A number of new needs have emerged with the advent of SOA. First of all, there was no standard way for an application to construct and deliver a service call. Secondly, there was no standard way to ensure the service would be delivered. Thirdly, it was not clear how this SOA environment could be managed and operated effectively. Fourthly . . . well, you get the idea; the list goes on and on.
SOA demands the existence of an enabling infrastructure layer known as middleware. Middleware provides all necessary services, independent of the underlying technical infrastructure. To satisfy this need, vendors began to define SOA architectures around a relatively abstract concept: the Enterprise Service Bus, or ESB. Now, there has never been disagreement about the need to have a foundational layer to support common SOA functions—an enterprise bus of sorts. The problem is that each vendor took it upon himself to define the specific capabilities and mechanisms of their proprietary ESB, oftentimes by repackaging preexisting products and rebranding them to better fit their sales strategies.
As a result, depending on the vendor, the concept of Enterprise Service Bus encompasses an amalgamation of integration and transformation technologies that enable the cooperative work of any number of environments: Service Location, Service Invocation, Service Routing, Security, Mapping, Asynchronous and Event Driven Messaging, Service Orchestration, Testing Tools, Pattern Libraries, Monitoring and Management, etc. Unfortunately, when viewed as an all-or-nothing proposition, ESB’s broad and fuzzy scope tends to make vendor offerings somewhat complex and potentially expensive.
The term ESB is now so generic and undefined that you should be careful not to get entrapped into buying a cornucopia of vendor products that are not going to be needed for your specific SOA environment. ESBs resemble more a Swiss army knife, with its many accessories, of which only a few will ever be used. Don’t be deceived; vendors will naturally try to sell you the complete superhighway, including rest stops, gas stations and the paint for the road signs, when all you really need is a quaint country road. You can be choosy and build your base SOA foundation gradually. Because of this, I am willfully avoiding use of the term “Enterprise Service Bus”, preferring instead to use the more neutral term, “SOA Fabric.”
Of all the bells and whistles provided by ESB vendors (data transformation, dynamic service location, etc.), the one key function the SOA Fabric should deliver is ensuring that the services and service delivery mechanisms are abstracted from the SOA clients.
A salient feature that vendors tell us ESBs are good for is their ability to integrate heterogeneous environments. However, if you think about it, since you are going through the process of transforming the technology in your company (the topic of this writings after all!), you should really strive to introduce a standard protocol and eliminate as many of legacy protocols as you can.
Ironically, a holistic transformation program should have the goal of deploying the most homogeneous SOA environment possible; thus obviating the need for most of the much touted ESB’s transformation and mapping functions. In a new system, SOA can be based upon canonical formats and common protocols; thus minimizing the need for data and service format conversion. This goal is most feasible when applied to the message flows occurring in you internal ecosystem.
Now, you may still need some of those conversion functions for several other reasons, migration and integration with external systems being the most obvious cases. If the migration will be gradual, and therefore requires the interplay of new services with legacy services, go ahead and enable some of the protocol conversion features provided by ESBs. The question would then be how important this feature is to you, and whether you wouldn’t be better off following a non-ESB integration mechanism in the interim. At least, knowing you will be using this particular ESB function only for migration purposes, you can try to negotiate a more generous license with the vendor.
There are cases whereby, while striving for a homogeneous SOA environment, you may well conclude that your end state architecture must integrate a number of systems under a federated view. Your end state architecture in this case will be a mix of hybrid technologies servicing autonomous problem domains. Under this scenario, it would be best to reframe the definition of the problem at hand from one of creating an SOA environment to one of applying Enterprise Application Integration (EAI) mechanisms. If your end state revolves more around integration EAI, it would be better suited to performing boundary-level mapping and transformation work. In this case, go and shop for a great EAI solution; not for an ESB.
If the vendor gives you the option of acquiring specific subsets of their ESB offering (at a reduced price) then that’s something worth considering. At the very least, you will need to provide support for service deployment, routing, monitoring, and management, even if you won’t require many of the other functions in the ESB package. Just remember to focus in deploying the fabric that properly matches your SOA objectives and not the one that matches your vendor’s sales quota.
A quick word regarding Open Source ESBs. . . There are many, but the same caveats I’ve used for vendor-based ESB’s apply. Open Source ESBs are not yet as mature, and the quality of functions they provide varies significantly according to the component. Focus on using only those components you can be sure will work in a reliable and stable manner or those which are not critical to the system. Remember you are putting in place components that will become part of the core fabric. Ask yourself, does it make sense in order to save a few dollars to use a relatively unsupported ESB component for a critical role (Service Invocation or Messaging, come to mind), versus using a more stable vendor solution?
