<?xml version="1.0" encoding="UTF-8"?>
<?asciidoc-toc?>
<?asciidoc-numbered?>
<book xmlns="http://docbook.org/ns/docbook" xmlns:xl="http://www.w3.org/1999/xlink" version="5.0" xml:lang="en">
<info>
<title>Cloud Native Applications</title>
<date>2020-08-04</date>
</info>
<preface>
<title></title>
<simpara><link xl:href="https://pivotal.io/platform-as-a-service/migrating-to-cloud-native-application-architectures-ebook">Cloud Native</link> is a style of application development that encourages easy adoption of best practices in the areas of continuous delivery and value-driven development.
A related discipline is that of building <link xl:href="https://12factor.net/">12-factor Applications</link>, in which development practices are aligned with delivery and operations goals&#8201;&#8212;&#8201;for instance, by using declarative programming and management and monitoring.
Spring Cloud facilitates these styles of development in a number of specific ways.
 The starting point is a set of features to which all components in a distributed system need easy access.</simpara>
<simpara>Many of those features are covered by <link xl:href="https://projects.spring.io/spring-boot">Spring Boot</link>, on which Spring Cloud builds. Some more features are delivered by Spring Cloud as two libraries: Spring Cloud Context and Spring Cloud Commons.
Spring Cloud Context provides utilities and special services for the <literal>ApplicationContext</literal> of a Spring Cloud application (bootstrap context, encryption, refresh scope, and environment endpoints). Spring Cloud Commons is a set of abstractions and common classes used in different Spring Cloud implementations (such as Spring Cloud Netflix and Spring Cloud Consul).</simpara>
<simpara>If you get an exception due to "Illegal key size" and you use Sun&#8217;s JDK, you need to install the Java Cryptography Extension (JCE) Unlimited Strength Jurisdiction Policy Files.
See the following links for more information:</simpara>
<itemizedlist>
<listitem>
<simpara><link xl:href="https://www.oracle.com/technetwork/java/javase/downloads/jce-6-download-429243.html">Java 6 JCE</link></simpara>
</listitem>
<listitem>
<simpara><link xl:href="https://www.oracle.com/technetwork/java/javase/downloads/jce-7-download-432124.html">Java 7 JCE</link></simpara>
</listitem>
<listitem>
<simpara><link xl:href="https://www.oracle.com/technetwork/java/javase/downloads/jce8-download-2133166.html">Java 8 JCE</link></simpara>
</listitem>
</itemizedlist>
<simpara>Extract the files into the JDK/jre/lib/security folder for whichever version of JRE/JDK x64/x86 you use.</simpara>
<note>
<simpara>Spring Cloud is released under the non-restrictive Apache 2.0 license.
If you would like to contribute to this section of the documentation or if you find an error, you can find the source code and issue trackers for the project at <link xl:href="https://github.com/spring-cloud/spring-cloud-commons/tree/master/docs/src/main/asciidoc">github</link>.</simpara>
</note>
</preface>
<chapter xml:id="_spring_cloud_context_application_context_services">
<title>Spring Cloud Context: Application Context Services</title>
<simpara>Spring Boot has an opinionated view of how to build an application with Spring.
For instance, it has conventional locations for common configuration files and has endpoints for common management and monitoring tasks.
Spring Cloud builds on top of that and adds a few features that probably all components in a system would use or occasionally need.</simpara>
<section xml:id="_the_bootstrap_application_context">
<title>The Bootstrap Application Context</title>
<simpara>A Spring Cloud application operates by creating a <quote>bootstrap</quote> context, which is a parent context for the main application.
It is responsible for loading configuration properties from the external sources and for decrypting properties in the local external configuration files.
The two contexts share an <literal>Environment</literal>, which is the source of external properties for any Spring application.
By default, bootstrap properties (not <literal>bootstrap.properties</literal> but properties that are loaded during the bootstrap phase) are added with high precedence, so they cannot be overridden by local configuration.</simpara>
<simpara>The bootstrap context uses a different convention for locating external configuration than the main application context.
Instead of <literal>application.yml</literal> (or <literal>.properties</literal>), you can use <literal>bootstrap.yml</literal>, keeping the external configuration for bootstrap and main context
nicely separate.
The following listing shows an example:</simpara>
<formalpara>
<title>bootstrap.yml</title>
<para>
<screen>spring:
  application:
    name: foo
  cloud:
    config:
      uri: ${SPRING_CONFIG_URI:http://localhost:8888}</screen>
</para>
</formalpara>
<simpara>If your application needs any application-specific configuration from the server, it is a good idea to set the <literal>spring.application.name</literal> (in <literal>bootstrap.yml</literal> or <literal>application.yml</literal>).
In order for the property <literal>spring.application.name</literal> to be used as the application&#8217;s context ID you
must set it in <literal>bootstrap.[properties | yml]</literal>.</simpara>
<simpara>If you want to retrieve specific profile configuration, you should also set <literal>spring.profiles.active</literal> in <literal>bootstrap.[properties | yml]</literal>.</simpara>
<simpara>You can disable the bootstrap process completely by setting <literal>spring.cloud.bootstrap.enabled=false</literal> (for example, in system properties).</simpara>
</section>
<section xml:id="_application_context_hierarchies">
<title>Application Context Hierarchies</title>
<simpara>If you build an application context from <literal>SpringApplication</literal> or <literal>SpringApplicationBuilder</literal>, then the Bootstrap context is added as a parent to that context.
It is a feature of Spring that child contexts inherit property sources and profiles from their parent, so the <quote>main</quote> application context contains additional property sources, compared to building the same context without Spring Cloud Config.
The additional property sources are:</simpara>
<itemizedlist>
<listitem>
<simpara><quote>bootstrap</quote>: If any <literal>PropertySourceLocators</literal> are found in the Bootstrap context and if they have non-empty properties, an optional <literal>CompositePropertySource</literal> appears with high priority.
An example would be properties from the Spring Cloud Config Server.
See <quote><xref linkend="customizing-bootstrap-property-sources"/></quote> for instructions on how to customize the contents of this property source.</simpara>
</listitem>
<listitem>
<simpara><quote>applicationConfig: [classpath:bootstrap.yml]</quote> (and related files if Spring profiles are active): If you have a <literal>bootstrap.yml</literal> (or <literal>.properties</literal>), those properties are used to configure the Bootstrap context.
Then they get added to the child context when its parent is set.
They have lower precedence than the <literal>application.yml</literal> (or <literal>.properties</literal>) and any other property sources that are added to the child as a normal part of the process of creating a Spring Boot application.
