REST API vs Web Service – Java Backend Interview

Direct Answer

The term web service is used in two ways. In everyday team discussions, it can mean any network service used by another program. The W3C Web Services Architecture uses a narrower definition: the service has a machine-processable interface described in WSDL and exchanges SOAP messages, typically over HTTP. A REST API follows REST's constraints and uniform interface. Under the everyday broad meaning it is a kind of web service, but it is not a SOAP web service.

Deep Dive & Architecture

Why this question causes confusion

“Web service” does not have one meaning in every team. In everyday development, it can mean any service that one program calls over a network. Under that broad meaning, REST APIs, SOAP services, and other network APIs can all be called web services.

The W3C Web Services Architecture uses a narrower historical meaning: a software system with a machine-readable service description, specifically WSDL, whose messages follow that description and use SOAP. Many enterprise interviews use “web service” in this narrower sense. When they do, the real question is REST versus SOAP.

What defines a REST API

A REST API is defined by REST's architectural constraints, not by a file format. It works with identified resources and representations, keeps interactions stateless, provides cache information, supports layers, and uses a uniform interface. In a complete REST interaction, messages are self-descriptive and hypermedia can show the client which transitions are available.

Most REST APIs use HTTP because HTTP already defines method semantics, status codes, headers, media types, caching, and conditional requests. REST does not require JSON. A resource may be represented as JSON, XML, HTML, an image, or another suitable media type.

What defines a SOAP service

SOAP 1.2 defines an XML messaging framework. Its envelope contains a header and body and can carry an application operation such as GetOrder. WSDL is commonly used to describe the available operations, message types, and service location. SOAP messages are often carried over HTTP, but SOAP supplies its own message model rather than using the REST uniform interface.

The correct comparison depends on the term

With the broad meaning, “REST API” is a more specific term inside “web service.” With the narrow enterprise meaning, REST API and SOAP web service are different styles. Neither style automatically provides security, speed, reliability, or transaction safety. Those qualities come from the complete protocol, standards, and system design—not from the label alone.

Code Example

A REST-style request identifies the order resource:

GET /orders/42 HTTP/1.1
Host: api.example.com
Accept: application/json

HTTP/1.1 200 OK
Content-Type: application/json
Cache-Control: private, max-age=60

{"id":42,"status":"PAID","total":1499}

A SOAP 1.2 service can model the same capability as an operation inside an XML envelope:

<?xml version="1.0" encoding="UTF-8"?>
<env:Envelope xmlns:env="http://www.w3.org/2003/05/soap-envelope">
  <env:Body>
    <GetOrder xmlns="https://example.com/orders">
      <id>42</id>
    </GetOrder>
  </env:Body>
</env:Envelope>

Both interfaces can return order 42. The first uses a resource identifier and HTTP's uniform semantics. The second uses a SOAP message whose application operation and XML types may be described by WSDL. The difference is the public contract, not the business data being requested.