In the end, if you are planning to use the protocol conversion features packaged in a vendor-provided or open source ESBs, I suggest you use them in a discrete, case-by-case basis, and not as an inherent component of your SOA fabric. This way, even as you face having to solve integration problems associated with the lack of standards, at least you won’t be forced into drinking the Kool-Aid associated with a particular vendor’s view of ESB!
Before I present a sample view of services as applied to a hypothetical airline reservation flow, I would like to cover yet other dimensions to the categorization of services: the manner of the service delivery and its disposition.
In SOA, as in life, for any given service request you will be deciding between these three service delivery patterns:
Asynchronous: The service request is posted by the client with the expectation that the action will take place at the server’s leisure and that the client will not expect a related response from the service call. As the client is not waiting for an immediate response, he/she can continue to do whatever it is that clients do. The service is posted asynchronously and possibly queued up in a wait area until the system (the server) is able to process it. Any response resulting from processing the asynchronous request will also be sent back asynchronously to the client. The service interface designer is responsible for defining and filling-in a correlation identifier, if there is a need to match a request with its response. An example of asynchronous exchanges is a request for support from a vendor via email, with the expectation (but not certainty!) of a reply sometime later. Implicit in the way asynchronous services are handled is the idea that a queuing system of some sort must exist in the system infrastructure to properly handle the various aspects related to this pattern: How do we ensure delivery of the request? How do we prioritize the handling of the service? In the email example, the mail server takes care of all these details.
Query/Reply: This is the predominant service pattern for transactional systems. Here, a service request is made with the expectation that a response to the request will be given immediately. The fact that the client actually waits for a response gives this pattern very definite sensitivities as far as performance is concerned. If email exchanges are a representative example of asynchronous exchanges, you can think of chat or even telephone exchanges as an example of Query/Reply. Instead of sending an email, you establish a two-way real-time dialogue between yourself and the service provider.
Event Driven. This pattern is also known as Pub/Sub because it relies on the Publish/Subscribe idea. In this pattern the service request is for an asynchronous response that will take place upon the satisfaction of the event criteria. This pattern is typically used in a manner similar to the synchronous pattern in that the calling client need not wait for a response, but on occasion, a design may call for the client issuing the subscription request to sit idly by until a response occurs.
With that covered, let me now display a sample generic service flow, orchestrated from a putative airline reservation application with a client requesting via a natural language interface, all available flights to a chosen destination. The example shows a number of service types interacting to construct the appropriate response.
Hopefully most of the diagram is self-explanatory. The services shown to the left of the vertical line are meant to indicate those that can be accessed by the outside world and are thus considered to be Access Services. The Natural Language Parse, for example, could well be an external service provided by another company on a SaaS basis. The Update Customer profile could be available to external B2B partners to update the profile as per commercial agreements. Clearly, the Find Best Fare service could also be made available externally if desired, but the example here depicts a Best Fare Service that is applying internal rules that we do not wish to expose to the world. The various services could be classified as follows:
CLASS/
PATTERN
QUERY/REPLY
ASYNC
PUB/SUB
Access
NATURAL LANGUAGE PARSER
(Atomic)
UPDATE CUSTOMER PROFILE
(Atomic)
Business
FIND BEST FARE
(Composite)
AVAILABILITY & PRICE
(Atomic)
GET PROMOTIONS
(Atomic)
AIRLINE SPECIALFARE
(Atomic)
System
ENCRYPT CUSTOMER DATA
(Atomic)
Understanding the attributes of each service will enable you to apply predefined standards for their use. For example, Access services will be expected to provide public interfaces and will be hardened for public use. Access services are also cases where you may have to keep state across multiple service calls. Such state information may have to be kept in non-volatile storage (disk, or replicated cache). Composite services will be allowed to keep some state, but only for the duration of the service call. Atomic services will have highly streamlined execution paths, including avoidance of state.
Ideally, everything would follow an asynchronous pattern in the sense that not having to wait for a response is the most flexible way to optimize system resources and meet service level agreements. However, the reality is that you may need to adjust the overall solution against this ideal. Designing a system to do asynchronous messaging whenever possible may be seen as a desired goal, but fact of the matter is that you will need to use the Query/Reply patterns more often than not, particularly in transactional environments, to ensure prompt responses. Alas, I have witnessed actual designs that attempt to force asynchronous patterns in transactional systems (making everything flow through queues); resulting in very odd behaviors and unsatisfactory performance.
Finally, there is another category of “services” I have not yet defined. These are the services needed to facilitate or simplify the workings of SOA. I refer to these services as meta-services, and they are usually called upon to perform a specific SOA activity such as ensuring transaction integrity, forking the delivery of a service request, routing a service request to an alternate location, and other tasks. In fact, there are various patterns identified for these types of services, but the most interesting aspect is that a portfolio of meta-services is being bundled and it comprises a large portion of what is rapidly evolving to be a separate SOA middleware enabling layer: The Enterprise Service Bus.
