2010/1/17 Harm Smit <[email protected]> > > > > > > > *De :* [email protected] [mailto: > [email protected]] *De la part de* Steve > Jones > *Envoyé :* vendredi 15 janvier 2010 21:34 > > *À :* [email protected] > *Objet :* Re: [service-orientated-architecture] Steve on RPC > > > > 2010/1/15 Harm Smit <[email protected]> > > > > *De :* [email protected] [mailto: > [email protected]] *De la part de* Steve > Jones > *Envoyé :* jeudi 14 janvier 2010 21:12 > > > *À :* [email protected] > > *Objet :* Re: [service-orientated-architecture] Steve on RPC > > Harm, > > You are talking about technology implementation. > > *[HS] Nope. I’m talking at the level of the model of your application, > independently of how you implement that model. Try using whatever modelling > language to express more rigorously what you’re now expressing verbally and > you will see that the RPC paradigm alone doesn’t fit the bill as soon as > event-driven interactions come into play. In my view, events are not just > implementation-level entities, they are an integral part of the conceptual > model.* >
I've done this and it does work... not at the logical level but at the conceptual level. > > > Events are important but.... > > * * > > The point is that as an architect you can have the communication with the > business in an RPC language and then choose the most appropriate > implementation approach. RPC as a conceptual model effectively just says > "There are domains of control and you hand over requests to those domains" > > *[HS] Yes, but it doesn’t allow you to say “These domains of control may > send you unsolicited event notifications, be prepared to receive them at any > time and to process an event of type x within y minutes, otherwise the roof > will come down (or your boss will be fired, or a financial penalty is > incurred, etc.)”. BTW, I know a real-world example of this kind of > situation: in case a major failure event occurs in a car production plant, > such that the entire production line is stopped for more than 30 minutes, > the car company’s CEO receives a phone call. All this is functional, not > implementational, and has obvious business level implications.* > > If we look at this in RPC style though that isn't really an event. > > *[HS] The event I’m talking about is the failure that causes the > production line to be stopped. It is received by some sort of orchestrator > (maybe several of them), say a Manufacturing Execution System, which > evaluates the failure, correlates it with other failures, sends warning > messages to operators, attempts corrective action or a degraded mode of > operation, etc. After 30 minutes, if an overall error condition persists, it > triggers a phone call to the CEO.* > In other words the manufacturing system MAKES A CALL to the operators (RPC call) and if that isn't fixed it MAKES A CALL to the CEO. These are both very simple to model as RPC calls. The MES finds an error then makes a couple of RPC calls. You can even go a level down and say that the production line calls the MES when there is an error. No events required. > * * > > * * > > Most events aren't purist "don't know who sent it" they are just the > smartest way to implement a "get this to X, Y, and Z". > > *[HS] “Don’t know who sent it” is associated to request messages,whereas > events are of the type "don’t know whom it is sent to”. This is an important > characteristic that allows preserving autonomy and modularity. If the > machine that originates the failure event had to know the names of the > orchestrators that receive the event, then that machine would become > dedicated to those specific event sinks, hence would not be reusable in some > other environment – in other words: would lose its autonomy. To me, autonomy > is a key attribute of SOA: a service should provide self-contained > functionality that makes sense even if used in a stand-alone manner. Hence, > a service should not know from whom it receives request messages and to whom > it sends event messages. Breaking this principle automatically results in > spaghetti structures.* > Again its the difference between business conceptual and implementation. At a business conceptual perspective its 100% important to know that events are being consumed (or else why send them?) and the business purpose that they are being consumed against. When you get to technology (machines) then choose EDA if you really do have a federated reception problem (if its simple machine to machine then simple mediation would achieve the same de-coupling). The point is that the same conceptual model could CHOOSE to use either events or mediation to deliver the autonomy rather than being tied to one approach. > * * > > So in this case what you are saying is that the manufacturing Service CALLS > the CEO Service and therefore the CEO Service has a "receive escalation > call" capability. > > *[HS] This isn’t the point, see above. BTW, “CALLing the CEO Service” is a > misnomer, since no reply should be expected.