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Framework for building Event-Driven Microservices

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spring-cloud/spring-cloud-stream

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Introduction

Spring Cloud Stream is a framework for building message-driven microservice applications.Spring Cloud Stream builds upon Spring Boot to create standalone, production-grade Spring applications and uses Spring Integration to provide connectivity to message brokers.It provides opinionated configuration of middleware from several vendors, introducing the concepts of persistent publish-subscribe semantics, consumer groups, and partitions.These are called binder implementations in the parlance of Spring Cloud Stream.Out of the box, Spring Cloud Stream provides binder implementations for Apache Kafka and RabbitMQ.While these two binder implementations are based on Message Channels, Spring Cloud Stream also provides another binder implementation for Kafka Streams that does not use message channels, but native Kafka Streams types such as KStream, KTable etc.

Below, you can find more information on how to use these various out-of-the-box binder implementations in your applications.

Apache Kafka Binder

Usage

To use Apache Kafka binder, you need to addspring-cloud-stream-binder-kafka as a dependency to your Spring Cloud Stream application, as shown in the following example for Maven:

<dependency>  <groupId>org.springframework.cloud</groupId>  <artifactId>spring-cloud-stream-binder-kafka</artifactId></dependency>

Alternatively, you can also use the Spring Cloud Stream Kafka Starter, as shown in the following example for Maven:

<dependency>  <groupId>org.springframework.cloud</groupId>  <artifactId>spring-cloud-starter-stream-kafka</artifactId></dependency>

Apache Kafka Streams Binder

Usage

To use Apache Kafka Streams binder, you need to addspring-cloud-stream-binder-kafka-streams as a dependency to your Spring Cloud Stream application, as shown in the following example for Maven:

<dependency>  <groupId>org.springframework.cloud</groupId>  <artifactId>spring-cloud-stream-binder-kafka-streams</artifactId></dependency>

RabbitMQ Binder

Usage

To use the RabbitMQ binder, you can add it to your Spring Cloud Stream application, by using the following Maven coordinates:

<dependency>  <groupId>org.springframework.cloud</groupId>  <artifactId>spring-cloud-stream-binder-rabbit</artifactId></dependency>

Alternatively, you can use the Spring Cloud Stream RabbitMQ Starter, as follows:

<dependency>  <groupId>org.springframework.cloud</groupId>  <artifactId>spring-cloud-starter-stream-rabbit</artifactId></dependency>

Resources

For more information, please visit theproject website:

Building

Basic Compile and Test

To build the source you will need to install JDK 17.

Spring Cloud uses Maven for most build-related activities, and youshould be able to get off the ground quite quickly by cloning theproject you are interested in and typing

$ ./mvnw install
Note
You can also install Maven (>=3.3.3) yourself and run themvn commandin place of./mvnw in the examples below. If you do that you alsomight need to add-P spring if your local Maven settings do notcontain repository declarations for spring pre-release artifacts.
Note
Be aware that you might need to increase the amount of memoryavailable to Maven by setting aMAVEN_OPTS environment variable witha value like-Xmx512m -XX:MaxPermSize=128m. We try to cover this inthe.mvn configuration, so if you find you have to do it to make abuild succeed, please raise a ticket to get the settings added tosource control.

The projects that require middleware (i.e. Redis) for testing generallyrequire that a local instance of [Docker](https://www.docker.com/get-started) is installed and running.

Documentation

The spring-cloud-build module has a "docs" profile, and if you switchthat on it will try to build asciidoc sources fromsrc/main/asciidoc. As part of that process it will look for aREADME.adoc and process it by loading all the includes, but notparsing or rendering it, just copying it to${main.basedir}(defaults to${basedir}, i.e. the root of the project). If there areany changes in the README it will then show up after a Maven build asa modified file in the correct place. Just commit it and push the change.

Working with the code

If you don’t have an IDE preference we would recommend that you useSpring Tools Suite orEclipse when working with the code. We use them2eclipse eclipse plugin for maven support. Other IDEs and toolsshould also work without issue as long as they use Maven 3.3.3 or better.

Activate the Spring Maven profile

Spring Cloud projects require the 'spring' Maven profile to be activated to resolvethe spring milestone and snapshot repositories. Use your preferred IDE to set thisprofile to be active, or you may experience build errors.

Importing into eclipse with m2eclipse

We recommend them2eclipse eclipse plugin when working witheclipse. If you don’t already have m2eclipse installed it is available from the "eclipsemarketplace".

