What Is SOAP (Simple Object Access Protocol)?

SOAP (Simple Object Access Protocol) is a messaging protocol for exchanging structured information in web services. It relies on XML for its message format and typically uses HTTP/HTTPS for message negotiation and transmission.

What Is SOAP (Simple Object Access Protocol)?

SOAP, or Simple Object Access Protocol, is a protocol used for exchanging structured information in the implementation of web services. It employs XML (Extensible Markup Language) to format its messages, ensuring that they are readable and understandable across different systems and platforms. The protocol typically uses HTTP (HyperText Transfer Protocol) or HTTPS (HTTP Secure) to facilitate communication, allowing messages to be sent and received over the internet.

How Does SOAP Work?

SOAP (Simple Object Access Protocol) works by facilitating communication between applications over a network, typically the internet. Here's how it functions:

Message structure. SOAP messages are XML-based and consist of three main parts: an envelope, a header (optional), and a body. The envelope is the outermost element that defines the start and end of the message. The header contains optional attributes related to the message, such as authentication information. The body includes the actual message content or the data being exchanged.
Encoding rules. SOAP uses a set of encoding rules to define the data types used in the message. These rules ensure that the data is represented in a consistent manner, making it understandable across different platforms and programming languages.
Transport protocol. Although SOAP is transport-independent, meaning it can be used over various transport protocols such as SMTP or FTP, it most commonly uses HTTP or HTTPS. This allows SOAP messages to be easily sent and received over the web.
Communication. When a SOAP request is sent, it typically includes a remote procedure call (RPC) or a message that a server can process. The server processes the request and sends back a SOAP response. This response also follows the same XML structure and includes the result of the processing.
Interoperability. The XML format of SOAP messages ensures that different systems can interpret the messages correctly regardless of their underlying technology. This interoperability is one of SOAP’s key strengths, enabling diverse systems to communicate seamlessly.
Error handling. SOAP includes a mechanism for error handling within its message structure. If an error occurs during the processing of a SOAP message, the server includes an error element in the SOAP response, providing information about the nature of the error.
Security. SOAP can leverage various security protocols to ensure the integrity and confidentiality of messages. When using HTTPS, messages are encrypted during transmission. Additionally, the WS-Security standard can be used to include security tokens, digital signatures, and encryption within the SOAP header.

SOAP History

SOAP (Simple Object Access Protocol) was developed in the late 1990s by Dave Winer, Don Box, Bob Atkinson, and Mohsen Al-Ghosein to enable communication between applications over the internet using XML. The protocol was initially designed to work with HTTP and quickly gained traction due to its ability to facilitate interoperability between different platforms and programming languages.

In 2000, SOAP 1.1 was submitted to the World Wide Web Consortium (W3C), which eventually led to the release of SOAP 1.2 in 2003, incorporating feedback and improvements. Over time, SOAP became a key component of the web services stack, commonly used alongside other standards like WSDL (Web Services Description Language) and UDDI (Universal Description, Discovery, and Integration).

Despite the rise of simpler RESTful services, SOAP remains widely used in enterprise environments where robust security, transactional reliability, and extensive functionality are critical.

SOAP Example Code

SOAP messages are written in XML, making them both human- and machine-readable. The structure of the SOAP message is highly standardized, ensuring consistency across different web services.

Each message includes an envelope element that contains a body and optionally a header. The body contains the message's main content, including the method being called and the parameters being passed or the response data. Namespace declarations help avoid element name conflicts and provide context for the elements used within the message.

Here is a simple example of a SOAP request and response. This example demonstrates how a client might request information from a web service that provides weather information.

SOAP Request

POST /WeatherService HTTP/1.1

Host: www.example.com

Content-Type: text/xml; charset=utf-8

Content-Length: length

SOAPAction: "http://www.example.com/GetWeather"

<?xml version="1.0"?>

<soap:Envelope xmlns:soap="http://www.w3.org/2003/05/soap-envelope" xmlns:ws="http://www.example.com/weather">

   <soap:Header/>

   <soap:Body>

      <ws:GetWeather>

         <ws:City>San Francisco</ws:City>

      </ws:GetWeather>

   </soap:Body>

</soap:Envelope>

Explanation

POST /WeatherService HTTP/1.1: This line indicates that the client is making a POST request to the /WeatherService endpoint on the server.
Host: www.example.com: Specifies the server's hostname.
Content-Type: text/xml; charset=utf-8: Indicates that the message body is in XML format.
Content-Length: length: The length of the message body.
SOAPAction: "http://www.example.com/GetWeather": An optional header that indicates the intent of the SOAP HTTP request.

