Udi Dahan   Udi Dahan – The Software Simplist
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Autonomous Services – a step beyond Service Orientation

Sunday, January 8th, 2006.

After starting to write a whitepaper on Workflow in Service Oriented Architectures, I wanted to reference some prior work published on autonomous services (so that the whitepaper wouldn’t turn into a book). Anyway, after some futile googling, I’ve decided to give in and write it up myself.

The tenets of Service Orientation as put forth by Microsoft include one about “autonomy”. The tenet states that “Services should be autonomous”. After some digging, I found out that the intent of the authors was “The teams that develop different services should not be dependent on each other”, or in shortened form “Autonomous Teams”. This revelation was surprising to me since the real meaning of the tenet was less profound than what I had imagined – autonomous computing.

The idea of autonomous computing has been around for some time and presents a view of the world in which computing units cooperate to achieve global goals yet are not dependent even on the existence of other computing units to function. (If you’re envisioning tiny robots playing soccer, you’re not far off.)

So when I first saw the autonomy tenet, I was thinking of autonomous services: services so loosely coupled that the correct functioning of a service would not be dependent on the correct functioning of other cooperating services. Services loosely coupled in time as well as in code. Obviously this would mean that if Service A needed to cooperate with Service B, and Service B was not even available, Service A would continue to function, and live up to its service-level-agreement. But before we start drifting off into the outer reaches of business-IT alignment, let’s bring this down to earth.

Before we get into a detailed analysis of the how, let’s first agree on the why. Despite being a historical trend, architectures these days tend to be more loosely coupled than before. Loose coupling being a good thing that enables us to better manage the complexity inherent in large software projects. The practical test of loose coupling in a system is changing the public interface of a class and seeing how much of the system doesn’t compile any more (dynamic languages aside). Service Orientation brings us tenets that, when followed, lead to more loosely coupled architectures than if we actively did not follow them. I think that we can agree then that if we could somehow achieve the loose coupling in time mentioned above, without paying an arm and a leg, that would move our architectures another step forward.

Looking back on the evolution of the field of distributed computing, we can see that, over time, less and less things are being assumed. It is now well understood that anything that goes over the network takes much longer than those calls that stay on the same machine, yet once systems were built that abstracted the network communication into looking just like local calls. The performance of those systems was matched only by their lifetime. With the advent of autonomous computing, the assumption that the called service is available and will respond in a timely fashion is called into question. In the real world, servers crash and network equipment goes up in smoke – we can no longer take for granted that communication will always be available, and that its quality will be good enough. In essence, this marks the end of synchronous RPC/RMI. The following code just won’t cut it in this brave new world:

localhost.service1 s1 = new localhost.service1();
orderReply = s1.HandleOrder(orderRequest);

If the service is unavailable, what will happen to our order request? Will it just get lost?
If the service takes a long time to respond, will our server tie up resources for the same amount of time? If this happens under peak load, might it cause our server to crash?

Performing the above code on a different thread won’t make any difference, autonomous services means the end of Request/Response as we know it.

“No Request/Response between services?!”, you ask incredulously.

The simple answer is “yes”, but there is another level of meaning to it. If you have two software entities that between them you just HAVE to have request/response communication, then they should be in the same service. This is where the real architectural guidance comes in.

In component-orientation and object-orientation, the division of the solution into the right number of parts, with each part having the right amount of responsibility was a kind of black magic passed from master to apprentice. Getting the boundaries right was paramount, but difficult. A number of litmus tests are used to catch the gross errors, and the rest is just gut. So too, the request/response test helps us catch gross errors in service boundary demarcation.

The interesting thing that happens after separating our services out this way is that we often end up with services that mirror the way the business side is structured. Voila, business-IT alignment with your hands closed and one eye tied behind your back! Well, it’s one step in the right direction anyway.

This leaves us with the original types of one-way communication (fire-and-forget, pub-sub, etc) and with one kind of two-way communication: duplex. Duplex is really just two one-way communications (A to B, then B to A) that are correlated. First, I send a message to you, mark it with an id number, and save that number. At some future point in time, you get the message, process it, and send a message back with its own id number. But, you’ll have to put my original id number on the message too, so that I’ll know that your message is a response to mine. At some even more distant point in the future, I get a message from you, look at it, and see that it is the long-awaited response to the request I sent way back when.

If I had to sum up the difference autonomous services bring to the styles of communication used between services, I’d say this: You get a message, look at it, and figure out what it means and what you should do. This isn’t an infrastructure issue. There application level timeouts to deal with (If I don’t get a response back in 3 days, then notify the supervisor), and long-running workflows to manage (next whitepaper ).