Note
Older versions of m2e do not support Maven 3.3, so once theprojects are imported into Eclipse you will also need to tellm2eclipse to use the right profile for the projects. If yousee many different errors related to the POMs in the projects, checkthat you have an up to date installation. If you can’t upgrade m2e,add the "spring" profile to yoursettings.xml. Alternatively you cancopy the repository settings from the "spring" profile of the parentpom into yoursettings.xml.

Importing into eclipse without m2eclipse

If you prefer not to use m2eclipse you can generate eclipse project metadata using thefollowing command:

$ ./mvnw eclipse:eclipse

The generated eclipse projects can be imported by selectingimport existing projectsfrom thefile menu.

Contributing

Spring Cloud is released under the non-restrictive Apache 2.0 license,and follows a very standard Github development process, using Githubtracker for issues and merging pull requests into master. If you wantto contribute even something trivial please do not hesitate, butfollow the guidelines below.

Developer Certificate of Origin (DCO)

All commits must include aSigned-off-by trailer at the end of each commit message to indicate the contributor agrees to theDeveloper Certificate of Origin.For additional details please review the followingblog post

Code of Conduct

This project adheres to the Contributor Covenantcode ofconduct. By participating, you are expected to uphold this code. Please reportunacceptable behavior tospring-code-of-conduct@pivotal.io.

Code Conventions and Housekeeping

None of these is essential for a pull request, but they will all help. They can also beadded after the original pull request but before a merge.

  • Use the Spring Framework code format conventions. If you use Eclipseyou can import formatter settings using theeclipse-code-formatter.xml file from theSpringCloud Build project. If using IntelliJ, you can use theEclipse Code FormatterPlugin to import the same file.

  • Make sure all new.java files to have a simple Javadoc class comment with at least an@author tag identifying you, and preferably at least a paragraph on what the class isfor.

  • Add the ASF license header comment to all new.java files (copy from existing filesin the project)

  • Add yourself as an@author to the .java files that you modify substantially (morethan cosmetic changes).

  • Add some Javadocs and, if you change the namespace, some XSD doc elements.

  • A few unit tests would help a lot as well — someone has to do it.

  • If no-one else is using your branch, please rebase it against the current master (orother target branch in the main project).

  • When writing a commit message please followthese conventions,if you are fixing an existing issue please addFixes gh-XXXX at the end of the commitmessage (where XXXX is the issue number).

Checkstyle

Spring Cloud Build comes with a set of checkstyle rules. You can find them in thespring-cloud-build-tools module. The most notable files under the module are:

spring-cloud-build-tools/
└── src    ├── checkstyle    │   └── checkstyle-suppressions.xml(3)    └── main        └── resources            ├── checkstyle-header.txt(2)            └── checkstyle.xml(1)

  1. Default Checkstyle rules

  2. File header setup

  3. Default suppression rules

Checkstyle configuration

Checkstyle rules aredisabled by default. To add checkstyle to your project just define the following properties and plugins.

pom.xml
<properties><maven-checkstyle-plugin.failsOnError>true</maven-checkstyle-plugin.failsOnError>(1)        <maven-checkstyle-plugin.failsOnViolation>true        </maven-checkstyle-plugin.failsOnViolation>(2)        <maven-checkstyle-plugin.includeTestSourceDirectory>true        </maven-checkstyle-plugin.includeTestSourceDirectory>(3)</properties><build>        <plugins>            <plugin>(4)                <groupId>io.spring.javaformat</groupId>                <artifactId>spring-javaformat-maven-plugin</artifactId>            </plugin>            <plugin>(5)                <groupId>org.apache.maven.plugins</groupId>                <artifactId>maven-checkstyle-plugin</artifactId>            </plugin>        </plugins>    <reporting>        <plugins>            <plugin>(5)                <groupId>org.apache.maven.plugins</groupId>                <artifactId>maven-checkstyle-plugin</artifactId>            </plugin>        </plugins>    </reporting></build>

  1. Fails the build upon Checkstyle errors

  2. Fails the build upon Checkstyle violations

  3. Checkstyle analyzes also the test sources

  4. Add the Spring Java Format plugin that will reformat your code to pass most of the Checkstyle formatting rules

  5. Add checkstyle plugin to your build and reporting phases

If you need to suppress some rules (e.g. line length needs to be longer), then it’s enough for you to define a file under${project.root}/src/checkstyle/checkstyle-suppressions.xml with your suppressions. Example:

projectRoot/src/checkstyle/checkstyle-suppresions.xml
<?xml version="1.0"?><!DOCTYPE suppressions PUBLIC"-//Puppy Crawl//DTD Suppressions 1.1//EN""https://www.puppycrawl.com/dtds/suppressions_1_1.dtd"><suppressions><suppress files=".*ConfigServerApplication\.java" checks="HideUtilityClassConstructor"/><suppress files=".*ConfigClientWatch\.java" checks="LineLengthCheck"/></suppressions>