See <quote><xref linkend="customizing-bootstrap-properties"/></quote> for instructions on how to customize the contents of these property sources.</simpara>
</listitem>
</itemizedlist>
<simpara>Because of the ordering rules of property sources, the <quote>bootstrap</quote> entries take precedence.
However, note that these do not contain any data from <literal>bootstrap.yml</literal>, which has very low precedence but can be used to set defaults.</simpara>
<simpara>You can extend the context hierarchy by setting the parent context of any <literal>ApplicationContext</literal> you create&#8201;&#8212;&#8201;for example, by using its own interface or with the <literal>SpringApplicationBuilder</literal> convenience methods (<literal>parent()</literal>, <literal>child()</literal> and <literal>sibling()</literal>).
The bootstrap context is the parent of the most senior ancestor that you create yourself.
Every context in the hierarchy has its own <quote>bootstrap</quote> (possibly empty) property source to avoid promoting values inadvertently from parents down to their descendants.
If there is a Config Server, every context in the hierarchy can also (in principle) have a different <literal>spring.application.name</literal> and, hence, a different remote property source.
Normal Spring application context behavior rules apply to property resolution: properties from a child context override those in
the parent, by name and also by property source name.
(If the child has a property source with the same name as the parent, the value from the parent is not included in the child).</simpara>
<simpara>Note that the <literal>SpringApplicationBuilder</literal> lets you share an <literal>Environment</literal> amongst the whole hierarchy, but that is not the default.
Thus, sibling contexts, in particular, do not need to have the same profiles or property sources, even though they may share common values with their parent.</simpara>
</section>
<section xml:id="customizing-bootstrap-properties">
<title>Changing the Location of Bootstrap Properties</title>
<simpara>The <literal>bootstrap.yml</literal> (or <literal>.properties</literal>) location can be specified by setting <literal>spring.cloud.bootstrap.name</literal> (default: <literal>bootstrap</literal>), <literal>spring.cloud.bootstrap.location</literal> (default: empty) or <literal>spring.cloud.bootstrap.additional-location</literal> (default: empty)&#8201;&#8212;&#8201;for example, in System properties.
Those properties behave like the <literal>spring.config.*</literal> variants with the same name.
With <literal>spring.cloud.bootstrap.location</literal> the default locations are replaced and only the specified ones are used.
To add locations to the list of default ones, <literal>spring.cloud.bootstrap.additional-location</literal> could be used.
In fact, they are used to set up the bootstrap <literal>ApplicationContext</literal> by setting those properties in its <literal>Environment</literal>.
If there is an active profile (from <literal>spring.profiles.active</literal> or through the <literal>Environment</literal> API in the
context you are building), properties in that profile get loaded as well, the same as in a regular Spring Boot app&#8201;&#8212;&#8201;for example, from <literal>bootstrap-development.properties</literal> for a <literal>development</literal> profile.</simpara>
</section>
<section xml:id="overriding-bootstrap-properties">
<title>Overriding the Values of Remote Properties</title>
<simpara>The property sources that are added to your application by the bootstrap context are often <quote>remote</quote> (from example, from Spring Cloud Config Server).
By default, they cannot be overridden locally.
If you want to let your applications override the remote properties with their own System properties or config files, the remote property source has to grant it permission by setting <literal>spring.cloud.config.allowOverride=true</literal> (it does not work to set this locally).
Once that flag is set, two finer-grained settings control the location of the remote properties in relation to system properties and the application&#8217;s local configuration:</simpara>
<itemizedlist>
<listitem>
<simpara><literal>spring.cloud.config.overrideNone=true</literal>: Override from any local property source.</simpara>
</listitem>
<listitem>
<simpara><literal>spring.cloud.config.overrideSystemProperties=false</literal>: Only system properties, command line arguments, and environment variables (but not the local config files) should override the remote settings.</simpara>
</listitem>
</itemizedlist>
</section>
<section xml:id="_customizing_the_bootstrap_configuration">
<title>Customizing the Bootstrap Configuration</title>
<simpara>The bootstrap context can be set to do anything you like by adding entries to <literal>/META-INF/spring.factories</literal> under a key named <literal>org.springframework.cloud.bootstrap.BootstrapConfiguration</literal>.
This holds a comma-separated list of Spring <literal>@Configuration</literal> classes that are used to create the context.
Any beans that you want to be available to the main application context for autowiring can be created here.
There is a special contract for <literal>@Beans</literal> of type <literal>ApplicationContextInitializer</literal>.
If you want to control the startup sequence, classes can be marked with an <literal>@Order</literal> annotation (the default order is <literal>last</literal>).</simpara>
<warning>
<simpara>When adding custom <literal>BootstrapConfiguration</literal>, be careful that the classes you add are not <literal>@ComponentScanned</literal> by mistake into your <quote>main</quote> application context, where they might not be needed.
Use a separate package name for boot configuration classes and make sure that name is not already covered by your <literal>@ComponentScan</literal> or <literal>@SpringBootApplication</literal> annotated configuration classes.</simpara>
</warning>
<simpara>The bootstrap process ends by injecting initializers into the main <literal>SpringApplication</literal> instance (which is the normal Spring Boot startup sequence, whether it is running as a standalone application or deployed in an application server).
First, a bootstrap context is created from the classes found in <literal>spring.factories</literal>.
Then, all <literal>@Beans</literal> of type <literal>ApplicationContextInitializer</literal> are added to the main <literal>SpringApplication</literal> before it is started.</simpara>
</section>
<section xml:id="customizing-bootstrap-property-sources">
<title>Customizing the Bootstrap Property Sources</title>
<simpara>The default property source for external configuration added by the bootstrap process is the Spring Cloud Config Server, but you can add additional sources by adding beans of type <literal>PropertySourceLocator</literal> to the bootstrap context (through <literal>spring.factories</literal>).
For instance, you can insert additional properties from a different server or from a database.</simpara>
<simpara>As an example, consider the following custom locator:</simpara>
<programlisting language="java" linenumbering="unnumbered">@Configuration
public class CustomPropertySourceLocator implements PropertySourceLocator {

