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RPC over DDS – Presentation by Gerardo Pardo-Castellote, CTO, RTI

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  1. 1. DDS RPC over DDS Gerardo Pardo-­‐Castellote, Ph.D. Chief Technology Officer, RTI Sumant Tambe, Ph.D. Senior SoBware Reseach Engineer, RTI October 2014
  2. 2. © 2014 Real-­‐Time InnovaJons, Inc. Outline • Goals • Background • Status • Details • More info
  3. 3. © 2014 Real-­‐Time InnovaJons, Inc. Goals • Provide standard and interoperable way means to support “RPC” communicaJon paQerns in DDS • Support the PaQers: – Request/Reply – Service InvocaJon – Remote Procedure Call – Remote Method InvocaJon • Support invocaJon of IDL-­‐specified interfaces over DDS – Provide a simpler way to migrate CORBA applicaJons to DDS – Support familiar “remote interface” programming style equivalent to Java RMI and Apache ThriB
  4. 4. Advantages of RPC over DDS • Client and Servers are decoupled © 2014 Real-­‐Time InnovaJons, Inc. – No startup dependencies • QoS enforcement to support service SLAs – Ownership–Redundant pool of servers – Lifespan–request is only valid for the next N sec – Durability-­‐request/replies will be eventually received and processed – CancellaJon– aBer a good quality response has arrived • Data-­‐Centricity – Explicit trace-­‐ability – InteracJon state can be Monitored, Logged, Audited, Stored, Manipulated • Watch by subscribing to requests and/or responses (wire-­‐tap) • One middleware – leverage DDS infrastructure – Suited for real-­‐Jme systems – MulJpla`orm, MulJlanguage, Interoperable
  5. 5. But I can already do it... True: • I can write my own RCP using request/reply Topics © 2014 Real-­‐Time InnovaJons, Inc. • RTI Connext already supports a “Request/Reply” equivalent to RPC • Some other vendors do similar things But… • The mapping from service to Topics and Types is ad-­‐ hoc. • These approaches are not portable/Interoperable • Gebng everything right is cumbersome at best
  6. 6. How can I do it myself? • Map Interfaces to types & Topics… © 2014 Real-­‐Time InnovaJons, Inc. – One Topic per interface, one per operaJon? – What are the types. How to map excepJons, output parameters, … – What happens if the interfaces change, e.g. operaJon is added or moved? • Implement necessary plumbing: • Setup request topic and type • Setup reply topic and type • Filter unwanted requests • Filter unwanted replies
  7. 7. But RPC is not a good/robust idea/ design paQern…. • RPC versus Desired State PaQern Goal State © 2014 Real-­‐Time InnovaJons, Inc. – Insight: OBen a request-­‐reply can be modeled as stateful data-­‐centric interacJons write Current State Affect Service Client Service Provider Desired State read write read read
  8. 8. RPC vs Desired State • Desired state is good & more robust if the “request” © 2014 Real-­‐Time InnovaJons, Inc. actually represents: – a request to change of system state – Take a long Jme to fulfill – Needs to be observable by others • However: – Many requests that do not represent changes to state: Database Query, Read a File… – People familiar for “Service” or “Request/Reply” paQern may find Directed State cumbersome –specially if extra robustness is unwarranted RPC over DDS provides best of both worlds!
