This blog post is based on an original article I wrote around 6 years ago. While the terms used to describe Service Oriented Architecture (SOA) have changed in recent years (to terms like "Cloud Computing" and "SaaS"), the basic tenets are still valid. Therefore, I am posting the original white paper here on Technoracle with a series of updates to the topics in an attempt to explore the concept of SOA within the context of
mobile application development in a subsequent series of blog posts. To be the first to read the new data, subscribe to this blog.
Service Oriented Architecture (SOA) and Specialized Messaging Patterns
1.0 Thesis
The widespread emergence of the Internet in the mid 1990s as a platform for electronic data
distribution and the advent of structured information have revolutionized our ability to deliver
information to any corner of the world. While the introduction of Extensible Markup Language
(XML) as a structured format was a major enabling factor, the promise offered by SOAP based
webservices triggered the discovery of architectural patterns that are now known as Service
Oriented Architecture (SOA). XML and the JavaScript Object Notation (JSON) format, enabled portable data but the architecture based on services was the real coup in the expansion of data sharing.
Service Oriented Architecture or SOA is an architectural paradigm and discipline that may be used to
build infrastructures enabling those with needs (consumers) and those with capabilities
(providers) to interact via services across disparate domains of technology and ownership.
Services act as the core facilitator of electronic data interchanges yet require additional mechanisms in order to function. Several new trends in the computer industry rely upon SOA as the
enabling foundation. These include the automation of Business Process Management (BPM),
composite applications (applications that aggregate multiple services to function), and the
multitude of new architecture and design patterns generally referred to as Web 2.0.
The latter, Web 2.0, is not defined as a static architecture. Web 2.0 can be generally characterized
as a common set of architecture and design patterns, which can be implemented in multiple
contexts. The list of common patterns includes the Mashup, Collaboration-Participation,
Software as a Service (SaaS), Semantic Tagging (folksonomy), and Rich User Experience (also
known as Rich Internet Application) patterns among others. These are augmented with themes
for software architects such as trusting your users and harnessing collective intelligence. Most
Web 2.0 architecture patterns rely on Service Oriented Architecture in order to function.
When designing Web 2.0 applications based on these patterns, architects often have highly
specialized requirements for moving data. Enterprise adoption of these patterns requires special
considerations for scalability, flexibility (in terms of multiple message exchange patterns), and
the ability to deliver these services to a multitude of disparate consumers. Architects often need
to expand data interchanges beyond simple request-response patterns and adopt more robust
message exchange patterns, triggered by multiple types of events. As a result, many specialized
platforms are evolving to meet these needs.
This white paper discusses specializations for advanced data exchanges within enterprise service
oriented environments and illustrates some of the common architectures of these new platforms.
2.0 An Introduction to Service Oriented Architecture
Service Oriented Architecture (SOA) is a paradigm for organizing and utilizing distributed
capabilities that may be under the control of different ownership domains and implemented
using various technology stacks. In general, entities (people and organizations) create capabilities to solve or support a solution for the problems they face in the course of their business. It is
natural to think of one person’s needs being met by capabilities offered by someone else; or, in
the world of distributed computing, one computer agent’s requirements being met by a computer
agent belonging to a different owner. The term owner here may be used to denote different
divisions of one business or perhaps unrelated entities in different countries.
There is not necessarily a one-to-one correlation between needs and capabilities; the granularity
of needs and capabilities vary from fundamental to complex, and any given need may require a
combination of numerous capabilities while any single capability may address more than one
need. One perceived value of SOA is that it provides a powerful framework for matching needs
and capabilities and for combining capabilities to address those needs by leveraging other
capabilities. One capability may be repurposed across a multitude of needs.
SOA is a “view” of architecture that focuses in on services as the action boundaries between the
needs and capabilities in a manner conducive to service discovery and repurposing.
2.1 Requirements for SOA
Figure 2-1 shows an example of an information system scenario that could benefit from a
migration to SOA. Within one organization, three separate business processes use the same
functionality, each encapsulating it within an application. In this scenario, the login function,
the ability to change the user name, and the ability to persist it are common tasks implemented
redundantly in all three processes. This is a suboptimal situation because the company has paid
to implement the same basic functionality three times.