SOAP Message Structure

<?xml version="1.0"?>: XML declaration that specifies the XML version.
soap:Envelope: The root element of a SOAP message.
- xmlns

="http://www.w3.org/2003/05/soap-envelope": Namespace declaration for SOAP.

xmlns

="http://www.example.com/weather": Namespace declaration for the web service.

soap:Header/: (Optional) Contains any header information, such as authentication details.
soap:Body: Contains the main message content.
- ws:GetWeather: The operation or method being called.
  - ws:CitySan Francisco</ws

>: Parameter being passed to the operation.

SOAP Response

HTTP/1.1 200 OK

Content-Type: text/xml; charset=utf-8

Content-Length: length

<?xml version="1.0"?>

<soap:Envelope xmlns:soap="http://www.w3.org/2003/05/soap-envelope" xmlns:ws="http://www.example.com/weather">

   <soap:Header/>

   <soap:Body>

      <ws:GetWeatherResponse>

         <ws:Temperature>68</ws:Temperature>

         <ws:Conditions>Sunny</ws:Conditions>

      </ws:GetWeatherResponse>

   </soap:Body>

</soap:Envelope>

Explanation

HTTP/1.1 200 OK: The HTTP status code indicating the request was successful.
Content-Type: text/xml; charset=utf-8: Indicates that the message body is in XML format.
Content-Length: length: The length of the message body.

SOAP Message Structure

<?xml version="1.0"?>: XML declaration that specifies the XML version.
soap:Envelope: The root element of a SOAP message.
- xmlns

="http://www.w3.org/2003/05/soap-envelope": Namespace declaration for SOAP.

xmlns

="http://www.example.com/weather": Namespace declaration for the web service.

soap:Header/: (Optional) Contains any header information, such as authentication details.
soap:Body: Contains the main message content.
- ws:GetWeatherResponse: The response to the operation or method called.
  - ws:Temperature68</ws

>: The temperature in San Francisco.

ws:ConditionsSunny</ws

>: The weather conditions in San Francisco.

SOAP Practical Applications

SOAP (Simple Object Access Protocol) is a protocol for exchanging structured information in web services. Its XML-based messaging framework and support for various communication protocols make it a robust choice for a variety of applications. Here are some practical applications of SOAP:

Enterprise application integration. SOAP is widely used in enterprise environments to integrate disparate applications. It allows different software systems to communicate with each other, regardless of their underlying platforms, by providing a standardized messaging protocol. This is particularly useful for large organizations that rely on a mix of legacy systems and modern applications.
Web services. SOAP forms the backbone of many web services, enabling applications to interact over the internet. It supports complex operations and provides built-in error handling, making it suitable for services that require high reliability and security, such as financial transactions and ecommerce platforms.
Cloud services. Many cloud service providers use SOAP to offer their services. SOAP's protocol independence and extensibility make it ideal for cloud environments where interoperability and scalability are critical. It is often used in conjunction with other standards like WSDL (Web Services Description Language) to describe the services and UDDI (Universal Description, Discovery, and Integration) for service discovery.
Distributed computing. In distributed computing environments, SOAP facilitates communication between different system components. It allows for the exchange of information and the execution of remote procedures, making it easier to build and manage distributed applications.
B2B integration. Business-to-business (B2B) integration often relies on SOAP to enable secure and reliable communication between companies. SOAP's support for various security protocols ensures that sensitive business data can be transmitted safely over the internet.
Mobile services. Although RESTful services are more common in mobile applications due to their simplicity, SOAP is still used in mobile services that require robust security and transactional support. It is often employed in mobile banking and other financial applications where data integrity and security are paramount.
Healthcare systems. In the healthcare industry, SOAP is used to enable interoperability between different healthcare systems and applications. It facilitates the exchange of patient data and other critical information, supporting standards like HL7 (Health Level Seven) for healthcare data exchange.

SOAP Advantages and Disadvantages

When considering the use of SOAP (Simple Object Access Protocol) for web services and application integration, it is important to weigh its advantages and disadvantages. Understanding the strengths and limitations of SOAP can help determine whether it is the right choice for a particular project or use case.

