SOAP & XML

SOAP is a strict XML protocol over many transports, with a formal WSDL contract, built-in WS-Security, and errors as SOAP Faults. REST is an HTTP style, usually JSON, with a looser contract, HTTPS-level security, and HTTP status codes for errors. REST is lighter; SOAP suits strict enterprise integration.

SOAP is a protocol for exchanging structured messages as XML over a network. Each message is an Envelope wrapping an optional Header for metadata like security or routing, and a mandatory Body carrying the actual call, its response, or a Fault on error.

WSDL is an XML contract describing a SOAP service — its operations, their input/output message types, the bindings that map them to a protocol, and the endpoint address. The server framework usually generates it; clients consume it to build matching stubs and call the service correctly.

An element is a node with its own tags that can hold text, child elements, and repeat; an attribute is a name-value pair on the opening tag. Use elements for structured or repeatable data that may grow; use attributes for simple, singular metadata like an id or a unit that won't need children.

HTML has a fixed tag set for displaying a page; XML lets you define your own tags to carry data, with no presentation meaning. Well-formed means the syntax is correct — tags closed and properly nested. Valid is stronger: well-formed and also conforming to a schema such as an XSD.

XSD is an XML Schema — an XML document defining the allowed structure of other XML: which elements and attributes may appear, their order, cardinality, and data types. Plain XML can only be well-formed; validating against an XSD also confirms it is valid, matching the agreed contract.

JSON is compact, has native arrays and objects, and dominates web APIs; XML is verbose but offers namespaces, attributes, comments, and mature schema tooling like XSD and XSLT. Pick XML where a legacy or regulated system demands it — banking, government, SOAP — or where rich validation matters most.

JSON Schema is a JSON document describing the allowed shape of other JSON: required fields, their types, value ranges, enums, string patterns, and nested structure. Tools validate a payload against it — the JSON-world equivalent of XSD for XML — and analysts use it to pin down an API contract.

For editing and validating XML against an XSD, use IDE plugins or online validators; for SOAP, a client like SoapUI or Postman imports the WSDL and generates sample requests. You test by sending crafted envelopes and checking the responses and Faults against the contract and the schema.

A SOAP Fault is the standard error element returned inside the Body. It holds a fault code (a category like Client or Server), a fault string for humans, and optional detail. REST signals errors through many HTTP status codes; SOAP uses one transport code, usually 500, and puts the error in the Fault.

There are three well-formedness errors. The attribute value in id=42 is unquoted; <price> is closed by </Price>, but XML tag names are case-sensitive; and <b> and <note> overlap instead of nesting. A parser stops at the first fault — the unquoted attribute — and reports a fatal not-well-formed error.

An XML namespace is a URI that qualifies element and attribute names so identically-named tags from different vocabularies don't collide in one document. A prefix is a short alias bound to that URI, letting you write soap:Body instead of the full URI. The URI only identifies; it is never fetched.