Figure 2.1 – three business processes within one company duplicating functionality
Moreover, such scenarios are highly inefficient and introduce maintenance complexity within IT
infrastructures. For example, consider an implementation in which the state of a user is not
synchronized across all three processes. In this environment users might have to remember
multiple login username/password tokens and manage changes to their profiles in three separate
areas. Additionally, if a manager wanted to deny a user access to all three processes, it is likely
that three different procedures would be required (one for each of the applications). Corporate IT
workers managing such a system would be effectively tripling their work –and spending more for
software and hardware systems.
In a more efficient scenario, common tasks would be shared across all three processes. This can
be implemented by decoupling the functionality from each process or application and building a
standalone authentication and user management application that can be accessed as a service. In
such a scenario, the service itself can be repurposed across multiple processes and applications
and the company owning it only has to maintain the functionality in one central place. This
would be a simple example of Service Oriented Architecture in practice. The resultant IT
infrastructure would resemble Figure 2.2.
Figure 2.2 – three business processes repurposing one service for common tasks.
In figure 2.2, the shared user account tasks have been separated from each process and implemented in a way that enables other processes to call them as a service. This allows the shared
functions to be repurposed across all three processes. The common service bus is really a virtual
environment whereby services are made available to all potential consumers on a fabric. This is
typically referred to as an Enterprise Service Bus (ESB) and has a collection of specialized
subcomponents including naming and lookup directories, registry-repositories, and service
provider interfaces (for connecting capabilities and integrating systems) as well as a standardized collection of standards and protocols to make communications seamless across all connected devices. Advanced ESB vendors have tools that can aggregate services into complex
processes and workflows.
In the preceding example of SOA, the complications were relatively minor as the entire infrastructure existed within one domain. In reality, enterprise SOA is much more difficult because
services may be deployed across multiple domains of ownership. To make interactions possible,
mechanisms have to be present to convey semantics, declare and enforce policies and contracts,
the ability to use constraints for data passed in and out of the services as well as expressions for
the behavior models of services. The ability to understand both the structure and semantics of
data passing between service endpoints is essential for all parties involved.
While most SOA examples are typically shown as a request-response interaction pattern, more
robust exchanges are required. Additionally, modern service platforms also need the flexibility
to support these advanced message exchange patterns. Before discussing the platform and
reference architecture, this white paper will briefly delve into SOA in more detail.
2.3 A Reference Model for Service Oriented Architecture
As with any other architecture, Service Oriented Architecture can be expressed in a manner that
is decoupled from implementation. Software architects generally use standardized conventions
for capturing and sharing knowledge. This group of conventions is often referred to as an
Architecture Description Language (ADL). There are also several normalized artifacts used to
facilitate a shared understanding of the structure of a system, its major components, the
relationships between them, and their externally visible properties. This white paper will make
use of two special types of these artifacts – a
Reference Model and
Reference Architecture.
A
Reference Model is an abstract framework for understanding significant entities and relationships between them. It may be used for the further development of more concrete artifacts such
as architectures and blueprints. Reference models themselves do not contain a sufficient level of
detail sufficient to enable the direct implementation of a system. In the case of a reference model
for SOA, the Organization for the Advancement of Structured Information Systems (OASIS) has
a standard
Reference Model for SOA, shown in Figure 2.3, that is not directly tied to any
standards, technologies, or other concrete implementation details.
In order for SOA to be meet these challenges, services must have accompanying service
descriptions to convey the meaning and real world effects of invoking the service. These
descriptions must additionally convey both semantics and syntax for both humans and applications to use.
Each service has an interaction model, which is the externally visible aspects of invoking a
service. In this paper, this will be decomposed further to examine the data service aspects of
SOA.
Figure 2.3 – the core OASIS Reference Model for Service Oriented Architecture
Visibility and
Real World Effect are also key concepts for SOA. Visibility is the capacity for
those with needs and those with capabilities to be able to see and interact with each other. This is
typically implemented by using a common set of protocols, standards, and technologies across
service providers and service consumers. For consumers to determine if they can interact with a
specific service,
Service Descriptions provide declarations of aspects such as functions and
technical requirements, related constraints and policies, and mechanisms for access or response. In many real world situations, service descriptions may be technically described in instances of
Web Services Description Language (WSDL) documents.
The descriptions must be in a form (or can be transformed to a form) in which their syntax and
semantics are widely accessible and understandable. The
execution context is the set of specific
circumstances surrounding any given interaction with a service and may affect how the service
is invoked.
