Chair of Software Engineering Languages in Depth Series: Java Programming Prof. Dr. Bertrand Meyer Web Services Marco Piccioni
What’s a web service? “A software system designed to support interoperable machine to machine interaction over a network” (W3C) A self-contained, self-describing client-server system that enforces communication via XML messages A web API accessible over a network, and executed on a remote system hosting the requested service Languages in Depth series: Java Programming 2
The Web services protocol stack A collection of protocols that are used to define, locate, implement, and make web services interact with each other Description protocol layer: WSDL Discovery protocol layer: UDDI XML messaging protocol layer: SOAP, … Transport protocol layer: HTTP(S), SMTP, FTP, RSS, XMPP Languages in Depth series: Java Programming 3
A high- level view 1 2 Endpoint Endpoint 3 Picture from http://en.wikipedia.org/wiki/Web_services Languages in Depth series: Java Programming 4
The WSDL protocol Web Services Description Language XML-based, platform independent 2.0 is endorsed by W3C Describes A web service public interface Protocol bindings details Configuration Used For generating client and server code By Microsoft for .NET Languages in Depth series: Java Programming 5
The WSDL protocol (2) Web services are described as collections of network endpoints, or ports A port type is defined: As an abstract collection of supported operations By associating a network address with a reusable binding Languages in Depth series: Java Programming 6
The WSDL protocol (3) A reusable binding is a concrete protocol and data format specification for a specific port type Through the binding, operations and messages are bound to a concrete network protocol and message format A message is an abstract description of the exchanged data Ports and messages abstract definition is separated from their implementation, allowing reuse Languages in Depth series: Java Programming 7
The UDDI protocol Universal Description, Discovery and Integration XML-based, platform independent A registry for businesses to list themselves on the www Each business publishes a list of services Each UDDI business registration consists of: White pages: address, contact and known identifiers Yellow pages: industrial categorizations Green pages: technical info about exposed services Languages in Depth series: Java Programming 8
The UDDI protocol (2) Written in Y2K, enforces a vision in which consumers link to providers through a public brokerage system By the end of 2005, 70% of Fortune 500 companies were planning to use it In January 2006, IBM, Microsoft and SAP announced they were closing their public UDDI nodes Languages in Depth series: Java Programming 9
The UDDI protocol (3) “ The idea was that companies could publish how they wanted to interact, and other companies could find that information and use it to establish a relationship. Needless to say, this isn't how companies do business. There's always a human element to establishing a relationship.” Jason Bloomberg, senior analyst at ZapThink Languages in Depth series: Java Programming 10
The UDDI protocol (3) Anyway, according to Microsoft and IBM The interoperability and robustness of the UDDI was proven What now? Mostly used from within-the firewall (intranet) Used as metadata management standard for SOA (more on this later) Languages in Depth series: Java Programming 11
The SOAP protocol Service Oriented Architecture Protocol (former Simple Object Access Protocol) Simple and extensible XML-based communication protocol Platform and language independent Basic message-wrapping framework Has bindings to underlying protocols like HTTP, HTTPS, SMTP, FTP, RSS, XMPP Languages in Depth series: Java Programming 12
Remote Procedure Call The most used SOAP messaging pattern Enables remote calls to be treated as local calls from the programmer In object-oriented terms: Remote Method Invocation Microsoft DCOM, .NET remoting, OMG‘s CORBA and Java RMI API all use RPC Languages in Depth series: Java Programming 13
Remote Procedure Call (2) Implements the classical client-server model of distributed computing An RPC is initiated by a client (network node) sending a request to a server (another network node) The server immediately responds, while the client is blocked Remote calls can fail due to unpredictable network issues Languages in Depth series: Java Programming 14
Java RPC-style implementation JAX-RPC (Java API for XML-based Remote Procedure Calls) provides a standard way for accessing and exposing RPC-style SOAP web services JAX-RPC 2.0 has been renamed to JAX-WS 2.0 (Java API for XML Web Services) Apache Axis, fully JAX-RPC compliant, is the most popular JAX-RPC flavor Languages in Depth series: Java Programming 15
High-level view of Java JAX A Java program invokes a method on a stub (local object representing the remote service) The stub invokes routines in the JAX-WS Runtime JAX-WS converts the RMI into a SOAP message JAX-WS transmits the message as an HTTP request Therefore the web service can be implemented on the server side as a servlet or EJB container Languages in Depth series: Java Programming 16
A general framework: SOA Service Oriented Architecture A message is the basic unit of communication Focus is on WSDL contract, specifying Header (name, version, owner, responsible, type…) Functional (functionality accomplished, service operations, invocation details) Non-functional (security constraints, allowed failure rate, transactional, service level agreement, semantics) Languages in Depth series: Java Programming 17
One more style of use: RESTful REpresentational State Transfer is a collection of network architecture principles Application state and functionality are divided into resources Every resource is uniquely addressable using a universal syntax for use in hypermedia links All resources share a uniform interface for the transfer of state between client and resource A constrained set of well-defined operations A constrained set of content types A client-server, stateless, cacheable and layered protocol Does non use SOAP Languages in Depth series: Java Programming 18
Quiz What could be an example of RESTful design? Languages in Depth series: Java Programming 19
Quiz solution What could be an example of RESTful design? Yes, the World Wide Web, using URIs, MIME types, HTTP, HTML, DNS, … Languages in Depth series: Java Programming 20
RESTful vs. RPC RESTful resources Commands are defined in simple terms of resources to be retrieved, stored, get, set Difficult to do many joins RPC commands Commands are defined in methods with varying complexity depending on “standard” RESTful nouns exchange resources and concepts RPC verbs exchange methods Languages in Depth series: Java Programming 21
Spring Web Services support Spring Web Services is a separate Spring subproject 1.5.1 released on 04/05/2008 Document-driven, contract-first development Loose coupling between contract and implementation Standard JAXP API support (DOM, SAX,…) Mappings from incoming XML requests to any object Flexible XML marshalling (using JAXB, Castor, …) Support for security (sign, encrypt, decrypt and authenticate against SOAP messages) Languages in Depth series: Java Programming 22
Contract-first and contract-last WS Contract-First approach Write contract (WSDL) first, and use the language (Java) to implement that contract Contract-Last approach Write (Java) code first (interfaces), and let the Web Service contract (WSDL) be generated from that Languages in Depth series: Java Programming 23
Issues with contract-last development Object/XML impedance mismatch Fragility Performance Reusability Versioning Languages in Depth series: Java Programming 24
Object-XML impedance mismatch XML (and XSD) are hierarchical, OO languages are modeled by graphs XSD extensions are not like inheritance A XSD restriction that uses a regular expression is lost Unportable types e.g. java.util.TreeMap, time and dates Cyclic graphs A refers to B that refers to A Languages in Depth series: Java Programming 25
Fragility and performance Not all SOAP stacks generate the same web service contract from a Java contract By automatically transforming Java into XML, there is no guarantee on what is sent over the wire (dependencies can be very deep and unwanted) Using contract-first you can choose what information has to be sent Languages in Depth series: Java Programming 26
Reusability and versioning Defining your schema in a separate file you can reuse it in different scenarios Starting from Java, when a contract evolves you have no choice but to assign a different name to your interface. The old contract must be kept around for clients that need the older version Starting from contracts, you can implement the old and the new version of the contract in one class using XSLT for conversions Languages in Depth series: Java Programming 27
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