Remote%20Procedure%20Calls%20and%20Web%20Services

1
Remote Procedure Calls and Web Services

• Zachary G. Ives
• University of Pennsylvania
• CIS 455 / 555 Internet and Web Systems
• June 5, 2018

2
Today

• Reminder HW2 Milestone 2 due tonight
• HW3 pre-release today

3
What Does MapReduce Do Well?

• What are its strengths?
• What about weaknesses?

4
MapReduce is a ParticularProgramming Model

• But its not especially general (though things
  like Pig Latin improve it)
• Suppose we have autonomous application components
  that wish to communicate
• Weve 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!)
• Lets first look at what happens if we need
  in-order messaging

5
Communication Mechanisms

• Weve 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!)
• Lets 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
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Message-Queuing Model (2)

• Basic interface to a queue in a message-queuing
  system.

Primitive Meaning
Put Append a message to a specified queue
Get Block until the specified queue is nonempty, and remove the first message
Poll Check a specified queue for messages, and remove the first. Never block.
Notify Install a handler to be called when a message is put into the specified queue.
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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.

11
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

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)

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A Remote Procedure Call Visualized
server waits for req.
function
server is busy
RPCServer
request
RPC Client
working
client blocks
time
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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 Fs return status and parameters
• A server stub emulates the caller
• Receives a request for F with parameters
• Unmarshals the parameters, invokes F
• Takes Fs return status (e.g., protection fault),
  return value, and marshals it back to the client

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Passing Value Parameters

• Steps involved in doing remote computation
  through RPC

2-8
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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 TrickyBecause of
References

• The situation when passing an object by reference
  or by value.

2-18
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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
• Its 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

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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 http//my.com/rest/service1
• e.g., POST http//my.com/rest/service1 body
  adds the body to the service

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The Standard for Web Services Three Parts

• Wire / messaging protocols
• Data encodings, RPC calls or document passing,
  etc.
• Describing what goes on the wire
• Schemas for the data
• Service discovery
• Means of finding web services

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The Protocol Stacks of Web Services
High-levelstate transition msgingdiagramsbetw
een modules
WS-AtomicTransaction,WS-Coordination
Other extensions
Orchestration (WS-BPEL)
SOAP Attachments
WS-Addressing
MessageSequencing
WS-Security
Service Capabilities(WS-Capability)
SOAP, XML-RPC
Directory (UDDI)
Service Description (WSDL)
XML
XML Schema
Inspection
Wire Format Stack
Description Stack
Discovery Stack
Enhanced expanded from a figure from IBMs Web
Services Insider,http//www-106.ibm.com/develope
rworks/webservices/library/ws-ref2/
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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/soapxml)
• A companion SOAP Attachments encapsulates other
  (MIME) data
• The header defines information about processing
  encoding, signatures, etc.
• Its extensible, and theres 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

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A SOAP Envelope

• ltSOAP-ENVEnvelopexmlnsSOAP-ENVhttp//www.w3.o
  rg/2001/12/soap-envelopexmlnsxsdhttp//www.w3
  .org/www.w3.org/2001/XMLSchema-instancegt
• ltSOAP-ENVHeadergt
• lttTransaction xmlnstwww.mytrans.com
  SOAP-ENVmustUnderstand1 /gt
• lt/SOAP-ENVHeadergt
• ltSOAP-ENVBodygt
• ltmPlaceOrder xmlnsmwww.somewhere/theregt
  ltorderno xsitypexsdstringgt12lt/ordernogtlt/mPl
  aceOrdergt
• lt/SOAP-ENVBodygt
• lt/SOAP-ENV Envelopegt

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Making a SOAP Call

• To execute a call to service PlaceOrder
• POST /PlaceOrder HTTP/1.1
• Host my.server.com
• Content-Type application/soapxml
  charsetutf-8
• Content-Length nnn
• ltSOAP-ENVEnvelopegt
• lt/SOAP-ENVEnvelopegt

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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
• ltSOAP-ENVBodygt
• ltSOAP-ENVFault xmlnsmynamespacegt
• ltfaultcodegtSOAP-ENVClientlt/faultcodegt
• ltfaultstringgtCould not parse messagelt/faultstrin
  ggt

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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

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

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Example WSDL

• ltservice namePOServicegt
• ltport bindingmyPOBindinggt
• ltsoapaddress locationhttp//yyy9000/POSvc/gt
• lt/portgt
• lt/servicegt
• ltbinding xmlnsmy namePOBindinggt
• ltsoapbinding stylerpc transporthttp//www.w3
  .org/2001/... /gt
• ltoperation namePOrdergt
• ltsoapoperation soapActionPOService/POBinding
  stylerpc /gt
• ltinput namePOrdergt
• ltsoapbody useliteral namespacePOService
  /gt
• lt/inputgt
• ltoutput namePOrderResultgt
• ltsoapbody useliteral namespacePOService
  /gt
• lt/outputgt
• lt/operationgt
• lt/bindinggt