    @Override
    public PropertySource&lt;?&gt; locate(Environment environment) {
        return new MapPropertySource("customProperty",
                Collections.&lt;String, Object&gt;singletonMap("property.from.sample.custom.source", "worked as intended"));
    }

}</programlisting>
<simpara>The <literal>Environment</literal> that is passed in is the one for the <literal>ApplicationContext</literal> about to be created&#8201;&#8212;&#8201;in other words, the one for which we supply additional property sources for.
It already has its normal Spring Boot-provided property sources, so you can use those to locate a property source specific to this <literal>Environment</literal> (for example, by keying it on <literal>spring.application.name</literal>, as is done in the default Spring Cloud Config Server property source locator).</simpara>
<simpara>If you create a jar with this class in it and then add a <literal>META-INF/spring.factories</literal> containing the following, the <literal>customProperty</literal> <literal>PropertySource</literal> appears in any application that includes that jar on its classpath:</simpara>
<screen>org.springframework.cloud.bootstrap.BootstrapConfiguration=sample.custom.CustomPropertySourceLocator</screen>
</section>
<section xml:id="_logging_configuration">
<title>Logging Configuration</title>
<simpara>If you are going to use Spring Boot to configure log settings than
you should place this configuration in `bootstrap.[yml | properties]
if you would like it to apply to all events.</simpara>
<note>
<simpara>For Spring Cloud to initialize logging configuration properly you cannot use a custom prefix.  For example,
using <literal>custom.loggin.logpath</literal> will not be recognized by Spring Cloud when initializing the logging system.</simpara>
</note>
</section>
<section xml:id="_environment_changes">
<title>Environment Changes</title>
<simpara>The application listens for an <literal>EnvironmentChangeEvent</literal> and reacts to the change in a couple of standard ways (additional <literal>ApplicationListeners</literal> can be added as <literal>@Beans</literal> by the user in the normal way).
When an <literal>EnvironmentChangeEvent</literal> is observed, it has a list of key values that have changed, and the application uses those to:</simpara>
<itemizedlist>
<listitem>
<simpara>Re-bind any <literal>@ConfigurationProperties</literal> beans in the context</simpara>
</listitem>
<listitem>
<simpara>Set the logger levels for any properties in <literal>logging.level.*</literal></simpara>
</listitem>
</itemizedlist>
<simpara>Note that the Config Client does not, by default, poll for changes in the <literal>Environment</literal>.
Generally, we would not recommend that approach for detecting changes (although you could set it up with a
<literal>@Scheduled</literal> annotation).
If you have a scaled-out client application, it is better to broadcast the <literal>EnvironmentChangeEvent</literal> to all the instances instead of having them polling for changes (for example, by using the <link xl:href="https://github.com/spring-cloud/spring-cloud-bus">Spring Cloud Bus</link>).</simpara>
<simpara>The <literal>EnvironmentChangeEvent</literal> covers a large class of refresh use cases, as long as you can actually make a change to the <literal>Environment</literal> and publish the event.
Note that those APIs are public and part of core Spring).
You can verify that the changes are bound to <literal>@ConfigurationProperties</literal> beans by visiting the <literal>/configprops</literal> endpoint (a normal Spring Boot Actuator feature).
For instance, a <literal>DataSource</literal> can have its <literal>maxPoolSize</literal> changed at runtime (the default <literal>DataSource</literal> created by Spring Boot is an <literal>@ConfigurationProperties</literal> bean) and grow capacity dynamically.
Re-binding <literal>@ConfigurationProperties</literal> does not cover another large class of use cases, where you need more control over the refresh and where you need a change to be atomic over the whole <literal>ApplicationContext</literal>.
To address those concerns, we have <literal>@RefreshScope</literal>.</simpara>
</section>
<section xml:id="refresh-scope">
<title>Refresh Scope</title>
<simpara>When there is a configuration change, a Spring <literal>@Bean</literal> that is marked as <literal>@RefreshScope</literal> gets special treatment.
This feature addresses the problem of stateful beans that only get their configuration injected when they are initialized.
For instance, if a <literal>DataSource</literal> has open connections when the database URL is changed via the <literal>Environment</literal>, you probably want the holders of those connections to be able to complete what they are doing.
Then, the next time something borrows a connection from the pool, it gets one with the new URL.</simpara>
<simpara>Sometimes, it might even be mandatory to apply the <literal>@RefreshScope</literal>
annotation on some beans which can be only initialized once. If a bean
is "immutable", you will have to either annotate the bean with <literal>@RefreshScope</literal>
or specify the classname under the property key
<literal>spring.cloud.refresh.extra-refreshable</literal>.</simpara>
<important>
<simpara>If you create a <literal>DataSource</literal> bean yourself and the implementation is a <literal>HikariDataSource</literal>, return the
most specific type, in this case <literal>HikariDataSource</literal>. Otherwise, you will need to set
<literal>spring.cloud.refresh.extra-refreshable=javax.sql.DataSource</literal>.</simpara>
</important>
<simpara>Refresh scope beans are lazy proxies that initialize when they are used (that is, when a method is called), and the scope acts as a cache of initialized values.
To force a bean to re-initialize on the next method call, you must invalidate its cache entry.</simpara>
<simpara>The <literal>RefreshScope</literal> is a bean in the context and has a public  <literal>refreshAll()</literal> method to refresh all beans in the scope by clearing the target cache.
The <literal>/refresh</literal> endpoint exposes this functionality (over HTTP or JMX).
To refresh an individual bean by name, there is also a <literal>refresh(String)</literal> method.</simpara>
<simpara>To expose the <literal>/refresh</literal> endpoint, you need to add following configuration to your application:</simpara>
<programlisting language="yaml" linenumbering="unnumbered">management:
  endpoints:
    web:
      exposure:
        include: refresh</programlisting>
<note>
<simpara><literal>@RefreshScope</literal> works (technically) on an <literal>@Configuration</literal> class, but it might lead to surprising behavior.
For example, it does not mean that all the <literal>@Beans</literal> defined in that class are themselves in <literal>@RefreshScope</literal>.
Specifically, anything that depends on those beans cannot rely on them being updated when a refresh is initiated, unless it is itself in <literal>@RefreshScope</literal>.
In that case, it is rebuilt on a refresh and its dependencies are re-injected. At that point, they are re-initialized from the refreshed <literal>@Configuration</literal>).</simpara>
</note>
</section>
<section xml:id="_encryption_and_decryption">
<title>Encryption and Decryption</title>
<simpara>Spring Cloud has an <literal>Environment</literal> pre-processor for decrypting property values locally.
It follows the same rules as the Config Server and has the same external configuration through <literal>encrypt.*</literal>.