Since SOA permits service providers and consumers to interact, it also provides a decision point
for any
policies and contracts that may be in force. The purpose of using a capability is to
realize one or more real world effects. At its core, an interaction is “an act” as opposed to “an
4object” and the result of an interaction is an effect (or a set/series of effects). Real world effects
are, then, couched in terms of changes to this shared state. This may specifically mutate the
shared state of data in multiple places within an enterprise and beyond.
The concept of
policy also must be applicable to data represented as documents and policies must
persist to protect this data far beyond enterprise walls. This requirement is a logical evolution of
the “locked file cabinet” model which has failed many IT organizations in recent years. Policies
must be able to persist with the data that is involved with services, wherever the data persists.
A contract is formed when at least one other party to a service oriented interaction adheres to the
policies of another. Service contracts may be either short lived or long lived.
2.3 Decomposing the Interaction Model
Whereas visibility introduces the possibilities for matching needs to capabilities (and vice versa),
interaction is the act of actually using a capability via the service. Typically mediated by the
exchange of messages, an interaction proceeds through a series of information exchanges and
invoked actions. There are many facets of interaction; but they are all grounded in a particular
execution context – the set of technical and business elements that form a path between those
with needs and those with capabilities. Architects building Rich Internet Applications (RIAs),
are faced with special considerations when designing their systems from this perspective. The
concept of “Mashups” surrounds a model whereby a single client RIA may actually provide a
view composed by binding data from multiple sources persisting in multiple domains across
many tiers.
Figure 2.4 – a decomposition of the Interaction Model (courtesy of OASIS Reference Model for SOA)
As depicted in Figure 2.4, the interaction model can be further decomposed into a data model
and behavior model. The data model is present in all service instances. Even if the value is
“null”, the service is still deemed to have a data model. The data models are strongly linked to the
behavior models. For example, in a Request-Response behavior model, the corresponding data
model would have two components – the input (service Request) data model and the output
(service Response) data model. Data models may be further specialized to match the behavior
model if it is other than “Request-Response”.
The behavior model is decomposable into the action model and the process model. The sequence
of messages flowing into and out of the service is captured in the action model while the service’s
5processing of those signals is captured in the processing model. The processing model is
potentially confusing as some aspects of it may remain invisible to external entities and its inner
working known only to the service provider.
3.0 A Reference Architecture for Service Oriented Architecture
A reference architecture is a more concrete artifact used by architects. Unlike the reference
model, it can introduce additional details and concepts to provide a more complete picture for
those who may implement a particular class. Reference architectures declare details that would
be in all instances of a certain class, much like an abstract constructor class in programming.
Each subsequent architecture designed from the reference architecture would be specialized for a
specific set of requirements. Reference architectures often introduce concepts such as cardinality, structure, infrastructure, and other types of binary relationship details. Accordingly,
reference models do not have service providers and consumers. If they did, then a reference
model would have infrastructure (between the two concrete entities) and it would not longer be a
model.
The reference model and the reference architecture are intended to be part of a set of guiding
artifacts that are used with patterns. Architects can use these artifacts in conjunction with
others to compose their own SOA. The relationships are depicted in Figure 3.1.
Figure 3.1 – The architectural framework for SOA (Courtesy of OASIS).
The concepts and relationships defined by the reference model are intended to be the basis for
describing reference architectures that will define more specific categories of SOA designs.
Specifically, these specialized architectures will enable solution patterns to solve particular
problems. Concrete architectures may be developed based upon a combination of reference
architectures, architectural patterns, and additional requirements, including those imposed by
technology environments. Architecture is not done in isolation; it must account for the goals,
motivation, and requirements that define the actual problems being addressed. While reference
architectures can form the basis of classes of solutions, concrete architectures will define specific
solution approaches.
Architects and developers also need to bind their own SOA to concrete standards technologies
and protocols at some point. These are typically part of the requirements process. For example,
when building a highly efficient client side Mashup application, a developer might opt for the
ActionScript Messaging Format (AMF) to provide the most efficient communication between
remote services and the client .
Neutrality
The reference architecture shown in Figure 3.2 is not tied to any specific technologies, standards,
or protocols. In fact, it would be equally applicable to a .NET or J2EE environment and can be
used with either the Web Service family of technologies, plain old XML-RPC (XML – Remote
Procedure Call), or a proprietary set of standards. This reference architecture allows developers
to make decisions and adopt technologies that are best suited to their specific requirements.
