Simple Object Access Protocol (SOAP) can be considered as the foremost messaging standard for use with web services. It is defined by the World Wide Web Consortium (W3C) at http://www.w3.org/TR/2000/NOTE-SOAP-20000508/ as follows:
SOAP is a lightweight protocol for exchange of information in a decentralized, distributed environment. It is an XML based protocol that consists of three parts: an envelope that defines a framework for describing what is in a message and how to process it, a set of encoding rules for expressing instances of application-defined datatypes, and a convention for representing remote procedure calls and responses.
The SOAP specification has been universally accepted as the standard transport protocol for messages processed by web services. There are two different versions of SOAP specification and both of them are widely used in service-oriented solutions. These two versions are SOAP v1.1 and SOAP v1.2.
Regardless of the SOAP specification version, the message format of a SOAP message still remains intact. A SOAP message is an XML document that consists of a mandatory SOAP envelope, an optional SOAP header, and a mandatory SOAP body.
The structure of a SOAP message is shown in the following diagram:
The SOAP Envelope is the wrapper element which holds all child nodes inside a SOAP message.
The SOAP Header element is an optional block where the meta information is stored. Using the headers, SOAP messages are capable of containing different types of supplemental information related to the delivery and processing of messages. This indirectly provides the statelessness for web services as by maintaining SOAP headers, services do not necessarily need to store message-specific logic. Typically, SOAP headers can include the following:
Message processing instructions
Security policy metadata
Addressing information
Message correlation data
Reliable messaging metadata
The SOAP body is the element where the actual message contents are hosted. These contents of the body are usually referred to as the message payload.
Let's have a look at a sample SOAP message and relate the preceding concepts through the following diagram:
In this example SOAP message, we can clearly identify the three elements; envelope, body, and header. The header element includes a set of child elements such as <wsa:To>, <wsa:ReplyTo>, <wsa:Address>, <wsa:MessageID>, and <wsa:Action>. These header blocks are part of the WS-Addressing specification. Similarly, any header element associated with WS-* specifications can be included inside the SOAP header element.
The <s:Body> element carries the actual message payload. In this example, it is the <p:echoString> element with a one child element.
Tip
When working with SOAP messages, identification of the version of SOAP message is one of the important requirements. At first glance, you can determine the version of the specification used in the SOAP message through the namespace identifier of the <Envelope> element. If the message conforms to SOAP 1.1 specification, it would be http://schemas.xmlsoap.org/soap/envelope/,otherwise http://www.w3.org/2003/05/soap-envelope is the name space identifier of SOAP 1.2 messages.