What is the difference between OpenAPI and WSDL? #

OpenAPI and WSDL (Web Services Description Language) are both interface description languages — they describe an API’s operations, inputs, outputs, and data types in a machine-readable format. Despite this shared purpose, they differ fundamentally in the protocols they describe, the architectural styles they support, the formats they use, and the ecosystem of tools built around them. Understanding these differences helps teams make informed choices about which standard applies to their situation.

Historical Context #

WSDL emerged in the early 2000s as part of the SOAP (Simple Object Access Protocol) web services ecosystem. WSDL 1.1 was standardized by the W3C in 2001; WSDL 2.0 followed in 2007. During the 2000s, SOAP and WSDL dominated enterprise integration — particularly in financial services, healthcare, and government systems.

OpenAPI (originally Swagger) emerged in 2011 to describe REST APIs, which had grown rapidly as an alternative to SOAP. Swagger was donated to the Linux Foundation in 2016 and became the OpenAPI Specification under the OpenAPI Initiative. OpenAPI 3.0 was released in 2017; OpenAPI 3.1 (aligned with JSON Schema) followed in 2021.

The two standards reflect two different eras of API development and two different architectural philosophies.

Protocol and Architectural Style #

This is the most fundamental difference:

WSDL describes services in terms of operations (remote procedure calls with typed inputs and outputs), bindings (how operations are transmitted, e.g., SOAP over HTTP), ports (the endpoint addresses), and messages (the data structures). This RPC model maps directly to SOAP’s operation-centric approach.

OpenAPI describes APIs in terms of paths (URL resources), operations (HTTP methods on those resources), parameters (URL, query, header, cookie), request bodies, and responses. This resource-oriented model maps to REST’s representation of state through URLs and standard HTTP verbs.

Document Format #

WSDL documents are XML. A minimal WSDL document is verbose and requires familiarity with XML namespaces, schema types, message definitions, port types, bindings, and services:

<?xml version="1.0" encoding="UTF-8"?>
<wsdl:definitions xmlns:wsdl="http://schemas.xmlsoap.org/wsdl/"
                  xmlns:soap="http://schemas.xmlsoap.org/wsdl/soap/"
                  xmlns:xs="http://www.w3.org/2001/XMLSchema"
                  xmlns:tns="http://example.com/users"
                  targetNamespace="http://example.com/users">

  <wsdl:types>
    <xs:schema targetNamespace="http://example.com/users">
      <xs:element name="GetUserRequest">
        <xs:complexType>
          <xs:sequence>
            <xs:element name="userId" type="xs:string"/>
          </xs:sequence>
        </xs:complexType>
      </xs:element>
      <xs:element name="GetUserResponse">
        <xs:complexType>
          <xs:sequence>
            <xs:element name="id" type="xs:string"/>
            <xs:element name="email" type="xs:string"/>
          </xs:sequence>
        </xs:complexType>
      </xs:element>
    </xs:schema>
  </wsdl:types>

  <wsdl:message name="GetUserInput">
    <wsdl:part name="parameters" element="tns:GetUserRequest"/>
  </wsdl:message>
  <wsdl:message name="GetUserOutput">
    <wsdl:part name="parameters" element="tns:GetUserResponse"/>
  </wsdl:message>

  <wsdl:portType name="UserServicePortType">
    <wsdl:operation name="GetUser">
      <wsdl:input message="tns:GetUserInput"/>
      <wsdl:output message="tns:GetUserOutput"/>
    </wsdl:operation>
  </wsdl:portType>

  <wsdl:binding name="UserServiceSoapBinding" type="tns:UserServicePortType">
    <soap:binding style="document" transport="http://schemas.xmlsoap.org/soap/http"/>
    <wsdl:operation name="GetUser">
      <soap:operation soapAction="GetUser"/>
      <wsdl:input><soap:body use="literal"/></wsdl:input>
      <wsdl:output><soap:body use="literal"/></wsdl:output>
    </wsdl:operation>
  </wsdl:binding>

  <wsdl:service name="UserService">
    <wsdl:port name="UserServicePort" binding="tns:UserServiceSoapBinding">
      <soap:address location="https://api.example.com/users"/>
    </wsdl:port>
  </wsdl:service>

</wsdl:definitions>

OpenAPI documents are YAML or JSON. The same operation is expressed much more concisely:

openapi: 3.1.0
info:
  title: User API
  version: 1.0.0
paths:
  /users/{userId}:
    get:
      summary: Get a user
      parameters:
        - name: userId
          in: path
          required: true
          schema:
            type: string
      responses:
        "200":
          description: The user
          content:
            application/json:
              schema:
                type: object
                properties:
                  id:
                    type: string
                  email:
                    type: string

The OpenAPI version is significantly more readable and requires less XML namespace and schema machinery.

Schema Languages #

WSDL uses XML Schema Definition (XSD) to define data types. XSD is a powerful but complex schema language designed specifically for XML documents.

OpenAPI uses JSON Schema (fully adopted in OpenAPI 3.1) to define data types. JSON Schema is widely understood, has a rich tooling ecosystem, and is more accessible to developers familiar with JSON.

Tooling Ecosystem #

Both standards have tooling ecosystems, but they differ significantly in scale and modernity:

The OpenAPI ecosystem is substantially larger, more actively maintained, and better integrated with modern development workflows.

Current Relevance #

WSDL and SOAP remain relevant in:

• Legacy enterprise systems — many large financial, healthcare, and government systems built in the 2000s use SOAP APIs that have not been replaced
• Strict contract enforcement — SOAP’s built-in WS-Security, WS-ReliableMessaging, and WS-AtomicTransaction standards provide features that REST+OpenAPI does not address natively
• Interoperability with older systems — some enterprise middleware requires SOAP

OpenAPI is the dominant standard for:

• New REST API development — virtually all new HTTP APIs are documented with OpenAPI
• Public-facing APIs — OpenAPI’s tooling, documentation, and SDK generation make it the standard for public APIs
• Microservices — lightweight REST/HTTP APIs in microservice architectures use OpenAPI

Conclusion #

OpenAPI and WSDL address the same fundamental problem — describing a service interface in a machine-readable way — but for entirely different protocol ecosystems. WSDL was built for SOAP’s operation-centric, XML-based world; OpenAPI was built for REST’s resource-centric, JSON-based world. For teams building new HTTP APIs today, OpenAPI is the default choice. WSDL remains important for maintaining or integrating with legacy SOAP services built during the enterprise web services era.

Last updated on April 30, 2026.