Figure 3.2 – A generic SOA Reference Architecture for implementing core Web 2.0 design patterns
(Courtesy of O’Reilly Media)
3.1 Service Tier
The server side component of the reference architecture has a number of commonly used
components. The Service Provider Interface is the main integration point whereby service
providers connect to capabilities that exist in internal systems in order to expose them as
services. These internal applications typically reside in a resource tier, a virtual collection of
capabilities that become exposed as services so consumers can access their functionality. Service
providers may integrate such capabilities using numerous mechanisms, including using other
services. In most cases, an enterprise will use the Application Programmatic Interface (API) of
the system as provided by the application vendor.
The
Service Invocation Layer is where services are invoked. A service may be invoked when an
external messages being received or, alternatively, it can be invoked by an internal system or by a
non-message based event (such as a time out). It is essential to understand that services may be
invoked via messages from multiple sets of standards and protocols working together. Common
examples of external service interface endpoints include:
• Asynchronous JavaScript and XML (AJAX),
• Simple Object Access Protocol (SOAP),
• XML Remote Procedure Call (XML-RPC),
• a watched folder being polled for content,
• an email endpoint, and
• other
REST(viii) style endpoints including plain old HTTP and HTTP/S.
Services may also be invoked by local consumers including environments like J2EE and language
specific interfaces (for example - Plain Old Java Objects or POJO’s).
Each service invocation is often handed to a new instance of a service container. The service
container is responsible for handling the service invocation request for its entire lifecycle, until
either it reaches a successful conclusion or failed end state. Regardless of its ultimate end state,
the service container may also delegate responsibilities for certain aspects of the service’s
runtime to other services for common tasks. These tasks typically include logging functions,
archiving, security, and authentication, among others.
To facilitate orchestration and aggregation of services into processes and composite applications,
a registry-repository is often used. During the process design phase, the registry-repository
provides a single view of all services and related artifacts. The repository provides a persistence
mechanism for artifacts during the runtime of processes and workflows. If multiple system
actors use and interact with a form, the repository can persist it while allowing access to
privileged individuals.
Design, development and governance tools are also commonly used by humans to deploy,
monitor, and aggregate multiple services into more complex processes and applications.
3.2 Client Tier
While much attention has been focused on the server side aspects of SOA, less has been written
about the new breed of clients evolving for consuming services. The clients have evolved to
embrace many common architecture and design patterns discussed in greater detail in the next
section. A highly visible example of this is the ability of most modern browsers to subscribe to
RSS feeds.
Figure 3.3 – client application architecture
As depicted in Figure 3.3, clients must have far more robust communications services than a
decade ago. In fact, any communication standards, protocols and technologies (such as SOAP),
ActionScript Messaging Format, or XML-RPC) have to be implemented on both sides to
facilitate proper communications. Client side communications buses also need to monitor the
state of communications including potentially both synchronous and asynchronous exchange
patterns.
The main controller of each client application must be capable of launching various runtime
environments. This is typically done via launching one or more virtual machines that can
interpret scripting languages or consume bytecode as in Adobe Flash. The architecture for these
virtual machines varies greatly depending upon the language used. Some compile an intermediate level bytecode just in time to run a program while others must be launched and make.
Representational State Transfer (REST) is an important component of Roy Fielding’s Dissertation
Architectural Styles and the Design of Network-based Software Architectures -
http://www.ics.uci.edu/~fielding/pubs/dissertation/top.htm
The Simple Object Access Protocol is a W3C Recommendation - http://www.w3.org/TR/soap/
multiple passes over a script (usually once to check it for errors, another time to run the script,
and a concurrent iteration to collect garbage and free up memory as it becomes possible to
reallocate.
Most modern clients have some form of
data persistence and
state management. Data persistence is not always done using a traditional database. Increasingly, NoSQL Graph Databases are being used to store nodes which offer greater flexibility and scalability, as well as increased performance. This usually
works in conjunction with the clients’ communications services to allow the controller to use
cached resources rather than attempting to synchronize states if communications are down.
Additionally, rendering and media functionality specific to one or more languages is used to
ensure the view of the application is built in accordance with the intentions of the application
developer.
