<h1>How to test microservice with mocks from NestJS 2022</h1>
<h2>What are disadvantages of using microservices?</h2>
With the microservice approach, there is increased complexity in the support, 
especially with the strategies for testing. 
As with every part of the software, we can cover them with the unit tests, but
due to the distributed nature of microservices, you can occur in the situation 
that there is a necessity to mock a lot of API calls, so the contract between 
microservices can be broken.
To be able to correctly 
<h2>Extract the OpenAPI schema from the NestJS application</h2>
In order to retrieve the schema from the NestJS application, you can create a 
script, which imports application module in the script and writes to the filesystem.
<pre><code class="hljs language-typescript">import { NestFactory } from &quot;@nestjs/core&quot;;
import { DocumentBuilder, SwaggerModule } from &quot;@nestjs/swagger&quot;;
import { AppModule } from &quot;../app.module&quot;;
import * as fs from &#x27;fs&#x27;;

const options = new DocumentBuilder()
 .setTitle(&quot;Title&quot;)
 .setDescription(&quot;description&quot;)
 .setVersion(&quot;1.0&quot;)
 .build();

NestFactory.create(AppModule).then((app) =&gt; {
 const document = SwaggerModule.createDocument(app, options);
 SwaggerModule.setup(&#x27;api&#x27;, app, document); 

 fs.writeFileSync(&quot;./swagger-spec.json&quot;, JSON.stringify(document)); // write down the OpenAPI document
 app.close(); // Exit the application without listening
});
</code></pre><h2>Run the mock server from OpenAPI definition</h2>
After extracting the schema, you can utilize <a href="https://github.com/stoplightio/prism">prism</a>
to start the mock server. 
<pre><code class="hljs language-bash">prism mock ./swagger-spec.json -p 3000 -h 0.0.0.0
</code></pre>After it, you should be able to see output, stating that mock server should be up and running
<img src="/blog/nestjs-mock-prism/screenshot-of-prism.png" alt="Screenshot of Prism running on the localhost">
This setup allows you to provide necessary minimum mock server, which should be able to meet the 
requirements of your contract tests. 
You can put obtained OpenAPI JSON file and execute command above in your CI/CD setup.
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