SOAP (Simple Object Access Protocol) is a messaging protocol that is used to exchange structured and typed information between applications over the internet. It was developed in the late 1990s as a means of providing a standard way for applications to communicate with each other.
At its core, SOAP is a protocol for exchanging messages between applications. It uses XML (eXtensible Markup Language) to define the structure of messages, and it is designed to be platform-independent and language-neutral, meaning that applications written in different programming languages can communicate with each other using SOAP.
The basic architecture of a SOAP message consists of an envelope, which contains information about the message itself, such as its version, encoding, and other metadata. The envelope also contains a body, which contains the actual message content. This content is typically in the form of an XML document, which can be used to describe data structures, request data from a remote server, or perform other actions.
A SOAP envelope is a mandatory part of a SOAP message. It is an XML document that contains information about the SOAP message and its content. The envelope is used to provide a standard structure for the message and to ensure that it can be correctly interpreted by the recipient.
The SOAP envelope is composed of two parts: the header and the body. The header contains any optional information about the message, such as authentication credentials, and is used to provide context for the message. The body contains the actual content of the message, such as the request or response data.
The SOAP envelope is needed for several reasons:
- Standardization: The SOAP envelope provides a standard way to structure SOAP messages, making it easier for developers to write and interpret SOAP messages across different platforms and programming languages.
- Interoperability: By providing a common structure for SOAP messages, the SOAP envelope ensures that messages can be correctly interpreted by any compliant SOAP implementation, regardless of the programming language or platform used.
- Extensibility: The SOAP envelope is designed to be extensible, allowing developers to add additional elements or data to the message as needed, without breaking compatibility with existing SOAP implementations.
- Security: The SOAP envelope can be used to provide authentication and encryption information within the message header, ensuring that the message is only accessible to authorized parties.
SOAP contract
In SOAP, a contract is a set of rules that defines the format of the SOAP messages exchanged between the client and the server. A SOAP contract specifies the operations that a web service provides, the format of the SOAP messages that must be sent and received for each operation, and any other requirements for using the service.
A SOAP contract typically consists of two parts: the abstract definition and the concrete definition.
The abstract definition of a SOAP contract is specified using WSDL (Web Services Description Language), which defines the interface of the web service. The WSDL file specifies the types of messages that can be sent and received by the service, the operations that can be performed, and the message exchange patterns. The abstract definition is independent of the underlying transport protocol used to exchange SOAP messages.
The concrete definition of a SOAP contract specifies the transport protocol used to exchange SOAP messages, such as HTTP or HTTPS. The concrete definition includes details such as the endpoint URL of the service, the HTTP headers required for authentication or other purposes, and any other transport-specific details.
A SOAP contract is an agreement between the client and server on the format and rules for exchanging SOAP messages. It ensures that both parties are using the same message format and understand how to interact with the web service, making it an important part of building interoperable web services.
Here is an example of a SOAP contract expressed using WSDL:
<definitions name="ExampleService"
targetNamespace="http://example.com/ExampleService"
xmlns="http://schemas.xmlsoap.org/wsdl/"
xmlns:soap="http://schemas.xmlsoap.org/wsdl/soap/"
xmlns:tns="http://example.com/ExampleService">
<!-- Define the data types used by the service -->
<types>
<xsd:schema targetNamespace="http://example.com/ExampleService/types">
<xsd:complexType name="Customer">
<xsd:sequence>
<xsd:element name="firstName" type="xsd:string"/>
<xsd:element name="lastName" type="xsd:string"/>
<xsd:element name="email" type="xsd:string"/>
</xsd:sequence>
</xsd:complexType>
</xsd:schema>
</types>
<!-- Define the operations supported by the service -->
<message name="GetCustomerRequest">
<part name="customerId" type="xsd:int"/>
</message>
<message name="GetCustomerResponse">
<part name="customer" type="tns:Customer"/>
</message>
<portType name="ExampleServicePortType">
<operation name="GetCustomer">
<input message="tns:GetCustomerRequest"/>
<output message="tns:GetCustomerResponse"/>
</operation>
</portType>
<!-- Define the SOAP bindings for the service -->
<binding name="ExampleServiceBinding" type="tns:ExampleServicePortType">
<soap:binding style="document"
transport="http://schemas.xmlsoap.org/soap/http"/>
<operation name="GetCustomer">
<soap:operation soapAction="http://example.com/ExampleService/GetCustomer"/>
<input>
<soap:body use="literal"/>
</input>
<output>
<soap:body use="literal"/>
</output>
</operation>
</binding>
<!-- Define the service endpoint -->
<service name="ExampleService">
<port name="ExampleServicePort" binding="tns:ExampleServiceBinding">
<soap:address location="http://example.com/ExampleService"/>
</port>
</service>
</definitions>
In this example, the SOAP contract defines a web service called “ExampleService” that supports a single operation called “GetCustomer”. The operation takes an input message called “GetCustomerRequest” that contains a single parameter of type “int”, and returns an output message called “GetCustomerResponse” that contains a single parameter of type “Customer”. The service is bound to the SOAP protocol using HTTP transport, and the endpoint URL is “http://example.com/ExampleService“.
This SOAP contract specifies the interface for the web service, along with the format and rules for exchanging SOAP messages, making it possible for clients to interact with the service in a standard way.
SOAP messages are usually transported over HTTP, although SOAP can be used with other transport protocols such as SMTP, FTP, or JMS as well.
