated with SCA components. These policies can be interaction(affects the interaction at runtime) or implementation(how components behave at runtime).Policyset can be attached to Component or Composite in SCDL and are enforced by SCA runtime.

Diagram 2 tells about SCA runtime relationship with other containers.

Bottom Line -

1.A primary goal of SCA composites is to provide a consistent way to assemble Spring,BPEL,Java different technology component into coherent applications and so make this diversity more manageable.
Although Sping,JEE(EJB,Jax-WS) has been providing lot of these features extensively & widely used by industry and also very less Runtimes(Tuscany, Fabric3) available for SCA makes a long road going ahead for SCA.
Even though the SCA is used as one of the frameworks to build SOA, but that's not the intent of SCA(from specifications).And i don't agree with ravi's
view on same.
2.If component Implementation class is changed from Java to BPEL, then with small changes in the SCDL configuration file the runtime will behave differently to execute this BPEL component(without changing the component definition).
3.Integration with Policy framework for components in SCDL.
3.Presently Can't create a component that spans multiple domains, and hence also exists vendor lock-in for SCA runtime for component.
4.SCA runtime are trying to work on adding extensions to runtime using OSGI.

So it looks like SCA specifications has been taken lot of points from existing technologies(Spring, EJB, JAX-WS) & applying configurable policies & bindings.

So wait for next post for more details on SCA.

--Amit G Piplani--

Reference -
SCA v/s JBI Article by Ravi Venkidapathy
Introducing SCA by David Chappell

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