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 <h1 id="service-contract-definition">Service Contract Definition</h1>
 <h2 id="concept-description">Concept Description</h2>
 <p>The Service Contract refers to the micro-service interface contract based on the OpenAPI specification. The interface definition between the server and the consumer. The java chassis provides two ways to define the contract: 'code first' and 'contract first'.</p>
 <ul>
 <li>
 <p>Code first<br />
 The Producer use of Jax-RS or SpringMVC's RESTful annotation declares the input and output parameters of the interface, or with the OpenAPI annotations, to add human-readable information, such as sample code, text descriptions, etc.; when the ServiceComb engine starts, according to these annotations generate a contract description and automatically upload it to the service center. <br />
 The producer can also be developed using the transparent RPC model, but since there is no RESTful annotation to guide how to generate the contract, at this time, the automatically generated contract very non-standard and not recommended. The consumer is called with a transparent RPC or RestTemplate. <br />
 Under the code first development model, developers do not have to handwritten contracts.  </p>
 </li>
 <li>
 <p>Contract first<br />
 In this scenario, instead of using the contract automatically generated by the framework, the contract file provided by the developer is directly used, which requires the developer to ensure the consistency of the contract and the code.</p>
 </li>
 </ul>
 <h2 id="scenario">Scenario</h2>
 <p>The service contract is used for decoupling between the server and the consumer. The server implements the service around the contract. The consumer invokes the service according to the contract, which can support the server and the consumer to implement in different programming languages.</p>
 <blockquote>
 <p><em><strong> Description: </strong></em>
 The service provider registers the interface contract with the service center at startup, which can be downloaded and used by the service consumer. The interface contract is micro-service-version level information. When multiple micro-service instances are started, if an instance registers the contract with the service center, the service center will not overwrite the existing contract with the contract information registered by the latecomer. Therefore, only modifying the service provider's interface information will not change the contract stored by the service center. For the service consumer, the acquired interface information is still old. To update the interface contract in the service center, you can choose to upgrade the microservice version number or delete existing microservice information (the latter is not recommended for use in a production environment).</p>
 </blockquote>
 <h3 id="configuration">Configuration</h3>
 <p>ServiceComb defines API in a .yaml file. It's recommended to use <a href="http://editor.swagger.io/#/">Swagger Editor</a> to define the APIs. This tool can check syntax and automaticlly generate an API document. For details about the API definition file format, see <a href="https://github.com/OAI/OpenAPI-Specification/blob/master/versions/2.0.md"> Official OpenAPI documentation</a>。</p>
 <p>The API definition file is located in "resources/microservices" or "resources/applications" directory. The directory structure is as follows:</p>
 <pre><code class="language-yaml">resources
   - microservices
     - serviceName #Microservice name
       - schemaId.yaml #schema ID
   - applications
     - appId #Application ID
       - serviceName #Service name
         - schemaId.yaml #Schema ID
 </code></pre>
 <blockquote>
 <p><strong>Note</strong>:</p>
 <ul>
 <li>ServiceComb's Swagger contract file should be saved using the UTF-8 character set. If the user saves the contract file with a different character set and the file contains Chinese characters, it may cause an unknown error.</li>
 </ul>
 </blockquote>
 <h3 id="sample-code">Sample Code</h3>
 <p>The contents of the schemaId.yaml file in the <code>resources/microservices</code> directory and the <code>resources/application</code> directory are as follows. The interface definitions in the file need to match the actual interface of the service.</p>
 <pre><code class="language-yaml">swagger: '2.0'
 info:
   title: hello
   version: 1.0.0
   x-java-interface: org.apache.servicecomb.samples.common.schema.Hello
 basePath: /springmvchello
 produces:
   - application/json

 paths:
   /sayhi:
     post:
       operationId: sayHi
       parameters:
         - name: name
           in: query
           required: true
           type: string
       responses:
         200:
           description: return value
           schema:
             type: string
         default:
           description: return a default result
           schema:
             type: string
   /sayhello:
     post:
       operationId: sayHello
       parameters:
         - name: person
           in: body
           required: true
           schema:
             $ref: &quot;#/definitions/Person&quot;
       responses:
         200:
           description: return value
           schema:
             type: string
         default:
           description: return a default result
           schema:
             type: string
 definitions:
   Person:
     type: &quot;object&quot;
     properties:
       name:
         type: &quot;string&quot;
         description: &quot;person name&quot;
     xml:
       name: &quot;Person&quot;
 </code></pre>
 <blockquote>
 <p><strong>NOTE</strong>：<br />
 * Contract in ServiceComb, it is recommended that basePath not include the web root of the web container, and the url pattern of the servlet.</p>
 </blockquote>
 <p>Because ServiceComb supports deployment decoupling, it can be deployed independently from the servlet container, or deployed to the servlet container using the war, or it can be run using the embedded servlet container.
 As long as the base path does not contain the web root and the url pattern, the actual url changes caused by the deployment mode modification, the ServiceComb consumer business code does not need to be perceived, and the framework will automatically adapt.</p>
 <blockquote>
 <ul>
 <li>info.x-java-interface needs to indicate the specific interface path, depending on the actual situation of the project.</li>
 <li>SchemaId can contain "." characters, but it is not recommended. This is because the configuration file used by ServiceComb is in yaml format. The "." symbol is used to split the configuration item name. If the SchemaId also contains ".", some configurations that support the contract level may not be recognized correctly.</li>
 <li>The "." character cannot be included in the naming of the OperationId.</li>
 </ul>
 </blockquote>


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