[Michael, there is a misunderstanding here which is my fault - I seem to have forgotten to include my initial quote brackets, "<<". What I quoted verbatim was Steve's blog. Gervas]
Gervas, let's start with that "Get Delivery Status" is not really a capability as well as "Get Forecast", they are operations of the interface as the most. That is, we have clearly distinguish between service capabilities and service interface operations. This is the first 'attack' on the RPC as a high level of 'a consistent view' - what is consistent from RPC perspective may be inconsistent from service perspective. "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" - from the service consumer perspective (the only correct viewpoint for analysing services), the Finance Service is responsible for the Forecast irrespective to whether it gets it from the Sales Service or not. From architectural viewpoint, the Finance Service has to interact with the Sales Service, and this is not necessary to an RPC call (may be MOM-based request); this depends on the Finance Service's Service Description. If "we need the Forecast to be updated daily", this is another reason for NOT using RPC between the Finance Service and the Sales Service. "Now when we start working through this at a systems level ..." utilisation of legacy systems in Mainframes and FTP/ETL is fine but this is not dictated by the RPC style (as you confirmed: "...but instead of making an RPC call to get that information we have decided...". That is, RPC is good but not really universal to become the Development Solution #1. This is the second 'attack' on the RPC. Provided description of "RPC into Events" calls for messaging solution bypassing anything related to RPC. Given event-driven scenario is typical 'push' scenario, which operates in contra-RPC model. This is the third 'attack' on the RPC. REST=RPC via special transport protocol, no doubts. But REST can play only the interface role for the service. REST is technology not a separate concept, and this concept is RPC. Finally, I totally disagree with the "Conceptual v Delivery" and, I hope, I have demonstrated on your examples limited usability of RPC for service design. This is why I repeat - service must be designed as service and its implementation must respect and preserve service constraints and boundaries. The horse pools the cart; the cart, does not matter if it is a phaeton or a royal carriage, does not push the horse. - Michael ________________________________ From: Gervas Douglas <[email protected]> To: [email protected] Sent: Thu, January 14, 2010 11:01:08 AM Subject: [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
