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Now, back to the business of today.

In my previous series, I explored everything you need to know and learn about API and API Designs: Different API Styles.

Today, we will discuss one API style called SOAP, we will look at how SOAP works and the different components of SOAP.

This comes from my Backend Engineering Hub under the API and API Design section. However, I’m only transferring the knowledge here and breaking it down in this series one topic at a time.

How Does SOAP Work?

In the world of API communication, Simple Object Access Protocol (SOAP) stands as one of the most robust and structured methods for exchanging data.

Introduced in the late 1990s, SOAP remains a powerful choice for enterprises requiring reliable and standardized communication between systems.

But how does SOAP work, and what makes it suitable for specific use cases?

Let's explore:

What is SOAP?

SOAP is a protocol for exchanging structured information to implement web services. Unlike REST, an architectural style, SOAP is a strict protocol defined by standards from the World Wide Web Consortium (W3C).

SOAP messages are XML-based, ensuring platform-agnostic communication between applications.

SOAP is ideal for scenarios requiring high security, ACID-compliant transactions, or complex operations, such as banking or telecommunication services.

Key Components of SOAP

SOAP works through a combination of the following components:

1. SOAP Envelope

The SOAP Envelope is the container for the message. It defines the start and end of the message and includes metadata about the message.

2. SOAP Header

The Header is optional and contains metadata such as authentication credentials, transaction ID, or routing information.

3. SOAP Body

The Body holds the actual message or the data to be processed. This is where requests and responses are encapsulated.

4. WSDL (Web Services Description Language)

WSDL is an XML-based contract that describes the operations a SOAP service offers, including the parameters, return types, and endpoints.

How SOAP Works

Step 1: Client Sends a Request

The client generates a SOAP request message, encapsulated in an XML document. This message includes the operation to be performed and the necessary parameters

<soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/">
  <soap:Header>
    <auth>
      <username>exampleUser</username>
      <password>securePassword</password>
    </auth>
  </soap:Header>
  <soap:Body>
    <GetUserDetails xmlns="http://example.com/">
      <UserID>123</UserID>
    </GetUserDetails>
  </soap:Body>
</soap:Envelope>

Step 2: Transmission via HTTP

The SOAP request is sent over HTTP or HTTPS to the server. Although HTTP is the most common transport protocol, SOAP can also work with SMTP, FTP, or other protocols.

Step 3: Server Processes the Request

On the server side:

• The SOAP message is parsed to understand the requested operation.

• The server performs the requested action using the provided parameters (e.g., fetching user data from a database).

• The server generates a SOAP response.

Step 4: Server Sends a Response

The response is sent back to the client in the form of another SOAP message.

Example SOAP Response:

<soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/">
  <soap:Body>
    <GetUserDetailsResponse xmlns="http://example.com/">
      <User>
        <UserID>123</UserID>
        <Name>John Doe</Name>
        <Email>[email protected]</Email>
      </User>
    </GetUserDetailsResponse>
  </soap:Body>
</soap:Envelope>

The client then parses the response and processes the returned data.

Advantages of SOAP

1. Platform Independence: SOAP is language and platform-agnostic, thanks to its reliance on XML.

2. Standardized Protocol: It follows strict standards, making it highly interoperable.

3. Security: SOAP supports WS-Security for encryption and authentication, making it ideal for secure transactions.

4. Built-In Error Handling: SOAP messages include fault elements to describe errors in a structured manner.

5. Extensibility: The header can carry additional metadata for custom functionality.

Disadvantages of SOAP

1. Verbosity: XML messages are larger compared to JSON, leading to higher overhead.

2. Complexity: SOAP’s rigid standards and XML syntax can be cumbersome to work with.

3. Performance: The verbosity and processing requirements of XML make SOAP slower than lightweight protocols like REST.

How Does SOAP Work?

In the world of API communication, Simple Object Access Protocol (SOAP) stands as one of the most robust and structured methods for exchanging data. Introduced in the late 1990s, SOAP remains a powerful choice for enterprises requiring reliable and standardized communication between systems. But how does SOAP work, and what makes it suitable for specific use cases? Let's explore.

What is SOAP?