  9. 9. Example: RobotControl © 2014 Real-­‐Time InnovaJons, Inc. module robot { @DDSService interface RobotControl { void command(Command command); float setSpeed(float speed) raises (TooFast); float getSpeed(); void getStatus(out Status status); }; }
  10. 10. RobotControl Client © 2014 Real-­‐Time InnovaJons, Inc. import*; public class TestRobot { static void createRobotClient() { RPCRuntime runtime = RPCRuntime.getInstance(TestRobot.class.getClassLoader()); ClientParams clientParams = runtime.createClientParams(); // optional RobotControlSupport.Client robotClient = RobotControlSupport.createClient( clientParams.withServiceName(”TestRobot”) .withInstanceName(“iRobot”)); // more configs available robotClient.waitForService(); // optional robotClient.getSpeed(); } }
  11. 11. RobotControl Service (Java) package robot; import robot.RobotControl; public class MyRobot implements RobotControl { public void command(Command command) { } public float setSpeed(float speed) { return 1; } public float getSpeed() { return 2; } public void getStatus(/* out */ Status status) { } © 2014 Real-­‐Time InnovaJons, Inc. } import*; public class TestRobot { public static void createRobotServer() { RPCRuntime runtime = RPCRuntime.getInstance(TestRobot.class.getClassLoader()); MyRobot myRobot = new MyRobot(); Server server = runtime.createServer(); // Configurable using ServerParams RobotControlSupport.Service service = RobotControlSupport.createService(myRobot, server); // Configurable using ServiceParams; // blocking } }
  12. 12. RobotControl Requester public class TestRobot { static void createRobotRequester() { © 2014 Real-­‐Time InnovaJons, Inc. RPCRuntime runtime = RPCRuntime.getInstance(TestRobot.class.getClassLoader()); RequesterParams reqParams = runtime.createRequesterParams(); // change params here if you like Requester<RobotControlSupport.RequestType, RobotControlSupport.ReplyType> requester = runtime.createRequester( RobotControlSupport.RequestType.class, RobotControlSupport.ReplyType.class, reqParams); RobotControlSupport.RequestType request = new RobotControlSupport.RequestType(); // populate request requester.sendRequest(request); } }
  13. 13. RobotControl Replier © 2014 Real-­‐Time InnovaJons, Inc. public class TestRobot { static void createRobotReplier() { RPCRuntime runtime = RPCRuntime.getInstance(TestRobot.class.getClassLoader()); ReplierParams repParams = runtime.createReplierParams(); // change params here if you like Replier<RobotControlSupport.RequestType, RobotControlSupport.ReplyType> replier = runtime.createReplier( RobotControlSupport.RequestType.class, RobotControlSupport.ReplyType.class, repParams); Sample<RobotControlSupport.RequestType> request = replier.receiveRequest(); } }
  14. 14. RobotControlSupport public abstract class RobotControlSupport { public final static class RequestType { // header; // robot.RobotControl_Call data; © 2014 Real-­‐Time InnovaJons, Inc. } public final static class ReplyType { // header; // robot.RobotControl_Return data } public interface Client extends RobotControl, RobotControlAsync, ClientEndpoint { } public interface Service extends ServiceEndpoint { } public static final RobotControlSupport.Client createClient(); public static final RobotControlSupport.Service createService(RobotControl impl); // more client and service factory methods here }
  15. 15. What needs to be specified • Mapping to DDS Topics & Filters © 2014 Real-­‐Time InnovaJons, Inc. – Each operaJon maps to a pair of topics? • Simplicity, Flexibility vs Scalability – Each interface to a pair of Topics? • What are the types? Inheritance? – Is publish-­‐subscribe allowed? e.g. audiJng, monitoring… • Mapping to DDS Data-­‐types – How to model operaJons? Parameters (in, our) return, excepJons, … – Fragility upon interfaces / operaJons changes? – Describable using X-­‐TYPES? • DDS API and DDS-­‐RTPS impact – Are extensions required? – Deployment on top of exisJng DDS • Language Bindings (C, C++, Java, C#, …)