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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

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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

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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 youre building
  and the interfaces that youre converting into
  Web Services

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Example Configuration File

• lt?xml version"1.0" encoding"UTF-8"?gt
• ltconfiguration xmlns"http//java.sun.com/xml/ns/j
  ax- rpc/ri/config"gt
• ltservice name"StockQuote" targetNamespace"http/
  /example.com/stockquote.wsdl" typeNamespace"http
  //example.com/stockquote/types"
  packageName"stockqt"gt
• ltinterface name"stockqt.StockQuoteProvider"
  servantName"stockqt.StockQuoteServiceImpl"/gt
• lt/servicegt
• lt/configurationgt

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Starting a WAR

• The Web Service version of a Java JAR file is a
  Web Archive, WAR
• Theres 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 http//java.sun.com/developer/technicalArticle
  s/WebServices/WSPack2/jaxrpc.html for a detailed
  example

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Finding a Web Service

• UDDI Universal Description, Discovery, and
  Integration registry
• Think of it as DNS for web services
• Its a replicated database, hosted by IBM, HP,
  SAP, MS
• UDDI takes SOAP requests to add and query web
  service interface data

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Whats 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

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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)

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Relationships between UDDI Structures
publisherAssertion
n
tModel
2
businessEntity
n
1
m
n
businessService
bindingTemplate
1
n
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Example UDDI businessEntity

• ltbusinessEntity businessKey0123
  xmlnsurnuddi-orgapi_v2gt
• ltdiscoveryURLsgt
• ltdiscoveryURL useTypebusinessEntitygt
• http//uddi.ibm.com/registery/uddiget?businessKey
  0123...
• lt/discoveryURLgt
• ltnamegtMy Bookslt/namegt
• ltdescriptiongtTechnical Book Wholesalerlt/descriptio
  ngt
• ltbusinessServicesgt
• lt/businessServicesgt
• ltidentifierBaggt
• lt! keyedReferences to tModels ?
• lt/identifierBaggt
• ltcategoryBaggt lt/categoryBaggt
• lt/businessEntitygt

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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
• Its not clear that it really does what people
  really want they want to find services like Y
  or that do Z

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The Problem With UDDI and Plenty of Other
Situations

• Theres no universal, unambiguous way of
  describing what I mean
• Relational database idea of normalization
  doesnt 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 ? ? ji ??????
  murg
• Note that this mapping may be imprecise or
  situation-specific!
• Calling someone a chicken, vs. a chicken thats a
  bird

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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 dont need to transfer all data
  instead, we want to use the data at one source to
  help answer a query over another source

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Lazy Evaluation A Virtual View
Browser/App
Server(s)
Source1.xml
XQuery
VirtualXML doc.
QueryForm
XQuery
Source2.xml
Source1.xml
XSLT
HTML
QueryResults
ComposedXQuery
Source2.xml
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Lets Look at Some SimpleMappings

• Beginning with examples of using XQuery to
  convert from one schema to another, e.g., to
  import data
• First lets review what our mappings need to
  accomplish

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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
  dont 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
• Its a tough job, but often things can be mapped

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Example Schemas

• Bobs Movie Database
• ltmoviegt lttitlegtlt/titlegt ltyeargtlt/yeargt
  ltdirectorgtlt/directorgt lteditorgtlt/editorgt
  ltstargtlt/stargtlt/moviegt

• Marys Art List
• ltworkOfArtgt ltidgtlt/idgt lttypegtlt/typegt
  ltartistgtlt/artistgt ltsubjectgtlt/subjectgt
  lttitlegtlt/titlegtlt/workOfArtgt

Want to map data from one schema to the other
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Mapping Bobs Movies ? Marys Art

• Start with the schema of the output as a
  template
• ltworkOfArtgt ltidgtilt/idgt lttypegtylt/typegt
  ltartistgtalt/artistgt ltsubjectgtslt/subjectgt
  lttitlegttlt/titlegtlt/workOfArtgt
• Then figure out where to find the values in the
  source, and create XPaths

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The Final Schema Mapping

• Marys Art ? Bobs Movies
• for m in doc(movie.xml)//movie, a in
  m/director/text(), i in m/title/text(),
  t in m/title/text()return ltworkOfArtgt
  ltidgtilt/idgt lttypegtmovielt/typegt
  ltartistgtalt/artistgt lttitlegttlt/titlegt lt/work
  OfArtgt

Note the absence of subjectWe had no reasonable
source,so we are leaving it out.