The security models used by different clients also vary somewhat. The usual tenets are to prevent
unauthorized and undetected manipulation of local resources. In distributed computing
architectures, identity (knowing who and what) is a major problem that requires a complex
architecture to address. Each client side application must be architected in accordance with the
acceptable level of risk based on the user requirements.
3.3 Architectural Conventions spanning multiple tiers
While examining the client and service tiers of the reference architecture, developers will note
some commonalities. Architects need to employ common models for determining what
constitutes an object, what constitutes an event, how an event gets noticed or captured, what
constitutes a change in state, and more. As a result, architecture must take note of several
common architectural models over all tiers of modern SOAs.
First and foremost, the core axioms of service oriented architecture should be observed. Services
themselves should be treated as subservient to the higher level system or systems that use them.
If you are deploying services to be part of an automated process management system, the
services themselves should not know (or care) what they are being used for.
Services that are designed otherwise are architecturally inelegant for a number of reasons.
First, if services were required to know the state of the overall process, state misalignment would
likely result if two services had differing states for even a fraction of a second. In such instances,
errors might be thrown when this is detected or worse, developers would have to rely on using a
series of synchronous calls to services rather than forking a process into asynchronous calls. As
depicted in Figure 3.4, services should remain agnostic to what they are used for. The state of a
process or other application using services should be kept within the higher layer of logic that
uses consumers to invoke the services.
Figure 3.4 – services within overall architecture
Second, if the overall process stalled or failed for some reason, each service used would have to be notified and rolled back to a previous state. Having services maintain or store the overall
state of a process that uses more than one service is an anti-pattern of SOA and should be
avoided.
Another core architectural convention is to keep the service consumers agnostic to how the
services are delivering their functionality. This results in a clean decoupling of components,
another architecturally elegant feature of modern service oriented systems. Having dependencies on knowing the internal working of the services functionality is another anti-pattern of SOA
and should also be avoided.
Service composition, the act of building an application out of multiple services, is likewise an
anti-pattern of SOA, if composition is defined as per Unified Modeling Language (UML) 2.0.
Composition is depicted as a “has a” relationship and the whole is composed of the parts. The
correct terminology should be service aggregation. Aggregation is a “uses a” type of relationship.
The differences are quite subtle but nevertheless important to grasp. In composition relationships, the life cycles of parts are tied to the lifecycle of the whole and when the whole no longer
exists, the parts no longer exist either. In aggregation, the parts exist independent of the whole
and can go on living after the entity that uses them no longer exists. This terminology is
common within both OOPSLA and UML. Regardless, the term “service composition” has been
misused widely within the computer industry and will likely prevail as a norm. Architects and
developers should pay close attention to the types of binary relationships between components in
loosely coupled, distributed systems and bear these definitions in mind.
3.4 Events
Architects and developers using the reference architecture within this paper should also consider
the event architecture. Events often must be detected and acted upon. Each specific programming language has a form of event architecture for detection, dispatching messages, and
capturing and linking behaviors to events. The main challenge presented in distributed, service
oriented systems is that the event model must traverse multiple environments and possibly span
multiple domains. Detecting an event in one domain, dispatching a message to a remote system
and linking the event to an action in a virtual machine running on the remote system presents
multiple challenges. Architects and developers must often bridge disparate systems. Having a
common model used by all systems makes the traversal of systems much easier for developers
and architects alike.
3.5 Objects
In much the same way they treat events, each disparate environment in a distributed service
oriented environment might have a distinct notion of what constitutes an object. Relying on
programming environments and languages that are aligned conceptually with respect to objects
(that is, “object-oriented”) makes the work of architects and developers much easier. Languages
such as JavaScript (specifically JavaScript Object Notation or JSON), Java, ActionScript, and
others have alignment on object concepts. (Note: ECMA’s ActionScript 3.0 is much more
object-oriented than previous incarnations and is strongly tied to Java). When a developer must
implement a pattern where an object’s state must be tracked in a remote location and action
taken upon a state change on the object, a common model for object and encapsulation is
important.
3.6 Architectural Patterns
As noted in the reference architecture in Figure 3.1, architecture and design patterns are an
important aspect of any architecture.
