Internet communication protocols are an integral part of the Internet of Things. Various tools are used to transfer and receive data among which an important interface are the APIs (Application Programming Interface). Here in this article, we will be shining light upon the SOAP (Simple Object Access Protocol) which is used for web services.
What is a Web service?
According to the definition, “A web service is a collection of open protocols and standards used for exchanging data between applications or systems built on top of open standards such as TCP/IP, HTTP, XML, Java and HTML”.
however, The working of a web service depends on XML to tag the data, SOAP for the transferring of messages and WSDL to deliver information about the availability of the service.
So, To know more about web services, you can visit the link: https://www.tutorialspoint.com/webservices/web_services_architecture.htm
Introduction to XML:
XML stands for an Extensible Markup language. thus, As the name suggests, XML is extensible, i.e., you are free to your own variables. It is very similar to HTML, but HTML has pre-defined tags.
The previous line must be making you wonder, why do we need XML when we already have HTML?
XML tags help identify the data in contrast to HTML tags that are used to display data. so, This makes it easier to use XML for data transfer by sending requests and responses. It can also be used to arrange the data for data handling.
The architecture of an XML document is given in the diagram below:
Src: click here
SOAP Web Service:
As mentioned earlier, SOAP stands for Simple Object Access Protocol. Which was later reduced to SOAP v1.2. although, It was developed to provide standard specifications for data exchange with the client applications. It builds an XML protocol over the HTTP.
however, The diagram below shows the various building blocks of a SOAP message (referred from W3 schools).
Describing the elements in detail:
- Envelope: It is thus the first bit of the building block which encloses the details of the message. It is thus a root element, which defines the beginning and end of the message.
so By rule, each envelope can contain only one body element.
though In the absence of an envelope, the v1.2-compliant SOAP processor generates a Version Mismatch fault.
- Header: The header element is a child element of the envelope. Information that is not part of the body element is put in the header. therefore, The header element requires to be using the same XML namespace by the SOAP specification.
The header element has the following child attributes:
- mustUnderstand: It is used to specify if the header entry must be processed or not.
- encodingStyle: This attribute is used to specify the data types that can also be used in the document. A SOAP message does not contain an encoding by default and has to be specified.
- actor: This attribute is also used to address the Header to an endpoint.
- Body element: This element contains the main information of the SOAP message that is processed by the client. Text that needs to be written in the body element must be inside the child elements of the body element. It is preferred if the child elements of the body are the same namespace-qualified.
- Fault element (optional): In case any error occurs during the processing, the fault element returns the relevant information about the error to the body. There can only be one fault element in the body. The following messages are part of the fault code.
- <faultCode>: used to return a class of predefined errors.
- <faultString>: Text used to describe the error
- <faultActor>: Text which tells which node of the message path caused the error.
- <detail>: Caries application-specific error messages.
SOAP Communication Protocol:
The standard remote procedure protocol used earlier had a few limitations.
- The server hosting and calling methods needed to be written in the same language that the message was written in.
- There was no standard protocol for efficient communication.
- Could be blocked easily by firewalls as they needed separate ports to facilitate client communication.
To overcome these obstacles, there were changes made to the existing model for SOAP.
- Data was encapsulated to a SOAP message in a method called Marshalling by the client
- The process of changing the data to original form for the client is called Unmarshalling which is done by the server.
SOAP vs REST:
REST is a lighter – weight alternative of the SOAP protocol. SOAP seemed syntax heavy to many developers because to run SOAP each time, an XML needs to be created. With REST, only the URL is required to make a request. REST protocols use the HTTP methods GET, PUT, POST, DELETE.
The differences between REST and SOAP are explained in the tabular column below:
| SOAP Protocol | REST Protocol |
| Independent of transport protocols | Uses HTTP |
| Better for a distributed enterprise environment. | Point to point communication |
| Has error handling methods | No built-in error handling but has an easy learning curve to build one. |
| Slow to process | Efficient (Can use smaller message formats). |
| Invokes RPC methods. | Calls services using HTTP methods |
Even though most web services have evolved to the REST services, many big-agencies still used SOAP because of its built-in security and reliability functions (For eg: Banks where security is more crucial than performance and speed of the message transfer.
Written by: Suchitra Srinivasan.
Reviewed by: Batta Pruthvi
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