Thus, you can use encrypted values in the form of <literal>{cipher}*</literal> and, as long as there is a valid key, they are decrypted before the main application context gets the <literal>Environment</literal> settings.
To use the encryption features in an application, you need to include Spring Security RSA in your classpath (Maven co-ordinates: "org.springframework.security:spring-security-rsa"), and you also need the full strength JCE extensions in your JVM.</simpara>
<simpara>If you get an exception due to "Illegal key size" and you use Sun&#8217;s JDK, you need to install the Java Cryptography Extension (JCE) Unlimited Strength Jurisdiction Policy Files.
See the following links for more information:</simpara>
<itemizedlist>
<listitem>
<simpara><link xl:href="https://www.oracle.com/technetwork/java/javase/downloads/jce-6-download-429243.html">Java 6 JCE</link></simpara>
</listitem>
<listitem>
<simpara><link xl:href="https://www.oracle.com/technetwork/java/javase/downloads/jce-7-download-432124.html">Java 7 JCE</link></simpara>
</listitem>
<listitem>
<simpara><link xl:href="https://www.oracle.com/technetwork/java/javase/downloads/jce8-download-2133166.html">Java 8 JCE</link></simpara>
</listitem>
</itemizedlist>
<simpara>Extract the files into the JDK/jre/lib/security folder for whichever version of JRE/JDK x64/x86 you use.</simpara>
</section>
<section xml:id="_endpoints">
<title>Endpoints</title>
<simpara>For a Spring Boot Actuator application, some additional management endpoints are available. You can use:</simpara>
<itemizedlist>
<listitem>
<simpara><literal>POST</literal> to <literal>/actuator/env</literal> to update the <literal>Environment</literal> and rebind <literal>@ConfigurationProperties</literal> and log levels.</simpara>
</listitem>
<listitem>
<simpara><literal>/actuator/refresh</literal> to re-load the boot strap context and refresh the <literal>@RefreshScope</literal> beans.</simpara>
</listitem>
<listitem>
<simpara><literal>/actuator/restart</literal> to close the <literal>ApplicationContext</literal> and restart it (disabled by default).</simpara>
</listitem>
<listitem>
<simpara><literal>/actuator/pause</literal> and <literal>/actuator/resume</literal> for calling the <literal>Lifecycle</literal> methods (<literal>stop()</literal> and <literal>start()</literal> on the <literal>ApplicationContext</literal>).</simpara>
</listitem>
</itemizedlist>
<note>
<simpara>If you disable the <literal>/actuator/restart</literal> endpoint then the <literal>/actuator/pause</literal> and <literal>/actuator/resume</literal> endpoints
will also be disabled since they are just a special case of <literal>/actuator/restart</literal>.</simpara>
</note>
</section>
</chapter>
<chapter xml:id="_spring_cloud_commons_common_abstractions">
<title>Spring Cloud Commons: Common Abstractions</title>
<simpara>Patterns such as service discovery, load balancing, and circuit breakers lend themselves to a common abstraction layer that can be consumed by all Spring Cloud clients, independent of the implementation (for example, discovery with Eureka or Consul).</simpara>
<section xml:id="_enablediscoveryclient">
<title>@EnableDiscoveryClient</title>
<simpara>Spring Cloud Commons provides the <literal>@EnableDiscoveryClient</literal> annotation.
This looks for implementations of the <literal>DiscoveryClient</literal> interface with <literal>META-INF/spring.factories</literal>.
Implementations of the Discovery Client add a configuration class to <literal>spring.factories</literal> under the <literal>org.springframework.cloud.client.discovery.EnableDiscoveryClient</literal> key.
Examples of <literal>DiscoveryClient</literal> implementations include <link xl:href="https://cloud.spring.io/spring-cloud-netflix/">Spring Cloud Netflix Eureka</link>, <link xl:href="https://cloud.spring.io/spring-cloud-consul/">Spring Cloud Consul Discovery</link>, and <link xl:href="https://cloud.spring.io/spring-cloud-zookeeper/">Spring Cloud Zookeeper Discovery</link>.</simpara>
<simpara>By default, implementations of <literal>DiscoveryClient</literal> auto-register the local Spring Boot server with the remote discovery server.
This behavior can be disabled by setting <literal>autoRegister=false</literal> in <literal>@EnableDiscoveryClient</literal>.</simpara>
<note>
<simpara><literal>@EnableDiscoveryClient</literal> is no longer required.
You can put a <literal>DiscoveryClient</literal> implementation on the classpath to cause the Spring Boot application to register with the service discovery server.</simpara>
</note>
<section xml:id="_health_indicator">
<title>Health Indicator</title>
<simpara>Commons creates a Spring Boot <literal>HealthIndicator</literal> that <literal>DiscoveryClient</literal> implementations can participate in by implementing <literal>DiscoveryHealthIndicator</literal>.
To disable the composite <literal>HealthIndicator</literal>, set <literal>spring.cloud.discovery.client.composite-indicator.enabled=false</literal>.
A generic <literal>HealthIndicator</literal> based on <literal>DiscoveryClient</literal> is auto-configured (<literal>DiscoveryClientHealthIndicator</literal>).
To disable it, set <literal>spring.cloud.discovery.client.health-indicator.enabled=false</literal>.
To disable the description field of the <literal>DiscoveryClientHealthIndicator</literal>, set <literal>spring.cloud.discovery.client.health-indicator.include-description=false</literal>.
Otherwise, it can bubble up as the <literal>description</literal> of the rolled up <literal>HealthIndicator</literal>.</simpara>
</section>
<section xml:id="_ordering_discoveryclient_instances">
<title>Ordering <literal>DiscoveryClient</literal> instances</title>
<simpara><literal>DiscoveryClient</literal> interface extends <literal>Ordered</literal>. This is useful when using multiple discovery
 clients, as it allows you to define the order of the returned discovery clients, similar to
how you can order the beans loaded by a Spring application. By default, the order of any <literal>DiscoveryClient</literal> is set to
<literal>0</literal>. If you want to set a different order for your custom <literal>DiscoveryClient</literal> implementations, you just need to override
the <literal>getOrder()</literal> method so that it returns the value that is suitable for your setup. Apart from this, you can use
properties to set the order of the <literal>DiscoveryClient</literal>
implementations provided by Spring Cloud, among others  <literal>ConsulDiscoveryClient</literal>, <literal>EurekaDiscoveryClient</literal> and
<literal>ZookeeperDiscoveryClient</literal>. In order to do it, you just need to set the
<literal>spring.cloud.{clientIdentifier}.discovery.order</literal> (or <literal>eureka.client.order</literal> for Eureka) property to the desired value.</simpara>
</section>
</section>
<section xml:id="_serviceregistry">
<title>ServiceRegistry</title>
<simpara>Commons now provides a <literal>ServiceRegistry</literal> interface that provides methods such as <literal>register(Registration)</literal> and <literal>deregister(Registration)</literal>, which let you provide custom registered services.
<literal>Registration</literal> is a marker interface.</simpara>
<simpara>The following example shows the <literal>ServiceRegistry</literal> in use:</simpara>
<programlisting language="java" linenumbering="unnumbered">@Configuration
@EnableDiscoveryClient(autoRegister=false)
public class MyConfiguration {
    private ServiceRegistry registry;

