magento-test/app/code/Magento/Webapi/etc/webapi_base.xsd

<?xml version="1.0" encoding="UTF-8"?>
<!--
/**
 * Structure description for webapi.xml configuration files.
 *
 * Copyright © Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */
-->
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
    <xs:element name="routes" type="routesType"/>

    <xs:complexType name="routesType">
        <xs:sequence>
            <xs:element name="route" type="routeType" minOccurs="0" maxOccurs="unbounded"/>
        </xs:sequence>
    </xs:complexType>

    <xs:complexType name="routeType">
        <xs:attribute name="method" use="required">
            <xs:simpleType>
                <xs:restriction base="xs:string">
                    <xs:enumeration value="GET"/>
                    <xs:enumeration value="PUT"/>
                    <xs:enumeration value="POST"/>
                    <xs:enumeration value="DELETE"/>
                </xs:restriction>
            </xs:simpleType>
        </xs:attribute>
        <xs:attribute name="url" type="xs:string" use="required"/>
        <xs:attribute name="secure" type="xs:boolean"/>
        <xs:attribute name="soapOperation" type="xs:string"/>
    </xs:complexType>

    <xs:complexType name="serviceType">
        <xs:attribute name="class" type="xs:string" use="required"/>
        <xs:attribute name="method" type="xs:string" use="required"/>
    </xs:complexType>

    <xs:complexType name="resourcesType" >
        <xs:sequence>
            <xs:element name="resource" type="resourceType" maxOccurs="unbounded"/>
        </xs:sequence>
    </xs:complexType>

    <xs:complexType name="resourceType">
       <xs:attribute name="ref" use="required">
           <xs:simpleType>
               <xs:restriction base="xs:string">
                   <xs:pattern value=".+(, ?.+)*"/>
               </xs:restriction>
           </xs:simpleType>
       </xs:attribute>
    </xs:complexType>

    <xs:complexType name="dataType" >
        <xs:sequence>
            <xs:element name="parameter" type="parameterType" minOccurs="0" maxOccurs="unbounded"/>
        </xs:sequence>
        <xs:attribute name="input-array-size-limit" type="xs:nonNegativeInteger" use="optional" />
    </xs:complexType>

    <xs:complexType name="parameterType">
        <xs:simpleContent>
            <xs:extension base="xs:string">
                <xs:attribute name="name" type="xs:string" use="required"/>
                <xs:attribute name="force" type="xs:boolean"/>
            </xs:extension>
        </xs:simpleContent>
    </xs:complexType>

</xs:schema>