Exercise_16_Call_UserService.md

Exercise 16: Call User Service (synchronous)

Learning Goal

After this exercise you know how to call an existing service synchronously within your Advertisement microservice.

The task of this exercise is to call the User service to find out whether the current user is a premium user. Only then this user is allowed to create an advertisement. Technically we are going to use RestTemplate. The RestTemplate is the central Spring class for client-side HTTP access. Conceptually, it is very similar to the JdbcTemplate, JmsTemplate, and the various other templates found in the Spring Framework. This means, for instance, that the RestTemplate is thread-safe once constructed, and that you can use callbacks to customize its operations.

Prerequisite

Continue with your solution of the last exercise. If this does not work, you can checkout the branch origin/solution-13-Use-SLF4J-Features.

• Note: The <<region>> needs to be replaced with eu10 or us10 depending on the trial environment where you have registered.

Step 1: Test User Service Using a REST Client

Before we start with the implementation we want to get familiar with the User service.

You can test the following REST service endpoints manually in the browser using the Postman Chrome plugin:

• https://opensapcp5userservice.cfapps.<<region>>.hana.ondemand.com/api/v1.0/users - returns all available users with their IDs.
• https://opensapcp5userservice.cfapps.<<region>>.hana.ondemand.com/api/v1.0/users/{ID} - returns the information for a user where {ID} is a placeholder for a user id, e.g. "42".

Step 2: Add Maven Dependency

Add the dependency to the Apache http client to your pom.xml using the XML view of Eclipse:

<!-- Apache HTTP Client (closeable, configurable) -->
<dependency>
    <groupId>org.apache.httpcomponents</groupId>
    <artifactId>httpclient</artifactId>
    <version>4.5.2</version>
    <exclusions>
        <exclusion>
            <artifactId>commons-logging</artifactId>
            <groupId>commons-logging</groupId>
        </exclusion>
    </exclusions>
</dependency>

• Note: After you've changed the Maven settings, don't forget to update your Eclipse project (Alt+F5)!

Step 3: Create a User Service Client

The User service client hides the call to the RESTful User Webservice, provides JSON parsing and error handling.

Create a new class UserServiceClient in package com.sap.bulletinboard.ads.services and copy the code from here. Explanation: This class should offer the information whether a given user (String id) is a premium User or not. It makes use of the RestTemplate that gets injected via the constructor and needs to be defined as well. The route/URI to the User service is retrieved from an environment variable using the @Value("${USER_ROUTE}") annotation. The path is set to the endpoint api/v1.0/users/{id}.

Create a new class RestTemplateConfig in package com.sap.bulletinboard.ads.config and copy the code from here.

Explanation

• This class makes use of the Apache HttpClientBuilder to build a CloseableHttpClient instance with a proxy, in case the environment variables http.proxyHost and http.proxyPort are set (for local execution).
• By default the RestTemplate establishes and closes a connection on every HTTP request. As the SSL handshake is time-consuming, we've configured an HTTP connection pool to reuse connections by keeping the sockets open.
• Furthermore we've configured timeouts to specify how long to wait until a connection is established and how long a socket should be kept open (i.e. how long to wait for the (next) data package). By default there are no timeout settings sepcified.
• Note that the configuration is application specific and the settings should be aligned with the hystrix settings (timout, threadpool size), which are introduced in Exercise 18.

Step 4: Integrate UserServiceClient.isPremiumUser()

The isPremiumUser check should be executed before a new Advertisement is created. So the next step is to introduce the check at the right place in the AdvertisementController class. Hint: Please hard-code the User id for now (use "42" as this user is a premium user) as we do not have a way to specify the information about the current user in the incoming request, yet.

Step 5: Test the Advertisement Microservice Locally

In this step we want to test the creation of an advertisement via Postman, which should call the User service.

Run in Eclipse IDE

Before you (re-)start your Tomcat webserver within Eclipse, you need to adapt the Tomcat configuration.

