Remote Procedure Calls and Web Services Zachary G. Ives University of Pennsylvania CIS 455 / 555 – Internet and Web Systems September 17, 2015

2 Today  Reminder HW2 Milestone 2 due tonight  HW3 “pre-release” today

What Does MapReduce Do Well?  What are its strengths?  What about weaknesses? 3

MapReduce is a Particular Programming Model … But it’s not especially general (though things like Pig Latin improve it) Suppose we have autonomous application components that wish to communicate We’ve already seen a few strategies:  Request/response from client to server  HTTP itself  Asynchronous messages  Router “gossip” protocols  P2P “finger tables”, etc. Are there general mechanisms and principles? (Of course!) … Let’s first look at what happens if we need in-order messaging 4

5 Communication Mechanisms We’ve already seen a few:  Request/response from client to server  HTTP itself  Asynchronous messages  Router “gossip” protocols  P2P “finger tables”, etc. Are there general mechanisms and principles? (Of course!) … Let’s first look at what happens if we need in-order messaging

6 Message-Queuing Model (1)  Four combinations for loosely-coupled communications using queues. 2-26

7 Message-Queuing Model (2)  Basic interface to a queue in a message-queuing system. PrimitiveMeaning PutAppend a message to a specified queue GetBlock until the specified queue is nonempty, and remove the first message PollCheck a specified queue for messages, and remove the first. Never block. NotifyInstall a handler to be called when a message is put into the specified queue.

8 General Architecture of a Message-Queuing System (1)  The relationship between queue-level addressing and network-level addressing.

9 General Architecture of a Message-Queuing System (2)  The general organization of a message-queuing system with routers. 2-29

10 Benefits of Message Queueing  Allows both synchronous (blocking) and asynchronous (polling or event-driven) communication  Ensures messages are delivered (or at least readable) in the order received  The basis of many transactional systems  e.g., Microsoft Message Queue (MMQ), IBM MQseries, etc.

Some Common Modes of Building Distributed Applications Data-intensive:  XQuery (fetch XML from multiple sites, produce new XML)  Turing-complete functional programming language  Good for Web Services; not much support for I/O, etc.  MapReduce (built over DHT or distributed file system)  Single filter (map), followed by single aggregation (reduce)  Languages over it: Sawzall, Pig Latin, Dryad, … Message passing / request-response:  e.g., over a DHT, sockets, or message queue  Communication via asynchronous messages  Processing in message handler loop Function calls:  Remote procedure call / remote method invocation 11

12 Fully Synchronous Request/Response: Remote Procedure Calls  Remote procedure calls have been around forever, including:  COM+  CORBA  Java RMI  The basic idea: put a function elsewhere in the system, call in distributed fashion but using standard languages, methods  An RPC API defines a format for:  Initiating a call on a particular server, generally in a reliable way  Sending parameters (marshalling) to the server  Receiving a return value, which may require marshalling as well  And an RPC call is synchronous (i.e., it generally blocks)

13 A Remote Procedure Call Visualized time working server is busy request function server waits for req. client blocks RPC Server RPC Client

14 How RPC Generally Works  You write an application with a series of functions  One of these functions, F, will be distributed remotely  You call a “stub generator”  A caller stub emulates the function F:  Opens a connection to the server  Requests F, marshalling all parameters  Receives F’s return status and parameters  A server stub emulates the caller:  Receives a request for F with parameters  Unmarshals the parameters, invokes F  Takes F’s return status (e.g., protection fault), return value, and marshals it back to the client

15 Passing Value Parameters  Steps involved in doing remote computation through RPC 2-8

16 RPC Components  Generally, you need to write:  Your function, in a compatible language  An interface definition, analogous to a C header file, so other people can program for F without having its source  Generally, software will take the interface definition and generate the appropriate stubs (In the case of Java, RMIC knows enough about Java to run directly on the source file)  The server stubs will generally run in some type of daemon process on the server  Each function will need a globally unique name or GUID

17 Parameter Passing Can Be Tricky Because of References  The situation when passing an object by reference or by value. 2-18

18 What Are the Hard Problems with RPC? Esp. Inter-Language RPC?  Resolving different data formats between languages (e.g., Java vs. Fortran arrays)  Reliability, security  Finding remote procedures in the first place  Extensibility/maintainability  (Some of these might look familiar from when we talked about data exchange!)