* > RPC calls don't have to have a return (think of it as "void") > > > If an event doesn't have a receiver/consumer its pointless and its the act > of consumption that is the capability on the receiving service. > > > > Lets put it this way. I've architected major manufacturing systems with > just the problem you talk about and we modeled it using an "RPC" style by > modeling the required consumption capabilities of services. This approach > clearly identified some current gaps where events were being sent but there > wasn't a clear reception process. > > > > * * > > in other words it fits a departmental or value network view of the world. > > > > Once you've agreed the interactions at a business level then as IT you can > choose the right implementation approach. > > > > The business doesn't know or care about CORBA, Web Services, REST, Events, > MQ Series, FTP or any of the other plumbing approaches that IT uses and > replaces, so why surface all of that to them? > > *[HS] In this list, “Events” is a red herring. Some of the other elements > in the list support event notification (such as CORBA Notification Services, > WS-Eventing, WS-Notification), but “Events” is not an implementation > technology by itself.* > > Some people would disagree and claim that EDA (event driven architecture) > is a way to go. > > *[HS] EDA is about architecture, not about implementation.* > And I'd disagree. EDA is an design and implementation approach, trying to model a business using EDA would be (IMO) madness. > * * > > Events is an implementation view that can be very easily represented using > textual descriptions in an RPC world. > > *[HS] We definitely disagree here. To me, events are at the same level as > requests and responses and in no way is this an implementation view, it’s > about design and modelling of interactions between services. As Eric > Newcomer stated it: “*We are modeling and designing services with various > message exchange patterns”*; request, response and event messages are the > constituents of these exchange patterns and the message is the primitive > element. RPC is a degenerate exchange pattern, in which a request message > and a response message are tied together in a synchronous manner.* > The MEP is the bit "between" the services and I completely agree that DESIGNING the services should use lots of different MEPs. My point is that to get a good architecture you really only need a bunch of interfaces and some sequence diagram style things to explain in an RPC way the objective of the system. Then in design choose the right implementation approach. The point I'm making is that having a consistent architectural model is incredibly valuable. Blurring that model with the "right" implementation approach isn't a great idea. Steve > > Steve > > > > > > > > Steve > > > > 2010/1/14 Harm Smit <[email protected]> > > > > “*RPC into Events* > > The next piece that is looked at is the sales to invoice process Here the > challenge is that historically there has been a real delay in getting > invoices out to customers and it needs to be tightened up much more. > Previously a batch has been sent at the end of each day to the logistics and > finance departments and they've run their own processing. This has led to > problems with customers being invoiced for products that aren't shipped and > a 48 hour delay in getting invoices out. > > The solution is to run an event based system where Sales sends out an event > on a new Sale, this is received by both Finance and the Logistics > department. The Logistics department then Ships the Product (Ship Product) > after which it sends a "Product Shipped" event which results in the Finance > department sending the invoice.” > > > > I don’t buy this shortcut. Since Finance and Logistics only think in terms > of RPC, they cannot be prepared to receive events; such a type of > interaction is simply not part of their model of the world. > > > > Indeed, eventing interactions, such as pub/sub, cannot be covered by the > simplistic RPC paradigm. Consequently, the idea of exclusively thinking in > terms of RPC interactions is fundamentally flawed, both at the functional > and at the implementation level. IMHO, the atomic interaction concept is > that of sending and receiving messages: “the message is the message”. > > > > Harm. > > > > *De :* [email protected] [mailto: > [email protected]] *De la part de* Gervas > Douglas > *Envoyé :* jeudi 14 janvier 2010 12:01 > *À :* [email protected] > *Objet :* [service-orientated-architecture] Steve on RPC > > > > > > Gregg Wonderly made a good comment on the Yahoo SOA list the other day > > *I think one of the still, largely unrecognized issues is that developers > really should be designing services as RPC interfaces, always. Then, > different service interface schemes, such as SOAP, HTTP (Rest et.al.), Jini, > etc., can be more easily become a "deployment" technology introduction > instead of a "foundation" technology implementation that greatly limits how > and under what circumstances a service can be used. Programming > Language/platform IDEs make it too easy to "just use" a single technology, > and then everything melds into a pile of 'technology' instead of a > 'service'.