1. Web Services

2. Definition Many definitions
“…technologies that allow for making connections”
“a software system designed to support interoperable m2m interaction over the network”
“..a service offered by an electronic device to another electronic device, communicating with each other via WWW”
Web services may use SOAP protocol over http (or ftp, smtp, ..) for their communication
http is utilized for m2m communication and for transferring information in XML or JSON
Has an interface described in WSDL or REST (WADL, RAML, OAS/OpenApi) machine-readable format
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3. Service
A function that is well defined, self contained, does not depend on the context or state of other services, can be accessed remotely and updated independently
Services carry out a function, such as producing data, validating a customer, or retrieve a credit card statement
Web services: connection technology of SOA
Most APIs in cloud computing are implemented as SOAP or RESTful Web services
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4. Web Services vs Websites
A Web site is typically intended for human consumption (human-to-computer interaction), whereas web services are typically intended for computer-to-computer interaction
A Web service may have GUI or API, a Web site may implement Web Services in the background
Paypal has both GUI and implements web services (e.g. for payments, use management etc.)
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5. A Service example
Basic service oriented architecture: The service provider returns a response message to the service consumer (client)
Simple data, message passing
The request and subsequent response connections are defined in some way that is understandable to both the service consumer and service provider
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6. Service Oriented Architecture (SOA)
Architecture based on services: All functional components of a system are viewed as services that communicate with each other through well defined interfaces by message passing
One of the keys to SOA architecture is that interactions occur with loosely coupled services that operate independently
SOA architecture allows for service reuse, making it unnecessary to start from scratch when developing an application or when upgrades and other modifications are needed
This is a benefit to businesses that seek ways to save time and money
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7. SOA Patterns A SOA building block can play one of the following roles
Service broker: Also known as service registry; Its main functionality is to make the information regarding the web services available to any potential requester (public or private brokers)
Service consumer: It locates entries in the broker registry (using find operations), binds to the service provider, invokes its web service
Service provider: It creates a web service and provides its information to the service registry, provides responses to consumers
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8. Service Registry (Broker)
UDDI is a registry for the discovery of services described by WSDL
WSDL is an XML-based description language for services
SOAP is the protocol for exchanging information in the communication of Web services over http
UDDI, WSDL, SOAP, XML are platform and language independent
Procedure:
A service provider describes its services using WSDL
Publishes its service in the registry (UDDI)
A service consumer issues queries to the registry to locate a service
The registry returns WSDL description
The consumer knows how to use and communicate with the service
The consumer uses SOAP to send a request to the service provider
The provider returns a response to the service consumer
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9. Web Services basic architecture
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10. SOA Principles 1/2
Standardized service contract: services adhere to the same design principles, usually expressed as WSDL, REST, … documents, XML schema
Service reference autonomy (loose coupling): services are independent and control the functionality they provide on their own
Service location transparency (loose coupling): services can be called from anywhere
Service abstraction: services as black boxes (inner logic is hidden)
Service granularity: services have adequate size and scope (provide functionality relevant to request)
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11. SOA Principles 2/2
Service composability: can be used to compose other services
Service discovery: come with communicative metadata by which they can be discovered and interpreted
Service re-usability: promote re-use of code
Service encapsulation: may get encapsulated or become part of a SOA
Service statelessness: services are stateless and simply return the requested value or an exception, each request is unrelated to any previous request
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12. SOA and Cloud Computing
Complementary technologies
While SOA exhibited varying success in the past, the movement to cloud computing provides some renewed value to SOA
Clouds are typically API or service-driven, and therefore service-oriented
As cloud computing becomes more popular, more enterprises will use SOA, which includes the use of service directories, orchestration, and other technologies related to SOA
FIWARE, ORACLE provide collections of RESTful services (IDM, CB)
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13. WSDL
XML format that describe services as a set of service end-points for data/message exchange
Service End-Point: Describes a service and how it is bound to specific IP addresses / interface
declares all abstract methods that are exposed to the client
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14. WSDL Documents (XML) Specifies location and methods of service using
<types> XML data types used by the service