When SOAP messages are transported over HTTP, they are typically sent using the POST method, with the message body containing the SOAP envelope. The HTTP headers can be used to specify additional information about the message, such as the content type, character encoding, and authentication credentials.
SOAP over HTTP is a popular choice for web services because HTTP is a widely supported and well-established protocol that is available on almost all platforms and devices. It is also firewall-friendly, which means that it can pass through firewalls and other network security devices easily, making it a good choice for building distributed applications that need to communicate across different networks.
SOAP security
- Encryption: Encryption is the process of transforming plaintext data into ciphertext using an algorithm, so that the data is unreadable to anyone who does not have the key to decrypt it. SOAP supports encryption of message payloads using a variety of algorithms, including AES (Advanced Encryption Standard) and RSA (Rivest-Shamir-Adleman). Encryption can help protect sensitive data from being intercepted and read by unauthorized parties.
- Digital Signatures: A digital signature is a mathematical algorithm that can be used to verify the authenticity and integrity of a message. It works by creating a unique hash value of the message, which is then encrypted using the sender’s private key. The recipient can then use the sender’s public key to decrypt the hash value and compare it to a newly calculated hash value of the received message. If the two hash values match, the message is considered to be authentic and unaltered. SOAP supports the use of digital signatures, which can help ensure that messages are not tampered with or forged in transit.
- Authentication: Authentication is the process of verifying the identity of a client or server. SOAP supports a variety of authentication mechanisms, including username/password authentication, X.509 certificates, and tokens. Username/password authentication is a simple mechanism that requires the client to provide a username and password, which are then verified by the server. X.509 certificates are digital certificates that are used to verify the identity of a client or server. Tokens are a lightweight form of authentication that can be used to provide temporary access to a resource.
- Authorization: Authorization is the process of determining what actions a client is authorized to perform. SOAP supports a variety of authorization mechanisms, including role-based access control (RBAC), attribute-based access control (ABAC), and policy-based access control (PBAC). RBAC is a simple mechanism that allows access to be granted based on a user’s role, while ABAC is a more flexible mechanism that allows access to be granted based on a user’s attributes, such as job title or department. PBAC is a more advanced mechanism that allows access to be granted based on a set of policies, which can be evaluated dynamically.
- WS-Security: WS-Security is a comprehensive framework for securing SOAP messages. It includes support for encryption, digital signatures, authentication, and authorization, as well as other security-related features such as message replay protection and timestamping. WS-Security is based on a set of standards developed by OASIS (Organization for the Advancement of Structured Information Standards) and is widely supported by SOAP implementations.
In summary, SOAP provides a comprehensive set of security features that can be used to build secure web services. By using encryption, digital signatures, authentication, authorization, and WS-Security, SOAP can help protect sensitive data and ensure that messages are not tampered with or forged in transit.
Here are some examples of SOAP messages:
Request message:
<soap:Envelope xmlns:soap="http://www.www.org/2003/05/soap-envelope" xmlns:xsd="http://www.www.org/2001/XMLSchema" xmlns:xsi="http://www.www.org/2001/XMLSchema-instance">
<soap:Header>
<AuthenticationHeader xmlns="http://example.com/authentication">
<Username>example_user</Username>
<Password>example_password</Password>
</AuthenticationHeader>
</soap:Header>
<soap:Body>
<GetWeatherRequest xmlns="http://example.com/weather">
<City>London</City>
</GetWeatherRequest>
</soap:Body>
</soap:Envelope>
In this example, a SOAP request message is being sent to a weather service. The message contains an authentication header and a body that specifies the city for which weather information is being requested.
Response message:
<soap:Envelope xmlns:soap="http://www.www.org/2003/05/soap-envelope" xmlns:xsd="http://www.www.org/2001/XMLSchema" xmlns:xsi="http://www.www.org/2001/XMLSchema-instance">
<soap:Body>
<GetWeatherResponse xmlns="http://example.com/weather">
<Temperature>12°C</Temperature>
<Conditions>Sunny</Conditions>
</GetWeatherResponse>
</soap:Body>
</soap:Envelope>
In this example, a SOAP response message is being sent back to the client that requested weather information. The message contains a body that specifies the temperature and conditions for the requested city.
Error message:
<soap:Envelope xmlns:soap="http://www.www.org/2003/05/soap-envelope" xmlns:xsd="http://www.www.org/2001/XMLSchema" xmlns:xsi="http://www.www.org/2001/XMLSchema-instance">
<soap:Body>
<soap:Fault>
<soap:Code>
<soap:Value>soap:Sender</soap:Value>
</soap:Code>
<soap:Reason>
<soap:Text xml:lang="en">Invalid city name</soap:Text>
</soap:Reason>
</soap:Fault>
</soap:Body>
</soap:Envelope>
In this example, a SOAP fault message is being sent back to the client that requested weather information. The message contains a fault element that specifies that the city name in the request was invalid.
One of the key advantages of SOAP is that it is highly extensible. This means that developers can define custom data types, complex object structures, and other features that are specific to their application. SOAP also provides a standard set of rules for handling errors, which makes it easier for developers to write robust and reliable applications.
However, one of the drawbacks of SOAP is that it can be quite complex to implement and use. Because SOAP messages are typically quite large, they can take longer to transmit over the network than other messaging protocols, such as REST. Additionally, SOAP requires a significant amount of overhead to establish and maintain connections between applications, which can make it less suitable for certain use cases.
Despite these drawbacks, SOAP remains an important protocol for many enterprise-level applications, particularly those that require a high degree of reliability and security.
