作者：DAVID CHAPPELL
SCA FUNDAMENTALS
COMPONENTS AND COMPOSITES(由component組成)
There must be a way to define components and to describe how they interact.
an SCA application can be accessed by software from the non-SCA world（如Servlet，JSP Service Data Objects）
An SCA composite is typically described in an associated configuration file
This file uses an XML-based format called the Service Component Definition Language (SCDL）
DOMAINS
an SCA runtime can allow a third party to create a container that plugs into that runtime to support a particular technology, such as BPEL.
A domain can contain one or more composites, each of which has components implemented in one or more processes running on one or more machines.
UNDERSTANDING COMPONENTS
a component is an instance of an implementation that has been appropriately configured.
The configuration, expressed in SCDL, defines how that component interacts with the outside world.
every component relies on a common set of abstractions, including services, references, properties, and bindings, to specify its interactions with the world outside itself。
SERVICES, REFERENCES, AND PROPERTIES
a component implemented in BPEL would likely describe its services using the Web Services Description Language (WSDL).
Each property contains a value that can be read by that component from the SCDL configuration file when it’s instantiated.
BINDINGS
A binding specifies exactly how communication should be done between an SCA component and something else.
in the same domain,the runtime determines what bindings to use
To communicate outside its domain，must specify one or more bindings for this communication
Each binding defines a particular protocol that can be used to communicate with this service or reference
AN EXAMPLE: SCA’S JAVA COMPONENT MODEL
the Spring Framework provides explicit support for services, references, and properties, and so mapping these into SCA’s similar concepts is straightforward
new component model for Java：Java component model.One important motivation was the need for an explicitly service-oriented approach.
Defining Services
SCA’s Java programming model relies on annotations rather than API calls。
Defining References
In SCA’s Java programming model, references are specified using the @Reference annotation
服務的具體實現是有runtime自動找的（dependency injection或inversion of control）
Defining Properties
@Property protected String region;
it indicates that a value should be read from the SCDL configuration file of the composite to which this component belongs
the goal is : providing a way to configure a component via values that are read at runtime.
Defining Bindings
bindings determine how a component communicates with the world outside its domain
SCA allows each remotable service and each reference to specify the protocols it supports using bindings
SOAP over HTTP-〉Web Services binding，queued messaging protocol-〉JMS binding
Internet Inter-ORB Protocol (IIOP)-〉EJB session bean binding
binary protocol->SCA binding
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
Defining Other Aspects of a Component
@OneWay 非阻塞 ，@Scope 控制構件生命期，@Callback 允許定義回調接口
CONFIGURING A COMPONENT
every SCA component relies on information in the SCDL configuration file
UNDERSTANDING COMPOSITES
If components are the atoms of SCA, then composites are the molecules.
WIRES AND PROMOTION
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.
a composite can also expose one or more services。These services are actually implemented by components within the composite。提升後的構件服務，能夠作爲composite外界可見的接口。
CONFIGURING A COMPOSITE
All of the relationships in a composite are expressed in the SCDL configuration file.
USING POLICY
let developers use policies to specify their intent—what they want to happen—then let something else figure out how to achieve this intent
Interaction policies:Modify how a component interacts with other components.指定安全，可靠性需求。一般是被應用到binding上的。
Implementation policies:Modify how a component behaves locally.指定構件必須運行在一個事務中。
policies can be declared in a SCDL configuration file
For Java SCA components, policies can also be defined using annotations assigned to interfaces,methods. @Confidentiality @Authentication
To define policies, SCA posits a policy administrator role in each domain.This administrator specifies what a particular policy means in her domain by specifying intents and policySets
PUTTING THE PIECES TOGETHER: ILLUSTRATING AN SCA APPLICATION
IMPLEMENTING SCA
The SCA specs say essentially nothing about how this technology should be implemented.
SCA runtime might provide a number of containers, one for each component technology it supports(Java component model, BPEL, and Spring).The SCA runtime provides all bindings,The runtime also defines all communication between components, regardless of how those components are implemented.
It’s also possible for the creator of an SCA runtime to use OSGi. OSGi defines how to package code into bundles, and while it’s also Java-only, some creators of SCA runtimes might allow extensions such as a new binding to be added as an OSGi bundle.
SOA Tools Platform Project, one of its goals is to create SCA-based development tools for Eclipse.
CONCLUSION