* > > > > The point here is that *conceptually* RPC is very easy for everyone to > understand and at the highest levels it provides a consistent view. Now > before people shriek that *"But RPC sucks"* I'll go through how it will > work. > > First off lets take a simple three service system where from an "RPC" > perspective we have the following: > > The Sales *Service* which has *capabilities* for "Buy Product" and "Get > Forecast" > > The Finance *Service* which has *capabilities* for "Report Sale" and "Make > Forecast" > > The Logistics *Service* which has *capabilities* for "Ship Product" and > "Get Delivery Status" > > There is also a customer who can "Receive Invoice" > > Now we get into the conceptual design stage and we want to start talking > through how these various services work and we use an "RPC language" to > start working out how things happen. > > *RPC into Push* > *When we call "Make Forecast" on the Finance Service it needs to ask the > Sales Service for its Forecast and therefore does a "Get Forecast" call on > the Sales Service. We need the Forecast to be updated daily.* > > Now when we start working through this at a systems level we see that the > mainframe solution of the Finance team is really old and creaky but it > handles batch processing really well. Therefore given our requirement for a > daily forecast what we do is take a nightly batch out of the CRM solution > and Push it into the Mainframe. Conceptually we are still doing exactly what > the RPC language says in that the data that the mainframe is processing has > been obtained from the Sales area, but instead of making an RPC call to get > that information we have decided in implementation to do it via Batch, FTP > and ETL. > > *RPC into Events* > The next piece that is looked at is the sales to invoice process Here the > challenge is that historically there has been a real delay in getting > invoices out to customers and it needs to be tightened up much more. > Previously a batch has been sent at the end of each day to the logistics and > finance departments and they've run their own processing. This has led to > problems with customers being invoiced for products that aren't shipped and > a 48 hour delay in getting invoices out. > > The solution is to run an event based system where Sales sends out an event > on a new Sale, this is received by both Finance and the Logistics department > . The Logistics department then Ships the Product (Ship Product) after which > it sends a "Product Shipped" event which results in the Finance department > sending the invoice. > > So while we have the conceptual view in RPC speak we have an implementation > that is in Event Speak. > > *RPC into REST* > The final piece is buying the products and getting the delivery status > against an order. The decision was made to do this via REST on a shiny new > website. Products are resources (of course), you add them to a shopping > basket (by POSTing the URI of the product into the basket) this shopping > basket then gets paid for and becomes an Order. The Order has a URI and you > just simply GET to have the status. > > So conceptually its RPC but we've implemented it using REST. > > *Conceptual v Delivery* > > The point here is that we can extend this approach of thinking about things > in RPC terms through an architecture and people can talk to each other in > this RPC language without having to worry about the specific implementation > approach. By thinking of simply "Services" and "Capabilities" and mentally > placing them as "Remote" calls from one service to another we can construct > a consistent architectural model. > > Once we've agreed on this model, that this is what we want to deliver, we > are then able to *design* the services using the most appropriate * > technology* approach. I'd contend that there really aren't any other > conceptual models that work consistently. A Process Model assumes steps, a > Data Model assumes some sort of entity relationship a REST model assumes its > all resources and an Event model assumes its all events. Translating between > these different conceptual models is much harder than jumping from a > conceptual RPC model that just assumes Services and Capabilities with the > Services "containing" the capabilities. > > So the basic point is that architecture, and particularly business > architecture, should always be RPC in flavour. Its conceptually easier to > understand and its the easiest method to transcribe into different > implementation approaches.>> > * > You can read Steven's blog at: > http://service-architecture.blogspot.com/2010/01/think-in-rpc-develop-in-anything.html > > Gervas* > > > > > > >