Patterns are recurring solutions to recurring problems. A pattern is composed of a problem, the
context in which the problem occurs, and the solution to resolve this problem. The focus of a
documented software architecture pattern is to illustrate a model to capture the structural
organization of a system, relate that to its requirements and highlight the key relationships
between entities within the system.
Patterns can be classified into three broad categories:
Architecture patterns
Architecture patterns are high level patterns on
how systems are laid out and how large systems
are divided. These typically account for the
major components, their externally visible
properties, the major functionality of each
component, and the relationships between
them.
Design patterns
Design patterns provide a scheme for refining
the subsystems or components of a software
system, and the relationships between them.
They describe a commonly recurring structure of
communicating components that solves a
general design problem within a specific
context.
Idioms
Idioms are the lowest-level patterns and may be
specific to a programming language. An idiom
guides the implementation aspects of
components and the relationships between
them, using features specific to a given language
or environment.
The modern day concept of patterns evolved from work by Christopher Alexander, the primary
author of a book called “A Pattern Language” (xiii) which had a great influence on object-oriented
programming. The basic concept of the book was a realization that patterns are the same when
architecting a city, a block, a house and a room. Each of these entities employs similar patterns.
The concepts of patterns in software architecture have been widely adopted since being modified
by the infamous Gang of Four and are now an accepted part of the engineering trade. There are also many industry standards and pseudo standards for architectural patterns meta models including the
MacKenzie-Nickull Meta-model for Architectural Patterns, which is widely used today.
4.0 Data and Message Exchange Patterns for Enterprise SOA
The most basic message exchange pattern is a common Request-Response where the parties can
simply communicate with each other. This is the basic building block of most SOA interactions
and is depicted below.
4.1 Request-Response
Request-Response is a pattern in which the service consumer uses configured client software to
issue an invocation request to a service provided by the service provider. The request results in
an optional response, as shown in Figure 4-1.
Figure 4-1. SOA Request-Response pattern
4.2 Request-Response via Service Registry (or Directory)
An optional service registry can be used within the architecture to help the client automatically
configure certain aspects of its service client. The service provider pushes changes regarding the
service’s details to the registry to which the consumer has subscribed. When the changes are
made, the service consumer is notified of these changes and can configure its service client to
talk to the service. This is represented conceptually in Figure 4-2.
Figure 4-2. SOA Request-Response pattern with a service registry
4.3 Subscribe-Push
A third pattern for interaction is called Subscribe-Push, shown in Figure 4-3. In this pattern, one
or more clients register subscriptions with a service to receive messages based on some criteria.
Regardless of the criteria, the externally visible pattern remains the same.
Subscriptions may remain in effect over long periods before being canceled or revoked. A
subscription may, in some cases, also register another service endpoint to receive notifications.
For example, an emergency management system may notify all fire stations in the event of a
major earthquake using a common language such as the OASIS Common Alerting Protocol
(CAP)(x v i).
Figure 4-3. SOA Subscribe-Push pattern
Note that this pattern can be triggered by a multitude of events. In figure 4-3, an auditable event
is triggering a message being sent to a subscribed client. The trigger could be a service consumer’s action, a timeout action, or a number of other actions that are not listed in the example
above. Each of these represents a specialization of the
Subscribe-Push pattern. This particular pattern is becoming dominent in mobile application development that uses SOA principles. Often a user has an application and the application pushes a notification to the user when a new piece of data or event is available that the user has expressed an interest in.
4.4 Probe and Match
A pattern used for discovery of services is the Probe and Match pattern. In this variation, shown
in Figure 4-4, a single client may multicast or broadcast a message to several endpoints on a
single fabric, prompting them to respond based on certain criteria. For example, this pattern
may be used to determine whether large numbers of servers on a server farm are capable of
handling more traffic by checking if they are scaled at less than 50% capacity. This variation of
the SOA message exchange pattern may also be used to locate specific services. There are caveats
with using such a pattern, as it may become bandwidth-intensive if used often. Utilizing a
registry or another centralized metadata facility may be a better option because the registry
interaction does not require sending the prob e() messages to all endpoints to find one. By
convention, they allow the query to locate the endpoint using a filter query or other search
algorithm.
Figure 4-4. SOA Probe and Match pattern
In the
Probe and Match scenario in Figure 4-4, the service client probes three services, yet only
the middle one returns an associated match() message. A hybrid approach could use the best of
both the registry and the probe and match models for locating service endpoints. In the future,
registry software could implement a probe interface to allow service location without requiring
wire transactions going to all endpoints and the searching mechanism could probe multiple
registries at the same time.