19 Web Services  Goal: provide an infrastructure for connecting components, building applications in a way similar to hyperlinks between data  It’s another distributed computing platform for the Web  Goal: Internet-scale, language-independent, upwards-compatible where possible  This one is based on many familiar concepts  Standard protocols: HTTP  Standard marshalling formats: XML-based, XML Schemas  All new data formats are XML-based

One Alternative: REST (Representational State Transfer)  Not really a standard – a style of development  Data is represented in XML, e.g., with a schema  Function call interface uses URIs  Server is to be stateless  And the HTTP request type specifies the operation  e.g., GET  e.g., POST {body} adds the body to the servicehttp://my.com/rest/service1 20

21 The “Standard” for Web Services: Three Parts 1.“Wire” / messaging protocols  Data encodings, RPC calls or document passing, etc. 2.Describing what goes on the wire  Schemas for the data 3.“Service discovery”  Means of finding web services

22 The Protocol Stacks of Web Services Enhanced + expanded from a figure from IBM’s “Web Services Insider”, Other extensions SOAP Attachments WS-Security WS-AtomicTransaction, WS-Coordination SOAP, XML-RPC XML XML Schema Service Description (WSDL) Service Capabilities (WS-Capability) Message Sequencing Orchestration (WS-BPEL) Inspection Directory (UDDI) Wire Format Stack Discovery Stack Description Stack WS-Addressing High-level state transition + msging diagrams between modules

23 Messaging Protocol: SOAP  Simple Object Access Protocol: XML-based format for passing parameters  Has a SOAP header and body inside an envelope  As a defined HTTP binding (POST with content-type of application/soap+xml)  A companion SOAP Attachments encapsulates other (MIME) data  The header defines information about processing: encoding, signatures, etc.  It’s extensible, and there’s a special attribute called mustUnderstand that is attached to elements that must be supported by the callee  The body defines the actual application-defined data

24 A SOAP Envelope 12

25 Making a SOAP Call  To execute a call to service PlaceOrder: POST /PlaceOrder HTTP/1.1 Host: my.server.com Content-Type: application/soap+xml; charset=“utf-8” Content-Length: nnn …

26 SOAP Return Values  If successful, the SOAP response will generally be another SOAP message with the return data values, much like the request  If failure, the contents of the SOAP envelop will generally be a Fault message, along the lines of: SOAP-ENV:Client Could not parse message …

27 How Do We Declare Functions?  WSDL is the interface definition language for web services  Defines notions of protocol bindings, ports, and services  Generally describes data types using XML Schema  In CORBA, this was called an IDL  In Java, the interface uses the same language as the Java code

28 A WSDL Service Service Port PortType Operation PortType Operation PortType Operation Binding

29 Web Service Terminology  Service: the entire Web Service  Port: maps a set of port types to a transport binding (a protocol, frequently SOAP, COM, CORBA, …)  Port Type: abstract grouping of operations, i.e. a class  Operation: the type of operation – request/response, one-way  Input message and output message; maybe also fault message  Types: the XML Schema type definitions

30 Example WSDL

31 JAX-RPC: Java and Web Services  To write JAX-RPC web service “endpoint”, you need two parts:  An endpoint interface – this is basically like the IDL statement  An implementation class – your actual code public interface BookQuote extends java.rmi.Remote { public float getBookPrice(String isbn) throws java.rmi.RemoteException; } public class BookQuote_Impl_1 implements BookQuote { public float getBookPrice(String isbn) { return 3.22; } }

32 Different Options for Calling  The conventional approach is to generate a stub, as in the RPC model described earlier  You can also dynamically generate the call to the remote interface, e.g., by looking up an interesting function to call  Finally, the “DII” (Dynamic Instance Invocation) method allows you to assemble the SOAP call on your own

33 Creating a Java Web Service  A compiler called wscompile is used to generate your WSDL file and stubs  You need to start with a configuration file that says something about the service you’re building and the interfaces that you’re converting into Web Services