    public MyConfiguration(ServiceRegistry registry) {
        this.registry = registry;
    }

    // called through some external process, such as an event or a custom actuator endpoint
    public void register() {
        Registration registration = constructRegistration();
        this.registry.register(registration);
    }
}</programlisting>
<simpara>Each <literal>ServiceRegistry</literal> implementation has its own <literal>Registry</literal> implementation.</simpara>
<itemizedlist>
<listitem>
<simpara><literal>ZookeeperRegistration</literal> used with <literal>ZookeeperServiceRegistry</literal></simpara>
</listitem>
<listitem>
<simpara><literal>EurekaRegistration</literal> used with <literal>EurekaServiceRegistry</literal></simpara>
</listitem>
<listitem>
<simpara><literal>ConsulRegistration</literal> used with <literal>ConsulServiceRegistry</literal></simpara>
</listitem>
</itemizedlist>
<simpara>If you are using the <literal>ServiceRegistry</literal> interface, you are going to need to pass the
correct <literal>Registry</literal> implementation for the <literal>ServiceRegistry</literal> implementation you
are using.</simpara>
<section xml:id="_serviceregistry_auto_registration">
<title>ServiceRegistry Auto-Registration</title>
<simpara>By default, the <literal>ServiceRegistry</literal> implementation auto-registers the running service.
To disable that behavior, you can set:
* <literal>@EnableDiscoveryClient(autoRegister=false)</literal> to permanently disable auto-registration.
* <literal>spring.cloud.service-registry.auto-registration.enabled=false</literal> to disable the behavior through configuration.</simpara>
<section xml:id="_serviceregistry_auto_registration_events">
<title>ServiceRegistry Auto-Registration Events</title>
<simpara>There are two events that will be fired when a service auto-registers.  The first event, called
<literal>InstancePreRegisteredEvent</literal>, is fired before the service is registered.  The second
event, called <literal>InstanceRegisteredEvent</literal>, is fired after the service is registered.  You can register an
<literal>ApplicationListener</literal>(s) to listen to and react to these events.</simpara>
<note>
<simpara>These events will not be fired if <literal>spring.cloud.service-registry.auto-registration.enabled</literal> is set to <literal>false</literal>.</simpara>
</note>
</section>
</section>
<section xml:id="_service_registry_actuator_endpoint">
<title>Service Registry Actuator Endpoint</title>
<simpara>Spring Cloud Commons provides a <literal>/service-registry</literal> actuator endpoint.
This endpoint relies on a <literal>Registration</literal> bean in the Spring Application Context.
Calling <literal>/service-registry</literal> with GET returns the status of the <literal>Registration</literal>.
Using POST to the same endpoint with a JSON body changes the status of the current <literal>Registration</literal> to the new value.
The JSON body has to include the <literal>status</literal> field with the preferred value.
Please see the documentation of the <literal>ServiceRegistry</literal> implementation you use for the allowed values when updating the status and the values returned for the status.
For instance, Eureka&#8217;s supported statuses are <literal>UP</literal>, <literal>DOWN</literal>, <literal>OUT_OF_SERVICE</literal>, and <literal>UNKNOWN</literal>.</simpara>
</section>
</section>
<section xml:id="_spring_resttemplate_as_a_load_balancer_client">
<title>Spring RestTemplate as a Load Balancer Client</title>
<simpara><literal>RestTemplate</literal> can be automatically configured to use a Load-balancer client under the hood.
To create a load-balanced <literal>RestTemplate</literal>, create a <literal>RestTemplate</literal> <literal>@Bean</literal> and use the <literal>@LoadBalanced</literal> qualifier, as shown in the following example:</simpara>
<programlisting language="java" linenumbering="unnumbered">@Configuration
public class MyConfiguration {

    @LoadBalanced
    @Bean
    RestTemplate restTemplate() {
        return new RestTemplate();
    }
}

public class MyClass {
    @Autowired
    private RestTemplate restTemplate;

    public String doOtherStuff() {
        String results = restTemplate.getForObject("http://stores/stores", String.class);
        return results;
    }
}</programlisting>
<caution>
<simpara>A <literal>RestTemplate</literal> bean is no longer created through auto-configuration.
Individual applications must create it.</simpara>
</caution>
<simpara>The URI needs to use a virtual host name (that is, a service name, not a host name).
The Ribbon client is used to create a full physical address.
See <link xl:href="https://github.com/spring-cloud/spring-cloud-netflix/blob/master/spring-cloud-netflix-ribbon/src/main/java/org/springframework/cloud/netflix/ribbon/RibbonAutoConfiguration.java">RibbonAutoConfiguration</link> for details of how the <literal>RestTemplate</literal> is set up.</simpara>
<important>
<simpara>In order to use a load-balanced <literal>RestTemplate</literal>, you need to have a load-balancer implementation in your classpath.
The recommended implementation is <literal>BlockingLoadBalancerClient</literal>
- add <literal>org.springframework.cloud:spring-cloud-loadbalancer</literal> in order to use it.
The
<literal>RibbonLoadBalancerClient</literal> also can be used, but it&#8217;s now under maintenance and we do not recommend adding it to new projects.</simpara>
</important>
<warning>
<simpara>If you want to use <literal>BlockingLoadBalancerClient</literal>, make sure you do not have
<literal>RibbonLoadBalancerClient</literal> in the project classpath, as for backward compatibility reasons, it will be used by default.</simpara>
</warning>
</section>
<section xml:id="_spring_webclient_as_a_load_balancer_client">
<title>Spring WebClient as a Load Balancer Client</title>
<simpara><literal>WebClient</literal> can be automatically configured to use a load-balancer client.
To create a load-balanced <literal>WebClient</literal>, create a <literal>WebClient.Builder</literal> <literal>@Bean</literal> and use the <literal>@LoadBalanced</literal> qualifier, as shown in the following example:</simpara>
<programlisting language="java" linenumbering="unnumbered">@Configuration
public class MyConfiguration {