Advantages

SOAP offers several advantages that make it a valuable protocol for web services and application integration:

Platform and language independence. SOAP messages are based on XML, which is both platform and language agnostic. Applications written in different programming languages and running on different platforms can communicate with each other seamlessly using SOAP.
Extensibility. SOAP is highly extensible, allowing developers to include additional features such as security, transaction management, and message routing. This extensibility is facilitated through the use of header elements in the SOAP envelope, which can carry metadata and control information.
Standardization. SOAP is a well-established standard that is widely supported by various tools and technologies. Standardization ensures compatibility and interoperability across different systems and vendors, making it a reliable choice for enterprise-level applications.
Security. SOAP provides robust security features through standards such as WS-Security, which supports encryption, digital signatures, and authentication. These features are critical for applications that require secure data transmission, such as financial services and healthcare systems.
Reliability. SOAP supports message acknowledgment and delivery guarantees, essential for applications requiring reliable communication and in scenarios where message delivery must be confirmed, and data integrity must be maintained.
Built-in error handling. SOAP has a built-in error handling mechanism that allows for detailed error reporting, helping developers diagnose and resolve issues more efficiently and improving the overall reliability of the communication process.
Protocol flexibility. Although SOAP is most commonly used with HTTP/HTTPS, it is designed to be transport protocol agnostic. This means it can be used over other protocols such as SMTP, FTP, and more, providing flexibility in how messages are transmitted.
Complex operations support. SOAP is well-suited for complex operations and scenarios that involve multiple steps or interactions. Its structured messaging framework can handle intricate processes and data exchanges, making it ideal for enterprise-level workflows.

Disadvantages

While SOAP (Simple Object Access Protocol) offers several advantages, it also comes with some notable disadvantages that impact its suitability for certain applications:

Complexity. SOAP's XML-based messaging format can be complex and verbose, making it more difficult to read and write compared to simpler protocols like REST. This complexity can lead to increased development time and a steeper learning curve for developers.
Performance overhead. The extensive use of XML in SOAP messages results in larger message sizes, which can lead to higher latency and slower performance. The parsing and processing of these XML messages also require more computational resources.
Strict standards. SOAP adheres to strict standards and protocols, which can be both an advantage and a disadvantage. While these standards ensure interoperability, they also limit flexibility and can make it more challenging to implement custom solutions.
Higher bandwidth usage. Due to its verbose XML format, SOAP messages consume more bandwidth compared to more lightweight protocols like REST. This can be a concern in scenarios where network bandwidth is limited or costly.
Complex error handling. While SOAP provides built-in error handling mechanisms, they can be complex to implement and manage. The detailed fault structures and error codes can be cumbersome for developers to handle properly.
Less human-readable. The XML format used by SOAP is less human-readable compared to JSON, which is commonly used by REST. This can make debugging and manual inspection of messages more difficult.
Limited browser support. SOAP is not natively supported by web browsers, making it less suitable for web-based applications that rely on direct communication with the server from client-side code. This limitation often necessitates additional middleware or workarounds.
Tight coupling. SOAP's use of WSDL (Web Services Description Language) for defining services can lead to tight coupling between the client and server. Any changes in the service interface can require updates to both the client and server code, reducing flexibility and increasing maintenance efforts.

SOAP vs. Other Solutions

Here is a comparison overview of SOAP and other computing solutions for prospective users and organizations.

REST

SOAP (Simple Object Access Protocol) and REST (Representational State Transfer) are two distinct approaches to web services.

SOAP is a protocol that relies on XML for message format and typically uses HTTP or HTTPS for transmission, known for its robustness, strict standards, and built-in security and error handling. It is often used in enterprise environments requiring high security and transactional reliability.

In contrast, REST is an architectural style that uses standard HTTP methods (GET, POST, PUT, DELETE) and supports multiple formats like JSON, XML, and plain text, making it simpler, more flexible, and lightweight. REST's stateless nature and ease of implementation make it ideal for web and mobile applications where performance and scalability are crucial.

RPC

SOAP is a protocol that uses XML for its message format and typically relies on HTTP or HTTPS for transport, offering extensive support for security, transactions, and interoperability across diverse platforms. It is highly standardized and designed for complex, enterprise-level applications.

RPC, on the other hand, is a simpler protocol that allows a program to execute procedures on a remote server as if they were local calls, often using binary or text-based formats and various transport mechanisms.

While RPC is more straightforward and faster due to its simplicity and less overhead, it lacks the built-in security, extensibility, and comprehensive standards of SOAP, making it less suitable for complex and heterogeneous environments.

GraphQL

SOAP emphasizes standardization, security, and transactional reliability, making it ideal for complex, enterprise-level applications requiring robust error handling and extensive security measures. However, its XML-based messaging can be verbose and less efficient in terms of performance and bandwidth usage.

In contrast, GraphQL is a query language for APIs that allows clients to request exactly the data they need, leading to more efficient data retrieval and less over-fetching. GraphQL's flexibility and efficiency make it well-suited for applications with dynamic frontend requirements and those that benefit from a more interactive and agile development approach.

While SOAP excels in environments needing strict standards and security, GraphQL is favored for its simplicity, flexibility, and performance efficiency.