34 Example Configuration File

35 Starting a WAR  The Web Service version of a Java JAR file is a Web Archive, WAR  There’s a tool called wsdeploy that generates WAR files  Generally this will automatically be called from a build tool such as Ant  Finally, you may need to add the WAR file to the appropriate location in Apache Tomcat (or WebSphere, etc.) and enable it  See WSPack2/jaxrpc.html for a detailed example WSPack2/jaxrpc.html

36 Finding a Web Service  UDDI: Universal Description, Discovery, and Integration registry  Think of it as DNS for web services  It’s a replicated database, hosted by IBM, HP, SAP, MS  UDDI takes SOAP requests to add and query web service interface data

37 What’s in UDDI White pages:  Information about business names, contact info, Web site name, etc. Yellow pages:  Types of businesses, locations, products  Includes predefined taxonomies for location, industry, etc. Green pages – what we probably care the most about:  How to interact with business services; business process definitions; etc  Pointer to WSDL file(s)  Unique ID for each service

38 Data Types in UDDI  businessEntity: top-level structure describing info about the business  businessService: name and description of a service  bindingTemplate: how to access the service  tModel (t = type/technical): unique identifier for each service-template specification  publisherAssertion: describes relationship between businessEntities (e.g., department, division)

39 Relationships between UDDI Structures publisherAssertion businessEntity businessServicebindingTemplate tModel n 2 1 n 1n m n

40 Example UDDI businessEntity My Books Technical Book Wholesaler … … <!– keyedReferences to tModels  …

41 UDDI in Perspective  Original idea was that it would just organize itself in a way that people could find anything they wanted  Today UDDI is basically a very simple catalog of services, which can be queried with standard APIs  It’s not clear that it really does what people really want: they want to find services “like Y” or “that do Z”

42 The Problem: With UDDI and Plenty of Other Situations There’s no universal, unambiguous way of describing “what I mean”  Relational database idea of “normalization” doesn’t convert concepts into some normal form – it just helps us cluster our concepts in meaningful ways  “Knowledge representation” tries to encode definitions clearly – but even then, much is up to interpretation The best we can do: describe how things relate  pollo = chicken = poulet = 雞 = 鸡 = j ī = मुर्गी = murg  Note that this mapping may be imprecise or situation-specific!  Calling someone a chicken, vs. a chicken that’s a bird

43 This Brings Us to XQuery,Whose Main Role Is to Relate XML Suppose we define an XML schema for our target data and our source data Can directly translate between XML schemas or structures  Describes a relationship between two items  Transform 2 into 6 by “add 4” operation  Convert from S1 to S2 by applying the query described by view V Often, we don’t need to transfer all data – instead, we want to use the data at one source to help answer a query over another source…

44 Lazy Evaluation: A Virtual View Source2.xml Source1.xml Virtual XML doc. XQuery Query Form Browser/App Server(s) Query Results XQuery Source2.xml Source1.xml Composed XQuery HTML XSLT

45 Let’s Look at Some Simple Mappings  Beginning with examples of using XQuery to convert from one schema to another, e.g., to import data  First: let’s review what our mappings need to accomplish…

46 Challenges of Mapping Schemas In a perfect world, it would be easy to match up items from one schema with another  Each element would have a simple correspondence to an element in the other schema  Every value would clearly map to a value in the other schema Real world: as with human languages, things don’t map clearly!  Different decompositions into elements  Different structures  Tag name vs. value  Values may not exactly correspond  It may be unclear whether a value is the same It’s a tough job, but often things can be mapped

47 Example Schemas Bob’s Movie Database … … … … … * * Mary’s Art List … … … … … * Want to map data from one schema to the other

48 Mapping Bob’s Movies  Mary’s Art Start with the schema of the output as a template: $i $y $a $s $t Then figure out where to find the values in the source, and create XPaths

49 The Final Schema Mapping Mary’s Art  Bob’s Movies for $m in doc(“movie.xml”)//movie, $a in $m/director/text(), $i in $m/title/text(), $t in $m/title/text() return $i movie $a $t Note the absence of subject… We had no reasonable source, so we are leaving it out.