<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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Testing on NestJS mocks

<h1>Postman Swagger CLI</h1>
<h2>Postman and Swagger overview</h2>
In short, postman is the utility, which helps you to make a request via the UI. 
There you can setup the URL, where you want to reach, the method (GET/POST/DELETE...),
query parameters and so on. Very useful for the API testing and can be easily shared
to other team members with the concept of the collections. 
Swagger on the other side, is more positioned to the API documentation and 
should be running on some server. With Swagger you can set up the descriptions
for the API, what parameters it can take, response structure and much more. 
<h2>CLI</h2>
To be able to spin up the Swagger which can be run on the server based of the
postman collections, an npm package <code>@decodeapps/postman-swagger</code> was created. 
It takes as an input the collections of requests from Postman and spins up the
Express server. 
To be able to quickly spin up such server, create a folder with some schema:
<pre><code class="hljs language-bash">mkdir swagger-examples
touch swagger-examples/example.json
</code></pre>And fill it with such content:
<pre><code class="hljs language-json">{
 &quot;info&quot;: {
 &quot;_postman_id&quot;: &quot;9adb14d7-625e-4a02-828d-a5e2d044bfd0&quot;,
 &quot;name&quot;: &quot;Example Collection&quot;,
 &quot;schema&quot;: &quot;https://schema.getpostman.com/json/collection/v2.1.0/collection.json&quot;
 },
 &quot;item&quot;: [
 {
 &quot;name&quot;: &quot;GET example.com&quot;,
 &quot;request&quot;: {
 &quot;method&quot;: &quot;GET&quot;,
 &quot;header&quot;: [],
 &quot;url&quot;: {
 &quot;raw&quot;: &quot;https://example.com&quot;,
 &quot;protocol&quot;: &quot;https&quot;,
 &quot;host&quot;: [
 &quot;example&quot;,
 &quot;com&quot;
 ]
 },
 &quot;description&quot;: &quot;Making the simple GET request to a website&quot;
 },
 &quot;response&quot;: []
 }
 ],
 &quot;protocolProfileBehavior&quot;: {}
}
</code></pre>And then startup the utility:
<pre><code class="hljs language-bash">npx @decodeapps/postman-swagger -d ./examples/
</code></pre>As a result, upon navigating to <code>http://localhost:5000</code>, you will see the following:
<img src='/blog/postman-swagger-screenshot.png' alt='Postman swagger screenshot' style="max-width: 800px" />


Visit <a href="https://github.com/FallenAngel97/postman-swagger">https://github.com/FallenAngel97/postman-swagger</a> to learn more about the implementation and possible parameters
Author of the background photo: Magda Ehlers: <a href="https://www.pexels.com/photo/2114014/">https://www.pexels.com/photo/2114014/</a>

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