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• Distributed Systems cover web services

Introduction

• A web service offers a service interface, allowing clients to interact with servers more generally than web browsers.
• Clients access web service operations via requests/replies formatted in XML, typically sent over HTTP.
• Users require secure methods to create, store, modify, and exchange documents online.
• TLS secure channels do not fulfill all necessary security needs.
• XML security aims to close security gaps.
• Web services give an infrastructure for richer, more structured client-server interoperability.
• Web services enable client programs in different organizations to interact without human supervision.
• External data representation and message marshalling between clients/web services use XML.
• SOAP (Simple Object Access Protocol) dictates XML usage rules for packaging messages for request-reply protocols.
• Web services are a main paradigm for distributed systems programming.
• They facilitate business-to-business integration
• They enable Service Oriented Architecture (SOA).
• Web services are adopted by the grid computing community, and exist internally or externally (in the cloud).
• Web Services began when Bill Gates introduced BizTalk in 1999, later renamed .NET.
• The vision to provide technologies enabling software in different locations and languages, on different platforms, to connect and interoperate.

Service-Oriented Architecture (SOA)

• SOA is a design approach in distributed systems integrating loosely coupled, reusable services.
• These self-contained and modular services communicate over a network to perform specific tasks.
• SOA prioritizes interoperability, flexibility, and scalability, making it ideal for complex, distributed systems.

Key Concepts of SOA

• Services are fundamental in SOA, each encapsulating a specific business function such as payment processing.
• Services are independent and are developed, deployed, and maintained separately.
• Loose coupling allows services to interact through well-defined APIs.
• Changes to one service do not directly impact others, as long as the interface remains consistent.
• Interoperability enables services to communicate using standard protocols like HTTP, SOAP, REST, or messaging queues.
• Services written in different languages or running on different platforms can work together.
• Reusability means services are designed for use across multiple applications or systems.
• This reduces redundancy and promotes efficiency.
• Discoverability Services are often registered in a service registry/directory (e.g., UDDI).
• This allows components to dynamically find and use them.
• Abstraction hides the internal implementation of a service from consumers.
• Consumers only need to know the service's interface and functionality.
• Composability means services can be combined or orchestrated for complex workflows or business processes.

Components of SOA

• The Service Provider creates and exposes the service.
• The Service Provider implements the service logic and makes it available for consumption.
• The Service Consumer uses the service.
• The Service Consumer invokes the service by sending requests and receiving responses.
• The Service Registry is where services are published and discovered.
• The Service Registry acts as a central repository for service descriptions.
• The Service Contract defines the interface, communication protocols, and data formats to interact with the service.
• The Enterprise Service Bus (ESB) is middleware that facilitates communication between services.
• The ESB Handles message routing, transformation, and mediation.

Communication Protocols in SOA

• SOAP (Simple Object Access Protocol) is a protocol for exchanging structured information using XML.
• SOAP is often used in enterprise-level SOA implementations.
• REST (Representational State Transfer) is a lightweight, stateless architecture using HTTP methods (GET, POST, PUT, DELETE).
• REST is commonly used in modern SOA implementations.
• Messaging Queues are used for asynchronous communication between services.
• RabbitMQ and Apache Kafka are examples of messaging queues.
• Two types (approaches): SOAP (Simple Object Access Protocol) Based Web Services
• Two types (approaches): REST style web services

SOAP-Based Web Services

• Include infrastructure and components like applications and directory or security services.
• Other components include web services, SOAP, URIs, XML, HTTP, SMTP.
• The application represents software providing or consuming web services, interacting via SOAP messages.
• The directory service acts as a web service repository with metadata, helping clients discover/locate available services and endpoints.
• Security choreography defines the sequence of security-related interactions between parties in web service communication, ensuring secure authentication, encryption, and data integrity during SOAP message exchange.
• WSDL (Web Services Description Language) is an XML-based language used to detail web service functionalities, operations, messages, and supported communication protocols.
• SOAP aids structured information exchange during web services implementation.
• SOAP Defines the XML message format and their exchange rules between applications.
• URLs (Uniform Resource Locators) identify web resources.
• URIs uniquely identify resources, with URNs being a subtype of URIs for persistent identification.
• XML structures data hierarchically and encodes SOAP message content in a machine-readable format.
• HTTP transfers data on the web and carries SOAP messages between clients/web services, often over the internet.
• SMTP sends/receives email messages and potentially transmits SOAP messages over email, less common than HTTP in web service communication.
• They provide service interfaces.
• Communicating using request and reply messages made of SOAP or some other XML document
• There is an Interface Definition Language (IDL) called WSDL (Web Service Definition Language)
• May be looked up in a web service UDDI registry
• UDDI is a Universal Directory and Discovery Service
• Languages are independent
• It may be synchronous or asynchronous
• In general, use either a synchronous request-reply pattern of communication or asynchronous messages.
• Allows for varied patterns via packaging of single one-way messages.
• SOAP holds RPC style parameters or entire documents and can be used over SMTP, TCP, UDP, or HTTP.
• Uniform Resource Identifiers (URIs) URLs
• Uniform Resource Locator URI's include location information, therefore resources pointed to by URL's are hard to move
• Uniform Resource Name URI's lack location information
• A uniform resource identifier (URI) could be a uniform resource locator (URL), uniform resource name (URN), or both.
• A 'travel agent service' combines other web services
• SOAP defines a scheme using XML to represent request and reply messages and communication of documents.
• The SOAP specification states, how XML should be use, rules of elements, and pattern combinations.
• SOAP's usual protocol is TCP.
• Each SOAP message is carried in an “envelope”
• The SOAP envelope contains an optional header along with a mandatory body

REST:

• REST (REpresentational State Transfer) is an architectural style that standardizes the way computer systems communicate over the web, making system integration easier.
• REST-compliant (RESTful) systems are stateless and separate client/server concerns.
• Statelessness ensures independence and self-containment in client-server requests, with the server not storing client data between requests.
• Separation of Client and Server enables independent implementation of the client and server.
• Code can be changed at any time.
• Scalability simplified by separating concerns.

REST Architectural Principles

• Consist of constraints:
  • Client - Server
  • Stateless
  • Cache
  • Uniform Interface
  • Layered System
  • Code-On-Demand (Optional)
• Principle: Addressability can be restricted to HTTP.
• Each HTTP request uses a URI.
• The format of a URI is scheme?//host:port/path?queryString#fragment
• The scheme may or may not be HTTP.
• The scheme may be FTP or HTTPS.
• The host field can be a DNS name or IP address.
• e.g. using HTTP implies port 80
• The path is a set of text segments delimited by the "/"
• The query String is a list of parameters represented as name=value pairs
• Each pair is delimited by an "&"
• The fragment shows a particular place in a document
• A space is represented with the '+' characters
• Other characters use % followed by two hex digits