	@Bean
	@LoadBalanced
	public WebClient.Builder loadBalancedWebClientBuilder() {
		return WebClient.builder();
	}
}

public class MyClass {
    @Autowired
    private WebClient.Builder webClientBuilder;

    public Mono&lt;String&gt; doOtherStuff() {
        return webClientBuilder.build().get().uri("http://stores/stores")
        				.retrieve().bodyToMono(String.class);
    }
}</programlisting>
<simpara>The URI needs to use a virtual host name (that is, a service name, not a host name).
The Ribbon client is used to create a full physical address.</simpara>
<important>
<simpara>If you want to use a <literal>@LoadBalanced WebClient.Builder</literal>, you need to have a loadbalancer
implementation in the classpath. It is recommended that you add the
<literal>org.springframework.cloud:spring-cloud-loadbalancer</literal> dependency to your project.
Then, <literal>ReactiveLoadBalancer</literal> will be used underneath.
Alternatively, this functionality will also work with spring-cloud-starter-netflix-ribbon, but the request
will be handled by a non-reactive <literal>LoadBalancerClient</literal> under the hood. Additionally,
spring-cloud-starter-netflix-ribbon is already in maintenance mode, so we do not recommned
adding it to new projects.</simpara>
</important>
<tip>
<simpara>The <literal>ReactorLoadBalancer</literal> used underneath supports caching. If <literal>cacheManager</literal> is detected,
cached version of <literal>ServiceInstanceSupplier</literal> will be used. If not, we will retrieve instances
from discovery service without caching them. We recommend <link xl:href="https://docs.spring.io/spring-boot/docs/current/reference/html/boot-features-caching.html">enabling caching</link> in your project
if you use <literal>ReactiveLoadBalancer</literal>.</simpara>
</tip>
<section xml:id="_retrying_failed_requests">
<title>Retrying Failed Requests</title>
<simpara>A load-balanced <literal>RestTemplate</literal> can be configured to retry failed requests.
By default, this logic is disabled.
You can enable it by adding <link xl:href="https://github.com/spring-projects/spring-retry">Spring Retry</link> to your application&#8217;s classpath.
The load-balanced <literal>RestTemplate</literal> honors some of the Ribbon configuration values related to retrying failed requests.
You can use <literal>client.ribbon.MaxAutoRetries</literal>, <literal>client.ribbon.MaxAutoRetriesNextServer</literal>, and <literal>client.ribbon.OkToRetryOnAllOperations</literal> properties.
If you would like to disable the retry logic with Spring Retry on the classpath, you can set <literal>spring.cloud.loadbalancer.retry.enabled=false</literal>.
See the <link xl:href="https://github.com/Netflix/ribbon/wiki/Getting-Started#the-properties-file-sample-clientproperties">Ribbon documentation</link> for a description of what these properties do.</simpara>
<simpara>If you would like to implement a <literal>BackOffPolicy</literal> in your retries, you need to create a bean of type <literal>LoadBalancedRetryFactory</literal> and override the <literal>createBackOffPolicy</literal> method:</simpara>
<programlisting language="java" linenumbering="unnumbered">@Configuration
public class MyConfiguration {
    @Bean
    LoadBalancedRetryFactory retryFactory() {
        return new LoadBalancedRetryFactory() {
            @Override
            public BackOffPolicy createBackOffPolicy(String service) {
        		return new ExponentialBackOffPolicy();
        	}
        };
    }
}</programlisting>
<note>
<simpara><literal>client</literal> in the preceding examples should be replaced with your Ribbon client&#8217;s name.</simpara>
</note>
<simpara>If you want to add one or more <literal>RetryListener</literal> implementations to your retry functionality, you need to
create a bean of type <literal>LoadBalancedRetryListenerFactory</literal> and return the <literal>RetryListener</literal> array
you would like to use for a given service, as shown in the following example:</simpara>
<programlisting language="java" linenumbering="unnumbered">@Configuration
public class MyConfiguration {
    @Bean
    LoadBalancedRetryListenerFactory retryListenerFactory() {
        return new LoadBalancedRetryListenerFactory() {
            @Override
            public RetryListener[] createRetryListeners(String service) {
                return new RetryListener[]{new RetryListener() {
                    @Override
                    public &lt;T, E extends Throwable&gt; boolean open(RetryContext context, RetryCallback&lt;T, E&gt; callback) {
                        //TODO Do you business...
                        return true;
                    }

                    @Override
                     public &lt;T, E extends Throwable&gt; void close(RetryContext context, RetryCallback&lt;T, E&gt; callback, Throwable throwable) {
                        //TODO Do you business...
                    }

                    @Override
                    public &lt;T, E extends Throwable&gt; void onError(RetryContext context, RetryCallback&lt;T, E&gt; callback, Throwable throwable) {
                        //TODO Do you business...
                    }
                }};
            }
        };
    }
}</programlisting>
</section>
</section>
<section xml:id="_multiple_resttemplate_objects">
<title>Multiple RestTemplate objects</title>
<simpara>If you want a <literal>RestTemplate</literal> that is not load-balanced, create a <literal>RestTemplate</literal> bean and inject it.
To access the load-balanced <literal>RestTemplate</literal>, use the <literal>@LoadBalanced</literal> qualifier when you create your <literal>@Bean</literal>, as shown in the following example:</simpara>
<programlisting language="java" linenumbering="unnumbered">@Configuration
public class MyConfiguration {

    @LoadBalanced
    @Bean
    RestTemplate loadBalanced() {
        return new RestTemplate();
    }