  16. 16. Client Application Service © 2014 Real-­‐Time InnovaJons, Inc. Architecture Service Implementa8on Lang. Bind. GUID2 Data Writer Client-­‐side Invoker Data Reader Data Reader Data Writer GUID1 SN1 Foo GUID2 SN2 GUID1 SN1 Bar Message-id1 Message-id2 Correlation-id1 Language Binding Bar Svc.method(in Foo); Content-­‐based Filter for filtering unwanted replies Foo Bar GUID1 Invoke Return Call Topic Return Topic Foo Bar
  17. 17. Basic and Enhanced Service Mappings • Basic © 2014 Real-­‐Time InnovaJons, Inc. – Highest portability – Enables RPC on top of exisJng (v1.3 or earlier) DDS ImplementaJons • Enhanced – Elegant, Clean – Leverages DDS-­‐XTypes for type matching, – Adds support for Robust service discovery, Implicit request/reply correlaJon Mapping Aspect Basic Service Mapping Profile Enhanced Service Mapping Profile Correla8on Informa8on (request-­‐id) Explicitly added to the data-­‐type Implicit. They appear on the Sample meta-­‐data. Topic Mapping One request topic and one reply topic per interface. 2*N for a hierarchy of N interfaces. One request and one reply topic per interface. Independent of interface hierarchy. Type Mapping Synthesized types compaJble with l e g a c y ( p r e D D S -­‐ X T y p e s ) implementaJons. Use faciliJes of DDS-­‐XTypes for type descripJons, annotaJons, and type-­‐ compaJbility checks. Discovery No special extensions. Robust service discovery (no sample loss due to transient states)
  18. 18. Language Bindings • Two © 2014 Real-­‐Time InnovaJons, Inc. types of language bindings – High-­‐level binding that provide func%on-­‐call semanJcs – Low-­‐level binding that are akin to send/receive (but sJll higher level than raw DDS read/take/write) • Strong separaJon between the data model and language binding Client ApplicaJon FuncJon-­‐call language binding DDS API Service Specification (IDL) Client ApplicaJon Request-­‐Reply language binding DDS API Service ImplementaJon FuncJon call language binding DDS API Service ImplementaJon Request-­‐Reply language binding DDS API Service-­‐specific Interoperable/Evolvable Data model (DDS/RTPS)
  19. 19. © 2014 Real-­‐Time InnovaJons, Inc. QoS Mapping • Default strict reliable (request and reply) – RELIABLE reliability – KEEP_ALL history – VOLATILE durability • Can be changed by the user on a per-­‐service level
  20. 20. © 2014 Real-­‐Time InnovaJons, Inc. @DDSService interface RobotControl { struct RobotControl_setSpeed_In { long s; }; struct RobotControl_getSpeed_In { boolean return_; }; float setSpeed(float speed) raises (TooFast); float getSpeed(); void start(); void stop(); }; @choice @autoid struct RobotControl_Request { RobotControl_setSpeed_In setSpeed; RobotControl_getSpeed_In getSpeed; RobotControl_start_In start; RobotControl_stop_In stop; @empty }; struct start_In { octet dummy_;}; @empty struct stop_In { octet dummy_;};
  21. 21. float setSpeed(float speed) raises (TooFast); float getSpeed(); void start(); void stop(); © 2014 Real-­‐Time InnovaJons, Inc. @DDSService interface RobotControl { }; struct RobotControl_setSpeed_Out { float return_; // Also inout and out params }; … @choice @autoid struct RobotControl_setSpeed_Result { RobotControl_setSpeed_Out out; SystemExcepJonCode sysx_; TooFast toofast_ex; }; … @choice @autoid struct RobotControl_Reply { RobotControl_setSpeed_Result setSpeed; RobotControl_getSpeed_Result getSpeed; RobotControl_start_Result start; RobotControl_stop_Result stop; };
  22. 22. © 2014 Real-­‐Time InnovaJons, Inc. Status • Ongoing standard at OMG • RFP was issued in June 2012 • IniJally 2 compeJng submissions: 1. Joint RTI + eProsima (later joined by TwinOaks) 2. PrismTech • Very different approaches – not converging – EvaluaJon team represenJng 5 companies formed to provide feedback • RecommendaJon was to simplify RTI/eProsima submission • PrismTech dropped their submission and joined RTI/eProsimas’/TwinOaks • Expected to be voted and adopted in Dec 2014 • Completed FuncJon-­‐Call and Request/Reply language binding – Java (hQps://­‐rpc-­‐java) – C++* (hQps://­‐rpc-­‐cxx) – eProsima has POC implementaJon of “basic” profile working with several DDS implementaJons
  23. 23. QuesJons? © 2014 Real-­‐Time InnovaJons, Inc.
  24. 24. © 2014 Real-­‐Time InnovaJons, Inc. Find out more…