If there is one thing to pay attention to in this whole “autonomous services paradigm” it is that the focus has shifted from between services to within a single service. In parting, I want to let you know that systems can be, and are being, built this way. It works. It better than works. Systems created this way are more robust to failures (seeing as they’re designed for failures makes it less impressive) and easier to manage. Give it a try. You didn’t really think that SOA would fizzle away into a bunch of WS specs, did you?

  
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7 Comments

  1. Ralf Says:

    To take you up on “If you have two software entities that between them you just HAVE to have req/resp communication, then they should be in the same service.”: This means, what Amazon and Google are offering, are no real services, because requesting some book information is a sync req/resp operation? Hm… As much as I would like your guideline, I think it´s somewhat limiting. Amazon´s service is pretty autonomous, but still req/resp style. Is this really wront?


  2. Arnon Rotem-Gal-Oz Says:

    Hi Ralf,
    While I am with Udi on the notion that implementation of request/reply is not an SOA practice as it increases coupling and prevents autonomy.
    I still believe a service can manifest itself in a request/reply fashion as long as it is actually implemented as a request/reaction, meaning, that there is no direct synchronous handling of the incoming message (request). Furthermore I expect the request/reply behavior to be policy driven (sort of a QOS property) – based on the knowledge and testing that the reaction time for the service can be perform fast-enough to be considered as a request/reply.

    Still, if you write a service and you bound yourself to another service, building on its request/reply manifestation – then you have a problem with the autonomy of your service

    Arnon


  3. Chris Sterling Says:

    I am not always sure that the use of the network allows enough flexibility as when used in conjunction with client-side execution. If you look at JavaScript, it seems to never go away and I believe it is due to the basic principal of mobile code and it’s perceived enhancement of the user experience.

    When speaking about server side code I think that mobile code must still be discussed to deal with SOA objectives. Jini is a great SOA and allows for mobility of code through JERI and the exporting smart proxies. A smart proxy may do all of the work and have no need to actually connect with a remote service. This architecture, or better yet “programming paradigm”, allows for runtime configuration of your proxy communication. Therefore you can communicate via a web service using XML/SOAP/REST, Hessian/Burlap from Coucho, JERI (an RMI implementation which is much improved containing transaction and security capabilities), or your own binary protocol of choice.

    I believe that XML and web services along with business process management is a powerful tool but is not a silver bullet. Many organizations have internal processes which are being service-oriented using an XML strategy without noticing the binary alternative. I hope that web services and XML are used more carefully and closer to the edge in the near future or we may see another fall from grace similar to that of EJB and CORBA.


  4. Joshua Ramirez Says:

    Ralf,

    In Udi’s article on MSDN;
    http://msdn.microsoft.com/en-us/library/bb245672.aspx

    He writes the following:

    Note that while a service may consume other services asynchronously, this consumption does not necessarily mean that it cannot expose a synchronous interface. Google and Amazon do exactly that. The Web services that they expose are synchronous in nature but it has no effect on their autonomy.

    The article is a very good read. Recommended 😉


  5. Madhu Says:

    what if a service needs data from a thrid party service. How should it be handled. Should we build some notify mechanism around it ?


  6. udidahan Says:

    Madhu,

    See some of my more recent blog posts on the topic.

    The short answer is that a “third party service” is actually a third party *system*. A service is a logical construct that can span multiple physical systems.

    Cheers.


  7. Mat Says:

    Yes definitely, a sevcrie (business capability) may include more than one bounded context. In many cases it does, in many cases it does not. In the example given, I would say the Node BC components would be part of the same sevcrie some parts of which are deployed on the nodes and some parts of which are deployed on the central server.VA:F [1.9.13_1145](from 0 votes)


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Eran Sagi, Software Architect at HP
“So far, I heard about Service Oriented architecture all over. Everyone mentions it – the big buzz word. But, when I actually asked someone for what does it really mean, no one managed to give me a complete satisfied answer. Finally in his excellent course “Advanced Distributed Systems”, I got the answers I was looking for. Udi went over the different motivations (principles) of Services Oriented, explained them well one by one, and showed how each one could be technically addressed using NService bus. In his course, Udi also explain the way of thinking when coming to design a Service Oriented system. What are the questions you need to ask yourself in order to shape your system, place the logic in the right places for best Service Oriented system.

I would recommend this course for any architect or developer who deals with distributed system, but not only. In my work we do not have a real distributed system, but one PC which host both the UI application and the different services inside, all communicating via WCF. I found that many of the architecture principles and motivations of SOA apply for our system as well. Enough that you have SW partitioned into components and most of the principles becomes relevant to you as well. Bottom line – an excellent course recommended to any SW Architect, or any developer dealing with distributed system.”

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