<message> data elements for each operation
<portType> operations that can be performed with the messages involved
<binding> defines protocol, operations and data format for each port type
Port: end-point as combination of binding + IP
Service: a collection of end-points
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15. Structure of WSDL document
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16. WSDL Example 1/2
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17. WSDL Name Spaces
xmlns: set default name spaces for the entity being described
targetNameSpace: the name space for your web service (points to the WSDL of the service)
tns: same URI as targetNameSpace
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18. WSDL Example 2/2
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19. Example Explanation 1/2 Definitions: “HelloService”
Type: Using built-in data types and data types defined in XML schema
Message:
“SayHelloRequest”: “firstName” parameter
“SayHelloResponse”: “greeting” return value
Port Type: “sayHello” operation that consists of a request and a response service
Binding: Direction to use the SOAP http protocol
Service: operations available at
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20. EncodingStyle Attribute
Can be used to indicate the encoding style used for the header entries, either simple or compound data types
Compound types: structure array, string, enumeration
Indicate the serialization rules used in a SOAP message
The attribute value is an ordered list of one or more URIs identifying the serialization rule or rules that can be used to de/serialize the SOAP message
Examples of values are:
"
"
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21. XML Serialization
Serialization is the process of converting an object into a form that can be transported
Serialize an object and transport it over the internet using http between a client and a serve
Deserialization reconstructs the object from the stream
XML serialization serializes only the public fields and property values of an object into an XML stream
XML serialization does not include type information
Eg., a Book object that exists in the Library namespace, there is no guarantee that it is deserialized into an object of the same type.
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22. SOAP
Simple Object Access Protocol (SOAP): a protocol based on XML for exchanging information among computers
Specifies how to communicate / pass messages using http
Initially used for remote procedure calls
Similar to CORBA, DCOM, Java RMI but SOAP messages are written in XML (language / platform independent)
Communication protocol for internet
Extends http for XML messaging specifying what information is sent
Provides data transport for Web services
Can also used for document exchange
Language / platform independent
Can be used with Java, PHP, …
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23. SOAP Message XML document with elements
Envelope: defines start/end of message, identifies the XML document as SOAP message
Header: attributes for processing the message
Body: message that contains call/response information
Fault: information about errors that occur while processing the message
All elements are declared in the default namespace for the SOAP envelope
Default name space for SOAP data
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24. SOAP Message Structure
Provides an “envelope” for sending Web service messages over internet (typically http)
Envelope parts:
Optional header
Body (message)
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25. SOAP Message Structure
XML namespaces disambiguate SOAP identifiers
from application specific identifiers.
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26. SOAP Header Element Application specific information
mustUnderstand Attribute: indicates whether the header is mandatory for the recipient
processsoap:mustUnderstand="0|1”
Actor Attribute: address of recipient of the Header soap:actor='
EncodingStyle Attribute: define the data types and serialization rules
soap:encodingStyle="
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27. SOAP Fault Element
Indicates error messages (optional) and has sub-elements
<faultcode> identifies the fault
VersionMissmatch: invalid namespace found
MustUnderstand: an element with “1” attributed was not understood
Client: message incorrectly formed or contained fault information
Server: problem with server so the message could not proceed
<faultstring> human readable information
<faultactor> who caused the fault
<detail> fault specific information
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28. SOAP Request for Price of Apples
m:GetPrice, m:Price are application specific
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29. SOAP Response
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30. SOAP example: GetQuotation request
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31. SOAP example: Quotation response
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32. SOAP example: GetStockPrice request
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33. SOAP example: GetStockPrice response
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34. Representation State Transfer (REST)
REST is a lightweight alternative to SOAP Like Web services
Platform independent
Runs on http
Language independent
Can be used with most programming environments like C#, Java, JavaScript, Perl, PHP, Python …
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35. Restful Web Services
In REST everything is a resource which is identified by its URI
A URI is of following format
<protocol>://<service-name>/<ResourceType>/<ResourceID>
A REST server provides access to resources and REST client accesses and presents resources
A RESTful web service usually defines a URI, provides resource representation such as JSON and a set of http methods
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36. How Simple is REST ?