4.5 Patterns for RIAs
Creating Rich Internet Applications (RIAs) requires a level of data management that goes
beyond the traditional Request-Response model. Providing a richer, more expressive experience
often requires more data-intensive interaction and introduces new challenges in managing data
between the client and server tiers.
Data synchronization is a key concept and requires states to be shared among multiple machines.
These are usually the clients who have subscribed to the state of an object somewhere within the
tier of a distributed system as depicted in Figure 4.5.
Figure 4.5 – data synchronization across multiple clients (courtesy James Ward).
4.6 Data paging
Some services automatically facilitate the paging of large data sets, enabling developers to focus
on core application business logic instead of worrying about basic data management infrastructure.
Modern service oriented clients and server infrastructures automatically handle temporary
disconnects, ensuring reliable delivery of data to and from the client application. Data paging is built into the model used by Neo4J and Mongo.db. Being graph databases, you can start by examining a specific node of data, then request incremental pieces of data based on your use case for the data.
4.7 Data push
Some services offer data-push capability, enabling data to automatically be pushed to the client
application without polling (contrast this pattern to the “Subscribe-Push pattern listed above).
This can be done via intuitive or inference methods to ensure data is provided as required. This
highly scalable capability can push data to thousands of concurrent users, providing up-to-the-second views of critical data, such as stock trader applications, live resource monitoring, shop
floor automation, and more.
Data push can be further specialized into broadcast, unicast, multicast, and several other
specializations of the basic pattern.
5.0 A Final Word
This white paper has been prepared to share ideas about data interaction patterns within SOA
and to illustrate some common concepts with a service oriented environment. It is based on
input provided by a number of people from different companies and is not considered the work
of any one company. It is free to share, use, quote, and post wherever and however you want.
Service Oriented Architecture will likely remain the mainstay of technology platforms for the
foreseeable future. It is our hope that the companies who have contributed to this will continue
to write more on specialized patterns of SOA.
This work is licensed under a Creative Commons Attribution 3.0 Unported License. You may
redistribute and quote from parts of this article however attribution is expected. There is no
need to seek explicit permission to reuse part of this paper or quote from it.
6.0 References
i. The Extensible Markup Language (XML) is a W3C Recommendation - http://www.w3.org/XML/ and JSON, described at http://www.json.org.
ii. Service Oriented Architecture is an architectural paradigm expressed as a Reference Model by OASIS at
http://www.oasis-open.org/committees/tc_home.php?wg_abbrev=soa-rm
iii. Web 2.0 is defined as a set of Design Patterns in the O’Reilly book Web 2.0 Design Patterns -
http://www.amazon.com/Web-2-0-Design-Patterns-entrepreneurs/dp/0596514433
(iv) AMF - http://osflash.org/documentation/amf
(v) .NET is a trademark and technology from Microsoft -
http://msdn2.microsoft.com/en-us/netframework/default.aspx
(vi.) Java 2 Enterprise Edition is a trademark of Sun Microsystems
(vii.) Asynchronous JavaScript And XML is described on Wikipedia in more details at
http://en.wikipedia.org/wiki/Ajax_(programming)
x. Unified Modeling Language is owned by the Object Management Group (OMG) and Described here -
http://www.omg.org/technology/documents/formal/uml.htm
xi. OOPSLA is an annual conference around Object Oriented Programming, Systems Languages and
Applications - http://www.oopsla.org
xii. JavaScript Object Notation (JSON) is RFC 4627 available at http://tools.ietf.org/html/rfc4627
10The modern day concept of patterns evolved from work by Christopher Alexander, the primary
author of a book called “A Pattern Language”
xiii. http://www.amazon.com/Pattern-Language-Buildings-Construction-Environmental/dp/0195019199
xiv. See “Design Patterns: Element of Reusable Object-Oriented Software” by Erich Gamma, Richard Helm,
Ralph Johnson, John Vlissides
xv. The Organization for the Advancement of Structured Information Systems (OASIS) at
http://www.oasis-open.org
xvi. OASIS CAP is a product of the OASIS Emergency Services Technical Committee -
http://www.oasis-open.org/committees/tc_home.php?wg_abbrev=emergency