    @Primary
    @Bean
    RestTemplate restTemplate() {
        return new RestTemplate();
    }
}

public class MyClass {
@Autowired
private RestTemplate restTemplate;

    @Autowired
    @LoadBalanced
    private RestTemplate loadBalanced;

    public String doOtherStuff() {
        return loadBalanced.getForObject("http://stores/stores", String.class);
    }

    public String doStuff() {
        return restTemplate.getForObject("https://example.com", String.class);
    }
}</programlisting>
<important>
<simpara>Notice the use of the <literal>@Primary</literal> annotation on the plain <literal>RestTemplate</literal> declaration in the preceding example to disambiguate the unqualified <literal>@Autowired</literal> injection.</simpara>
</important>
<tip>
<simpara>If you see errors such as <literal>java.lang.IllegalArgumentException: Can not set org.springframework.web.client.RestTemplate field com.my.app.Foo.restTemplate to com.sun.proxy.$Proxy89</literal>, try injecting <literal>RestOperations</literal> or setting <literal>spring.aop.proxyTargetClass=true</literal>.</simpara>
</tip>
</section>
<section xml:id="_multiple_webclient_objects">
<title>Multiple WebClient Objects</title>
<simpara>If you want a <literal>WebClient</literal> that is not load-balanced, create a <literal>WebClient</literal> bean and inject it.
To access the load-balanced <literal>WebClient</literal>, use the <literal>@LoadBalanced</literal> qualifier when you create your <literal>@Bean</literal>, as shown in the following example:</simpara>
<programlisting language="java" linenumbering="unnumbered">@Configuration
public class MyConfiguration {

    @LoadBalanced
    @Bean
    WebClient.Builder loadBalanced() {
        return WebClient.builder();
    }

    @Primary
    @Bean
    WebClient.Builder webClient() {
        return WebClient.builder();
    }
}

public class MyClass {
    @Autowired
    private WebClient.Builder webClientBuilder;

    @Autowired
    @LoadBalanced
    private WebClient.Builder loadBalanced;

    public Mono&lt;String&gt; doOtherStuff() {
        return loadBalanced.build().get().uri("http://stores/stores")
        				.retrieve().bodyToMono(String.class);
    }

    public Mono&lt;String&gt; doStuff() {
        return webClientBuilder.build().get().uri("http://example.com")
        				.retrieve().bodyToMono(String.class);
    }
}</programlisting>
</section>
<section xml:id="loadbalanced-webclient">
<title>Spring WebFlux WebClient as a Load Balancer Client</title>
<section xml:id="webflux-with-reactive-loadbalancer">
<title>Spring WebFlux WebClient with Reactive Load Balancer</title>
<simpara><literal>WebClient</literal> can be configured to use the <literal>ReactiveLoadBalancer</literal>.
If you add <literal>org.springframework.cloud:spring-cloud-loadbalancer</literal> to your project,
 <literal>ReactorLoadBalancerExchangeFilterFunction</literal> is auto-configured if <literal>spring-webflux</literal> is on the classpath.
The following example shows how to configure a <literal>WebClient</literal> to use reactive load balancer under the hood:</simpara>
<programlisting language="java" linenumbering="unnumbered">public class MyClass {
    @Autowired
    private ReactorLoadBalancerExchangeFilterFunction lbFunction;

    public Mono&lt;String&gt; doOtherStuff() {
        return WebClient.builder().baseUrl("http://stores")
            .filter(lbFunction)
            .build()
            .get()
            .uri("/stores")
            .retrieve()
            .bodyToMono(String.class);
    }
}</programlisting>
<simpara>The URI needs to use a virtual host name (that is, a service name, not a host name).
The <literal>ReactorLoadBalancerClient</literal> is used to create a full physical address.</simpara>
</section>
<section xml:id="_spring_webflux_webclient_with_non_reactive_load_balancer_client">
<title>Spring WebFlux WebClient with non-reactive Load Balancer Client</title>
<simpara>If you you don&#8217;t have <literal>org.springframework.cloud:spring-cloud-loadbalancer</literal> in your project,
but you do have spring-cloud-starter-netflix-ribbon, you can still use <literal>WebClient</literal> with <literal>LoadBalancerClient</literal>. <literal>LoadBalancerExchangeFilterFunction</literal>
will be auto-configured if <literal>spring-webflux</literal> is on the classpath. Please note, however, that this is
uses a non-reactive client under the hood.
The following example shows how to configure a <literal>WebClient</literal> to use load balancer:</simpara>
<programlisting language="java" linenumbering="unnumbered">public class MyClass {
    @Autowired
    private LoadBalancerExchangeFilterFunction lbFunction;

    public Mono&lt;String&gt; doOtherStuff() {
        return WebClient.builder().baseUrl("http://stores")
            .filter(lbFunction)
            .build()
            .get()
            .uri("/stores")
            .retrieve()
            .bodyToMono(String.class);
    }
}</programlisting>
<simpara>The URI needs to use a virtual host name (that is, a service name, not a host name).
The <literal>LoadBalancerClient</literal> is used to create a full physical address.</simpara>
<simpara>WARN: This approach is now deprecated.
We suggest you use <link linkend="webflux-with-reactive-loadbalancer">WebFlux with reactive Load-Balancer</link>
instead.</simpara>
</section>
<section xml:id="_passing_your_own_load_balancer_client_configuration">
<title>Passing your own Load-Balancer Client configuration</title>
<simpara>You can also use the <literal>@LoadBalancerClient</literal> annotation to pass your own load-balancer client configuration, passing the name of the load-balancer client and the configuration class, like so:</simpara>
<programlisting language="java" linenumbering="unnumbered">@Configuration
@LoadBalancerClient(value = "stores", configuration = StoresLoadBalancerClientConfiguration.class)
public class MyConfiguration {

	@Bean
	@LoadBalanced
	public WebClient.Builder loadBalancedWebClientBuilder() {
		return WebClient.builder();
	}
}</programlisting>
<simpara>It is also possible to pass together multiple configurations (for more than one load-balancer client) via the <literal>@LoadBalancerClients</literal> annotation, as shown below:</simpara>
<programlisting language="java" linenumbering="unnumbered">@Configuration
@LoadBalancerClients({@LoadBalancerClient(value = "stores", configuration = StoresLoadBalancerClientConfiguration.class), @LoadBalancerClient(value = "customers", configuration = CustomersLoadBalancerClientConfiguration.class)})
public class MyConfiguration {

