Simple Object Access Protocol

Simple Object Access Protocol
Name	Simple Object Access Protocol
Developer	Microsoft, IBM, BEA Systems, UserLand Software, Iona Technologies
Released	1998
Operating system	Cross-platform
Platform	XML-based
Genre	Web service protocol
License	Various

Simple Object Access Protocol

Simple Object Access Protocol is a protocol specification for exchanging structured information in distributed computing environments using XML, designed to enable interoperability among disparate Microsoft-based systems, IBM middleware, Sun Microsystems platforms, and heterogeneous enterprise applications such as those produced by BEA Systems, Oracle Corporation, and TIBCO Software. Initially driven by vendors including Microsoft, UserLand Software, Iona Technologies, and IBM, the specification aimed to provide a lightweight envelope for remote procedure calls and message exchange among services implemented on Windows NT, Solaris, AIX, and HP-UX systems, integrating with standards from W3C and industry consortia like the OASIS and the W3C SOAP Working Group.

Overview

The specification defines an XML-based message framework that runs primarily over HTTP and SMTP transports, allowing interoperability between disparate products such as Microsoft .NET Framework, Apache Axis, IBM WebSphere Application Server, and Oracle WebLogic Server. It established a standard message envelope and encoding rules to carry remote procedure calls and responses between clients and servers, influencing later specifications like WS-Security, WSDL 1.1, and WS-* family efforts endorsed by consortia including the W3C and OASIS.

History and Development

Work on the protocol began in the late 1990s when vendors including Microsoft, UserLand Software, Iona Technologies, and DevelopMentor collaborated to create a simple XML messaging convention to interoperate with existing technologies like CORBA, DCOM, and earlier RPC mechanisms. In 1999–2000 the specification was submitted to standards bodies; W3C participants and companies such as IBM and BEA Systems contributed to evolving the model. The development path intersected with the creation of WSDL 1.1 at W3C and the emergence of UDDI registries supported by organizations like Microsoft and IBM, shaping a broader Web Services stack adopted by enterprises including General Electric, Siemens, and Accenture.

Architecture and Protocol Details

The architecture specifies a message envelope, an optional header block for metadata and extensibility, and a body block for application data, enabling intermediaries such as HTTP proxy servers and enterprise gateways like IBM Tivoli and BEA AquaLogic to process headers for routing, security, and reliability. SOAP messages are commonly transported over HTTP/1.1 andSMTP and can be bound into higher-level descriptions using WSDL to define service endpoints consumed by platforms such as Microsoft .NET, Java Platform, Standard Edition, and Apache Tomcat. The model supports RPC-style interactions and document-style messaging, both of which were used by systems integrating with SAP applications and PeopleSoft deployments.

Message Structure and Serialization

Messages follow an XML schema with an Envelope element, optional Header, and Body; the Body contains payload encoded according to interoperability rules that were later formalized alongside WSDL and XML Schema (XSD). Serialization conventions vary among implementations: Microsoft’s stacks used literal and encoded use, while Java stacks like Apache Axis and JAX-WS offered serialization mappings for JAXB-based data binding. The standard permits MIME packaging (as in SOAP with Attachments) and mechanisms like MTOM for efficient binary serialization used by media services developed by vendors such as Adobe Systems and RealNetworks.

Security and Reliability Extensions

Native protocol features are minimal; security and reliability were addressed by companion specifications including WS-Security, WS-ReliableMessaging, and WS-Policy, promoted by vendors such as IBM, Microsoft, BEA Systems, and Oracle Corporation. These extensions specify message-level security tokens, XML Signature, XML Encryption, and reliable delivery semantics adopted by enterprise products like Microsoft BizTalk Server, IBM MQ Series, and Oracle SOA Suite, and incorporated into governance frameworks used by financial institutions such as JPMorgan Chase and Goldman Sachs.

Implementations and Interoperability

Implementations span languages and runtimes: Microsoft .NET Framework and WCF provided native support; Java ecosystems used Apache Axis, Apache CXF, and GlassFish libraries; open-source projects include gSOAP for C/C++ and Zolera SoapServer variants. Interoperability testing between vendors occurred at interoperability events hosted by consortia like OASIS and testing labs from Sun Microsystems and IBM, while commercial middleware from IBM WebSphere, Oracle WebLogic, and SAP NetWeaver incorporated adapters to bridge SOAP messages with legacy systems such as IDocs and BAPI interfaces.

Usage and Criticism

SOAP gained rapid adoption in enterprise integration, used by corporations such as Amazon (company), eBay, PayPal, Salesforce, and Microsoft for early web services before the prevalence of alternatives like REST and JSON. Critics from communities around Roy Fielding and Amazon Web Services argued that SOAP’s complexity, extensive extension stack (WS-*), and verbosity led to development and performance overhead, prompting a migration toward lighter-weight protocols exemplified by Representational State Transfer and JSON-based APIs promoted by companies like Twitter and Facebook. Despite criticism, SOAP remains used in regulated industries and legacy systems maintained by organizations such as NASA, Boeing, and Lockheed Martin where formal contracts, payload validation, and message-level security are required.

Category:Web services protocols