It’s advisable to copy the${spring-cloud-build.rootFolder}/.editorconfig and${spring-cloud-build.rootFolder}/.springformat to your project. That way, some default formatting rules will be applied. You can do so by running this script:

$ curl https://raw.githubusercontent.com/spring-cloud/spring-cloud-build/master/.editorconfig -o .editorconfig$ touch .springformat

IDE setup

Intellij IDEA

In order to setup Intellij you should import our coding conventions, inspection profiles and set up the checkstyle plugin.The following files can be found in theSpring Cloud Build project.

spring-cloud-build-tools/
└── src    ├── checkstyle    │   └── checkstyle-suppressions.xml(3)    └── main        └── resources            ├── checkstyle-header.txt(2)            ├── checkstyle.xml(1)            └── intellij                ├── Intellij_Project_Defaults.xml(4)                └── Intellij_Spring_Boot_Java_Conventions.xml(5)

  1. Default Checkstyle rules

  2. File header setup

  3. Default suppression rules

  4. Project defaults for Intellij that apply most of Checkstyle rules

  5. Project style conventions for Intellij that apply most of Checkstyle rules

Code style
Figure 1. Code style

Go toFile →Settings →Editor →Code style. There click on the icon next to theScheme section. There, click on theImport Scheme value and pick theIntellij IDEA code style XML option. Import thespring-cloud-build-tools/src/main/resources/intellij/Intellij_Spring_Boot_Java_Conventions.xml file.

Code style
Figure 2. Inspection profiles

Go toFile →Settings →Editor →Inspections. There click on the icon next to theProfile section. There, click on theImport Profile and import thespring-cloud-build-tools/src/main/resources/intellij/Intellij_Project_Defaults.xml file.

Checkstyle

To have Intellij work with Checkstyle, you have to install theCheckstyle plugin. It’s advisable to also install theAssertions2Assertj to automatically convert the JUnit assertions

Checkstyle

Go toFile →Settings →Other settings →Checkstyle. There click on the+ icon in theConfiguration file section. There, you’ll have to define where the checkstyle rules should be picked from. In the image above, we’ve picked the rules from the cloned Spring Cloud Build repository. However, you can point to the Spring Cloud Build’s GitHub repository (e.g. for thecheckstyle.xml :https://raw.githubusercontent.com/spring-cloud/spring-cloud-build/master/spring-cloud-build-tools/src/main/resources/checkstyle.xml). We need to provide the following variables:

Important
Remember to set theScan Scope toAll sources since we apply checkstyle rules for production and test sources.

Duplicate Finder

Spring Cloud Build brings along thebasepom:duplicate-finder-maven-plugin, that enables flagging duplicate and conflicting classes and resources on the java classpath.

Duplicate Finder configuration

Duplicate finder isenabled by default and will run in theverify phase of your Maven build, but it will only take effect in your project if you add theduplicate-finder-maven-plugin to thebuild section of the projecst’spom.xml.

pom.xml
<build>    <plugins>        <plugin>            <groupId>org.basepom.maven</groupId>            <artifactId>duplicate-finder-maven-plugin</artifactId>        </plugin>    </plugins></build>

For other properties, we have set defaults as listed in theplugin documentation.

You can easily override them but setting the value of the selected property prefixed withduplicate-finder-maven-plugin. For example, setduplicate-finder-maven-plugin.skip totrue in order to skip duplicates check in your build.

If you need to addignoredClassPatterns orignoredResourcePatterns to your setup, make sure to add them in the plugin configuration section of your project:

<build>    <plugins>        <plugin>            <groupId>org.basepom.maven</groupId>            <artifactId>duplicate-finder-maven-plugin</artifactId>            <configuration>                <ignoredClassPatterns>                    <ignoredClassPattern>org.joda.time.base.BaseDateTime</ignoredClassPattern>                    <ignoredClassPattern>.*module-info</ignoredClassPattern>                </ignoredClassPatterns>                <ignoredResourcePatterns>                    <ignoredResourcePattern>changelog.txt</ignoredResourcePattern>                </ignoredResourcePatterns>            </configuration>        </plugin>    </plugins></build>
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