SOAP to query a phone book for the details of a given user
REST (no “body”, its just a URL sent to server using an http POST request)
Can handle requests with multiple parameters
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37. Definition of REST 1/2
Client-Server: Client and server application can evolve separately without any dependency on each other (also SOA)
Stateless: The service doesn't store information from previous requests (also SOA)
A request from a client contains the necessary information to interact with the service
Cacheable: Response messages are labeled as cacheable or non-cacheable
If cacheable, a client is given the right to cache and reuse response data for later, equivalent requests
Performance improvement: reduces the number of requests to server as clients may instantly reuse existing information without the need to send a new request
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38. Definition of REST 2/2
Uniform Interface (HATEOAS): A client needs min or no prior information on how to interact with server
Server's responses include hyperlinks that a client can use to discover all the available actions and resources it needs
Simplifies the service architecture
Every component of the service can evolve independently
Layered System: Architecture is composed of layers
Each component cannot “see” beyond the immediate layer with which it is interacting
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39. HATEOAS A client sends a message in the form of a HTTP request ..
.. server responds in form of HTTP response
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40. REST vs SOAP Unlike SOAP services, REST is not bound to XML
CSV: comma separated values
JSON: JavaScript Object Notation (key value pairs)
Html is acceptable, human readable format, not machine processable
SOAP advantages include type safety of XML requests (although REST can still use XML)
REST advantage is ease of implementation, lightweight approach, better performance
REST: The service producer and service consumer need to have a common understanding of the context as well as the content being passed along as there is no standard set of rules to describe the REST Web services interface
SOAP: WSDL to define messages, operations, bindings and URI of service which can be more useful for publishing services to the world
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41. Representation of Resources
REST treats every content as a resource
Text files, html pages, images, binary data ..
REST uses XML
And JSON
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42. REST - Statelessness
A RESTful Web service should not keep a client state on server
Will always return user whose id is 1
Advantages of Statelessness
Web services can treat each method request independently
Web services need not to maintain client's previous interactions
It simplifies application design
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43. HTTP Methods
Following well known HTTP methods are commonly used in REST based architecture
GET - Provides a read only access to a resource
PUT - Used to create a new resource
DELETE - Used to remove a resource.
POST - Used to update a existing resource or create a new resource.
OPTIONS - Used to get the supported operations on a resource
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44. More REST Examples / http Methods
A Web service for “user management” with functionalities
Define REST service in Java using
Jersey REST Web Services framework for developing RESTful Web services in Java
Tomcat Web server for Java Servlet, http Web server in which Java code will run
PHP, Node.js, Perl, Python … can used as well to define RESTful services
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45. HTTP Method, URI and Operation
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46. HTTP Request Verb: HTTP methods (GET, POST, DELETE, PUT)
HTTP request’s major parts:
Verb: HTTP methods (GET, POST, DELETE, PUT)
URI: to identify the resource on server
HTTP Version: for example HTTP v1.1 .
Request Header: Contains metadata for the HTTP Request message as key-value pairs. For example, client ( or browser) type, format supported by client, format of message body, cache settings etc.
Request Body: Message content or Resource representation.
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47. HTTP Response HTTP Response parts:
Status/Response Code: 404 means resource not found and 200 means response is ok.
HTTP Version: for example HTTP v1.1 .
Response Header: Contains metadata for the HTTP Response message as key-value pairs. For example, content length, content type, response date, server type etc.
Response Body: Response message content or Resource representation.
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48. References http://W3schools.com
Web services, Service Oriented Architectures and Cloud Computing, Douglas K. Bary,
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