	@Bean
	@LoadBalanced
	public WebClient.Builder loadBalancedWebClientBuilder() {
		return WebClient.builder();
	}
}</programlisting>
</section>
</section>
<section xml:id="ignore-network-interfaces">
<title>Ignore Network Interfaces</title>
<simpara>Sometimes, it is useful to ignore certain named network interfaces so that they can be excluded from Service Discovery registration (for example, when running in a Docker container).
A list of regular expressions can be set to cause the desired network interfaces to be ignored.
The following configuration ignores the <literal>docker0</literal> interface and all interfaces that start with <literal>veth</literal>:</simpara>
<formalpara>
<title>application.yml</title>
<para>
<screen>spring:
  cloud:
    inetutils:
      ignoredInterfaces:
        - docker0
        - veth.*</screen>
</para>
</formalpara>
<simpara>You can also force the use of only specified network addresses by using a list of regular expressions, as shown in the following example:</simpara>
<formalpara>
<title>bootstrap.yml</title>
<para>
<screen>spring:
  cloud:
    inetutils:
      preferredNetworks:
        - 192.168
        - 10.0</screen>
</para>
</formalpara>
<simpara>You can also force the use of only site-local addresses, as shown in the following example:
.application.yml</simpara>
<screen>spring:
  cloud:
    inetutils:
      useOnlySiteLocalInterfaces: true</screen>
<simpara>See <link xl:href="https://docs.oracle.com/javase/8/docs/api/java/net/Inet4Address.html#isSiteLocalAddress--">Inet4Address.html.isSiteLocalAddress()</link> for more details about what constitutes a site-local address.</simpara>
</section>
<section xml:id="http-clients">
<title>HTTP Client Factories</title>
<simpara>Spring Cloud Commons provides beans for creating both Apache HTTP clients (<literal>ApacheHttpClientFactory</literal>) and OK HTTP clients (<literal>OkHttpClientFactory</literal>).
The <literal>OkHttpClientFactory</literal> bean is created only if the OK HTTP jar is on the classpath.
In addition, Spring Cloud Commons provides beans for creating the connection managers used by both clients: <literal>ApacheHttpClientConnectionManagerFactory</literal> for the Apache HTTP client and <literal>OkHttpClientConnectionPoolFactory</literal> for the OK HTTP client.
If you would like to customize how the HTTP clients are created in downstream projects, you can provide your own implementation of these beans.
In addition, if you provide a bean of type <literal>HttpClientBuilder</literal> or <literal>OkHttpClient.Builder</literal>, the default factories use these builders as the basis for the builders returned to downstream projects.
You can also disable the creation of these beans by setting <literal>spring.cloud.httpclientfactories.apache.enabled</literal> or <literal>spring.cloud.httpclientfactories.ok.enabled</literal> to <literal>false</literal>.</simpara>
</section>
<section xml:id="enabled-features">
<title>Enabled Features</title>
<simpara>Spring Cloud Commons provides a <literal>/features</literal> actuator endpoint.
This endpoint returns features available on the classpath and whether they are enabled.
The information returned includes the feature type, name, version, and vendor.</simpara>
<section xml:id="_feature_types">
<title>Feature types</title>
<simpara>There are two types of 'features': abstract and named.</simpara>
<simpara>Abstract features are features where an interface or abstract class is defined and that an implementation the creates, such as <literal>DiscoveryClient</literal>, <literal>LoadBalancerClient</literal>, or <literal>LockService</literal>.
The abstract class or interface is used to find a bean of that type in the context.
The version displayed is <literal>bean.getClass().getPackage().getImplementationVersion()</literal>.</simpara>
<simpara>Named features are features that do not have a particular class they implement, such as "Circuit Breaker", "API Gateway", "Spring Cloud Bus", and others. These features require a name and a bean type.</simpara>
</section>
<section xml:id="_declaring_features">
<title>Declaring features</title>
<simpara>Any module can declare any number of <literal>HasFeature</literal> beans, as shown in the following examples:</simpara>
<programlisting language="java" linenumbering="unnumbered">@Bean
public HasFeatures commonsFeatures() {
  return HasFeatures.abstractFeatures(DiscoveryClient.class, LoadBalancerClient.class);
}

@Bean
public HasFeatures consulFeatures() {
  return HasFeatures.namedFeatures(
    new NamedFeature("Spring Cloud Bus", ConsulBusAutoConfiguration.class),
    new NamedFeature("Circuit Breaker", HystrixCommandAspect.class));
}

@Bean
HasFeatures localFeatures() {
  return HasFeatures.builder()
      .abstractFeature(Foo.class)
      .namedFeature(new NamedFeature("Bar Feature", Bar.class))
      .abstractFeature(Baz.class)
      .build();
}</programlisting>
<simpara>Each of these beans should go in an appropriately guarded <literal>@Configuration</literal>.</simpara>
</section>
</section>
<section xml:id="_spring_cloud_compatibility_verification">
<title>Spring Cloud Compatibility Verification</title>
<simpara>Due to the fact that some users have problem with setting up Spring Cloud application, we&#8217;ve decided
to add a compatibility verification mechanism. It will break if your current setup is not compatible
with Spring Cloud requirements, together with a report, showing what exactly went wrong.</simpara>
<simpara>At the moment we verify which version of Spring Boot is added to your classpath.</simpara>
<simpara>Example of a report</simpara>
<screen>***************************
APPLICATION FAILED TO START
***************************

Description:

Your project setup is incompatible with our requirements due to following reasons:

- Spring Boot [2.1.0.RELEASE] is not compatible with this Spring Cloud release train


Action:

Consider applying the following actions:

- Change Spring Boot version to one of the following versions [1.2.x, 1.3.x] .
You can find the latest Spring Boot versions here [https://spring.io/projects/spring-boot#learn].
If you want to learn more about the Spring Cloud Release train compatibility, you can visit this page [https://spring.io/projects/spring-cloud#overview] and check the [Release Trains] section.</screen>
<simpara>In order to disable this feature, set <literal>spring.cloud.compatibility-verifier.enabled</literal> to <literal>false</literal>.
If you want to override the compatible Spring Boot versions, just set the
<literal>spring.cloud.compatibility-verifier.compatible-boot-versions</literal> property with a comma separated list
of compatible Spring Boot versions.</simpara>
</section>
</chapter>
</book>