SOAP is a protocol for exchanging structured information to implement web services. Unlike REST, an architectural style, SOAP is a strict protocol defined by standards from the World Wide Web Consortium (W3C). SOAP messages are XML-based, ensuring platform-agnostic communication between applications.

SOAP is ideal for scenarios requiring high security, ACID-compliant transactions, or complex operations, such as banking or telecommunication services.

Key Components of SOAP

SOAP works through a combination of the following components:

1. SOAP Envelope

The SOAP Envelope is the container for the message. It defines the start and end of the message and includes metadata about the message.

2. SOAP Header

The Header is optional and contains metadata such as authentication credentials, transaction ID, or routing information.

3. SOAP Body

The Body holds the actual message or the data to be processed. This is where requests and responses are encapsulated.

4. WSDL (Web Services Description Language)

WSDL is an XML-based contract describing the SOAP service's operations, including the parameters, return types, and endpoints.

How SOAP Works

Step 1: Client Sends a Request

The client generates a SOAP request message, encapsulated in an XML document. This message includes the operation to be performed and the necessary parameters.

Example SOAP Request:

<soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/">
  <soap:Header>
    <auth>
      <username>exampleUser</username>
      <password>securePassword</password>
    </auth>
  </soap:Header>
  <soap:Body>
    <GetUserDetails xmlns="http://example.com/">
      <UserID>123</UserID>
    </GetUserDetails>
  </soap:Body>
</soap:Envelope>

Step 2: Transmission via HTTP

The SOAP request is sent over HTTP or HTTPS to the server. Although HTTP is the most common transport protocol, SOAP can also work with SMTP, FTP, or other protocols.

Step 3: Server Processes the Request

On the server side:

• The SOAP message is parsed to understand the requested operation.

• The server performs the requested action using the provided parameters (e.g., fetching user data from a database).

• The server generates a SOAP response.

Step 4: Server Sends a Response

The response is sent back to the client in the form of another SOAP message.

Example SOAP Response:

<soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/">
  <soap:Body>
    <GetUserDetailsResponse xmlns="http://example.com/">
      <User>
        <UserID>123</UserID>
        <Name>John Doe</Name>
        <Email>[email protected]</Email>
      </User>
    </GetUserDetailsResponse>
  </soap:Body>
</soap:Envelope>

The client then parses the response and processes the returned data.

Advantages of SOAP

1. Platform Independence: SOAP is language and platform-agnostic, thanks to its reliance on XML.

2. Standardized Protocol: It follows strict standards, making it highly interoperable.

3. Security: SOAP supports WS-Security for encryption and authentication, making it ideal for secure transactions.

4. Built-In Error Handling: SOAP messages include fault elements to describe errors in a structured manner.

5. Extensibility: The header can carry additional metadata for custom functionality.

Disadvantages of SOAP

1. Verbosity: XML messages are larger compared to JSON, leading to higher overhead.

2. Complexity: SOAP’s rigid standards and XML syntax can be cumbersome to work with.

3. Performance: The verbosity and processing requirements of XML make SOAP slower than lightweight protocols like REST.

Use Cases for SOAP

Despite the rise of REST and GraphQL, SOAP remains a critical protocol in industries where security, reliability, and standardization are paramount.

Common Scenarios:

1. Financial Transactions: Banking APIs often use SOAP for secure, ACID-compliant operations.

2. Telecommunications: SOAP is used for provisioning services and managing billing systems.

3. Government Services: Standardization and security make SOAP a preferred choice for inter-governmental systems.

4. Enterprise Applications: ERP and CRM systems often integrate using SOAP.

Comparison: SOAP vs. REST

SOAP continues to serve as a backbone for enterprise-level APIs where security, reliability, and interoperability are non-negotiable. Its strict protocol, built-in error handling, and extensibility make it a preferred choice in high-stakes industries, despite its verbosity and complexity.

While modern API developers often lean toward REST or GraphQL for flexibility, understanding how SOAP works equips engineers to maintain and integrate with legacy systems or secure, regulated environments.

By mastering SOAP, developers can bridge the gap between legacy systems and modern applications, ensuring robust communication and data integrity across platforms.

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