• Open the Servers view.
• Double-click the server instance and select the Open launch configuration link.
• Open the Edit configuration dialog.
• switch to the Environment tab and add the following environment variables:
  • USER_ROUTE=https://opensapcp5userservice.cfapps.<<region>>.hana.ondemand.com
• In case you're sitting behind a proxy, you need to configure the proxy of your RestTemplate. Therefore switch to the Arguments tab and add the proxy settings to the VM arguments:
  • -Dhttp.proxyHost=<<your proxy host>> -Dhttp.proxyPort=<<your proxy port>>

When are proxy settings required? If you run your service locally within a corporate network that has a proxy, the host opensapcp5userservice.cfapps.<<region>>.hana.ondemand.com cannot be resolved. If you apply the proxy settings to the Java process (via VM arguments) then the proxy is used which is able to resolve the host name. Settings in Eclipse are separate from the settings in the shell (bash). In case you run your application locally in the command line, you need to configure the proxy settings (http.proxyHost and http.proxyPort) as part of your pom.xml file.

^{Note: In case you are getting a null-pointer-exception because USER_ROUTE==null, you probably created the UserServiceClient with new instead of @Inject. The latter is necessary since annotations in a class are not interpreted when you create the instance yourself with new.}

Alternatively: Run on Command Line

As described in Exercise 1 you can also deploy the service on an embedded Tomcat using Maven.

Step 6: Fix Tests

As we do not want our JUnit tests (AdvertisementControllerTest) to call third-party services, we need to introduce mocks for the UserServiceClient and as well for the PropertySourcesPlaceholderConfigurer to specify the USER_ROUTE variable. Similar to stub objects, mocks are object instances that just mock the original behavior and can be configured to behave in a certain way.

Create a new @Configuration annotated class TestAppContextConfig in test package com.sap.bulletinboard.ads.config and copy the code from here.

@Configuration
public class TestAppContextConfig {

    @Bean
    static PropertySourcesPlaceholderConfigurer placeholderConfigurer() {
        // provide a mock so that the @Value annotation can be resolved even if the environment variable is not set
        PropertySourcesPlaceholderConfigurer pspc = new PropertySourcesPlaceholderConfigurer();
        Properties properties = new Properties();
        properties.setProperty("USER_ROUTE", ""); // ensures that USER_ROUTE is initialized with no "real" route
        pspc.setProperties(properties);
        return pspc;
    }
    
    @Bean
    @Primary // preferred bean when there are multiple candidates
    UserServiceClient userServiceClient() {
        // return a UserServiceClient mock that just returns "true" for isPremiumUser, without issuing a network request
        UserServiceClient userServiceClient = Mockito.mock(UserServiceClient.class);
        Mockito.when(userServiceClient.isPremiumUser(Mockito.anyString())).thenReturn(true);
        return userServiceClient;
    }
}

Note: The tests will only run, when the placeholderConfigurer bean gets instantiated before the userServiceClient. One option to ensure that is to have only one placeholderConfigurer bean registered in the ApplicationContext. You can make use of Spring Profiles here: annotate the productive placeholderConfigurer bean, declared in the WebAppContextConfig class, with @Profile("cloud") to make sure, that it is never loaded as part of your Test-ApplicationContext.

Explanation:

• The PropertySourcesPlaceholderConfigurer is already registered in the WebAppContextConfig class; it resolves @Value annotations against the current Spring Environment and needs to provide other (dummy) values in the test context.
• Note: As of now the mock for the UserServiceClient.isPremiumUser method always returns true, i.e., the user is always a premium user. All your JUnit tests rely on that. If you analyze the Code Coverage for the JUnit tests, you will see that there is no JUnit test that tests the correct behaviour when the "User is unknown" or the "User is not a premium User" and as of now the user ID is hardcoded to “42”.

Step 7: Push to Cloud Foundry

When pushing the application to Cloud Foundry, the USER_ROUTE needs to be configured as a system environment variable. This can easily be done in the manifest.yml file by adding another entry under env:

env:
    USER_ROUTE: 'https://opensapcp5userservice.cfapps.<<region>>.hana.ondemand.com'

Used Frameworks and Tools

• Postman REST Client (Chrome Plugin)
• Mockito - Mocking Framework
• JDoc Spring RestTemplate
• Apache Rest Client

Further Reading

• JSON Conversion using JacksonJsonProvider
• Spring RestTemplate
• Mockito User Guide
• Spring Configuration

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