If you recall the SOA taxonomy that I presented earlier, I will include these Enterprise Service Bus elements as part of the Service Fabric, to be discussed at a later date.
It is one thing to say SOA is the most natural way to architect systems; another to figure out how SOA should be implemented. The way we define the SOA structure (its “taxonomy”) is important because it has a direct impact on the best organizational governance needed for its successful use.
While there are many ways to split the SOA-cake, in my experience it makes sense to borrow from the world around us. After all, we have already established that there is nothing new under the sun when it comes to SOA, and that humans have been using the service model quite naturally for thousands of years. Also, humanity has tested a variety of social structures that allegedly have improved over time. It’s easy to be cynical when looking at the issues facing us today, but feudal structures are no longer considered appropriate (at least by most of us in western societies), and sacrificing prisoners to appease the gods is frowned upon these days. It makes sense to look at how we operate today as a possible model for defining a proper SOA taxonomy.
Let’s start with the use of language, (human language, mind you; not the computer programming-kind). Think of sentences with their nouns and predicates and the concept of service interface can emerge naturally from it. “Give me the lowest fare for a flight to New York in April,” or “Find the address for Mr. John Jones,” are service requests you could make equally well to your assistant or to a software program.
Just as a language’s sentence has to follow specific grammatical rules, how we articulate the request follows an implied structure intended to guarantee the request is understood and that it is in fact actionable. Satisfaction of the service requires an agreement as to how the service request is going to be handled and an expectation of who is going to act on the service.
Acting upon language instructions would be impossible without a specialization framework. When you call for a taxi, you don’t expect a hot-dog cart to show up, and if you need to summon emergency services, you dial 911 and not the pizza delivery service (you can tell I am kind of hungry as I write this!)
In fact, much of the social fabric related to the various roles and institutions usually referred to as ‘civilization’ are nothing more than a broad framework for the services we deliver or receive.
The streets we traverse, the sewage and plumbing systems beneath it, and the way electricity is delivered via power grids are infrastructure elements we all take for granted in support of the ‘civilization’ framework.
Finally, requesting services via the right language, using the means of civilization and the needed infrastructure would be all for naught if we were only capable to utter nonsensical commands (‘ride me to the moon on your bike’), or even requests not followed by the magic words (‘here is my credit card’). There are protocols that must be followed to ensure the entire system works as expected and these represent the social and legal ecosystem from which everything else flows.
This is essence the SOA taxonomy I will be discussing in more detail next. The elements encompassed by SOA are no different from the Language-Civilization-Infrastructure-Protocols pattern I’ve just discussed. For SOA, the equivalent pattern is Services-Framework-Foundation-Techniques:
a.The Services as the language of SOA. It is not the same thing ordering a meal at McDonalds as it is at a five star restaurant (yes, I’m still hungry!). There are services and then there are services.A clear understanding of what constitutes a service is essential to the SOA approach.
b.The emerging SOA Framework as the civilization. Trying to approach SOA in the same mindset as a traditional design is not feasible. SOA demands the establishment of new actors and their roles. Here I’ll discuss the proposed introduction of a common enterprise service bus (ESB) as a potential common transport for all SOA interactions, and the guidelines related to the access of data by services.
c.The physical Foundation. SOA is a beautiful approach, but it still relies on actual wires and moving bits and bytes—a suitable infrastructure. Here I will cover the distributed model, and the systemic approaches to scaling and managing SOA.
d.The Techniques needed to make SOA work. Imagine that you are given the opportunity to race against a professional NASCAR driver using a superior car that of the professional. Or that you are to compete against Tiger Woods in a round of golf using a better golf club. Odds are that, even with better equipment, you won’t win. Ultimately, you can have the best equipment, but it’s the way you use it that makes a difference in the results. Good equipment, like good infrastructure and services, can only be leveraged with appropriate techniques that only expert hands and minds can apply.
This blog covers the practical techniques, trials and tribulations associated with the transformation of IT systems from legacy technologies to systems using SOA and modern open systems. I also include the occasional interlude with rants about technology in general.
About Me
Name: Israel del Rio
Location: Atlanta, GA, United States
Israel has been recognized by Computerworld as one of their Premiere 100 IT honorees. Israel is a business and technology leader who has contributed the technology vision as key strategist and designer behind the enterprise technology roadmaps of large hospitality and travel companies. Israel has also have developed and deployed various mission-critical systems and in the process, he's been instrumental in creating and building effective and skilled development organizations.
