Create an Apache Storm topology in Java
Learn how to create a Java-based topology for Apache Storm. You create a Storm topology that implements a word-count application. You use Maven to build and package the project. Then, you learn how to define the topology using the Flux framework.
After completing the steps in this document, you can deploy the topology to Apache Storm on HDInsight.
Note
A completed version of the Storm topology examples created in this document is available at https://github.com/Azure-Samples/hdinsight-java-storm-wordcount.
Prerequisites
Maven (https://maven.apache.org/download.cgi): Maven is a project build system for Java projects.
A text editor or IDE.
Configure environment variables
The following environment variables may be set when you install Java and the JDK. However, you should check that they exist and that they contain the correct values for your system.
JAVA_HOME - should point to the directory where the Java runtime environment (JRE) is installed. For example, in a Unix or Linux distribution, it should have a value similar to
/usr/lib/jvm/java-8-oracle. In Windows, it would have a value similar toc:\Program Files (x86)\Java\jre1.8PATH - should contain the following paths:
JAVA_HOME (or the equivalent path)
JAVA_HOME\bin (or the equivalent path)
The directory where Maven is installed
Create a Maven project
From the command line, use the following command to create a Maven project named WordCount:
mvn archetype:generate -DarchetypeArtifactId=maven-archetype-quickstart -DgroupId=com.microsoft.example -DartifactId=WordCount -DinteractiveMode=false
Note
If you are using PowerShell, you must surround the-D parameters with double quotes.
mvn archetype:generate "-DarchetypeArtifactId=maven-archetype-quickstart" "-DgroupId=com.microsoft.example" "-DartifactId=WordCount" "-DinteractiveMode=false"
This command creates a directory named WordCount at the current location, which contains a basic Maven project. The WordCount directory contains the following items:
pom.xml: Contains settings for the Maven project.src\main\java\com\microsoft\example: Contains your application code.src\test\java\com\microsoft\example: Contains tests for your application.
Remove the generated example code
Delete the generated test and the application files:
- src\test\java\com\microsoft\example\AppTest.java
- src\main\java\com\microsoft\example\App.java
Add Maven repositories
HDInsight is based on the Hortonworks Data Platform (HDP), so we recommend using the Hortonworks repository to download dependencies for your Apache Storm projects. In the pom.xml file, add the following XML after the <url>http://maven.apache.org</url> line:
<repositories>
<repository>
<releases>
<enabled>true</enabled>
<updatePolicy>always</updatePolicy>
<checksumPolicy>warn</checksumPolicy>
</releases>
<snapshots>
<enabled>false</enabled>
<updatePolicy>never</updatePolicy>
<checksumPolicy>fail</checksumPolicy>
</snapshots>
<id>HDPReleases</id>
<name>HDP Releases</name>
<url>http://repo.hortonworks.com/content/repositories/releases/</url>
<layout>default</layout>
</repository>
<repository>
<releases>
<enabled>true</enabled>
<updatePolicy>always</updatePolicy>
<checksumPolicy>warn</checksumPolicy>
</releases>
<snapshots>
<enabled>false</enabled>
<updatePolicy>never</updatePolicy>
<checksumPolicy>fail</checksumPolicy>
</snapshots>
<id>HDPJetty</id>
<name>Hadoop Jetty</name>
<url>http://repo.hortonworks.com/content/repositories/jetty-hadoop/</url>
<layout>default</layout>
</repository>
</repositories>
Add properties
Maven allows you to define project-level values called properties. In the pom.xml, add the following text after the </repositories> line:
<properties>
<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
<!--
This is a version of Storm from the Hortonworks repository that is compatible with HDInsight 3.6.
-->
<storm.version>1.1.0.2.6.1.9-1</storm.version>
</properties>
You can now use this value in other sections of the pom.xml. For example, when specifying the version of Storm components, you can use ${storm.version} instead of hard coding a value.
Add dependencies
Add a dependency for Storm components. Open the pom.xml file and add the following code in the <dependencies> section:
<dependency>
<groupId>org.apache.storm</groupId>
<artifactId>storm-core</artifactId>
<version>${storm.version}</version>
<!-- keep storm out of the jar-with-dependencies -->
<scope>provided</scope>
</dependency>
At compile time, Maven uses this information to look up storm-core in the Maven repository. It first looks in the repository on your local computer. If the files aren't there, Maven downloads them from the public Maven repository and stores them in the local repository.
Note
Notice the <scope>provided</scope> line in this section. This setting tells Maven to exclude storm-core from any JAR files that are created, because it is provided by the system.
Build configuration
Maven plug-ins allow you to customize the build stages of the project. For example, how the project is compiled or how to package it into a JAR file. Open the pom.xml file and add the following code directly above the </project> line.
<build>
<plugins>
</plugins>
<resources>
</resources>
</build>
This section is used to add plug-ins, resources, and other build configuration options. For a full reference of the pom.xml file, see http://maven.apache.org/pom.html.
Add plug-ins
For Apache Storm topologies implemented in Java, the Exec Maven Plugin is useful because it allows you to easily run the topology locally in your development environment. Add the following to the <plugins> section of the pom.xml file to include the Exec Maven plugin:
<plugin>
<groupId>org.codehaus.mojo</groupId>
<artifactId>exec-maven-plugin</artifactId>
<version>1.5.0</version>
<executions>
<execution>
<goals>
<goal>exec</goal>
</goals>
</execution>
</executions>
<configuration>
<executable>java</executable>
<includeProjectDependencies>true</includeProjectDependencies>
<includePluginDependencies>false</includePluginDependencies>
<classpathScope>compile</classpathScope>
<mainClass>${storm.topology}</mainClass>
<cleanupDaemonThreads>false</cleanupDaemonThreads>
</configuration>
</plugin>
Another useful plug-in is the Apache Maven Compiler Plugin, which is used to change compilation options. The changes the Java version that Maven uses for the source and target for your application.
For HDInsight 3.4 or earlier, set the source and target Java version to 1.7.
For HDInsight 3.5, set the source and target Java version to 1.8.
Add the following text in the <plugins> section of the pom.xml file to include the Apache Maven Compiler plugin. This example specifies 1.8, so the target HDInsight version is 3.5.
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-compiler-plugin</artifactId>
<version>3.3</version>
<configuration>
<source>1.8</source>
<target>1.8</target>
</configuration>
</plugin>
Configure resources
The resources section allows you to include non-code resources such as configuration files needed by components in the topology. For this example, add the following text in the <resources> section of the `pom.xml file.
<resource>
<directory>${basedir}/resources</directory>
<filtering>false</filtering>
<includes>
<include>log4j2.xml</include>
</includes>
</resource>
This example adds the resources directory in the root of the project (${basedir}) as a location that contains resources, and includes the file named log4j2.xml. This file is used to configure what information is logged by the topology.
Create the topology
A Java-based Apache Storm topology consists of three components that you must author (or reference) as a dependency.
Spouts: Reads data from external sources and emits streams of data into the topology.
Bolts: Performs processing on streams emitted by spouts or other bolts, and emits one or more streams.
Topology: Defines how the spouts and bolts are arranged, and provides the entry point for the topology.
Create the spout
To reduce requirements for setting up external data sources, the following spout simply emits random sentences. It is a modified version of a spout that is provided with the Storm-Starter examples.
Note
For an example of a spout that reads from an external data source, see one of the following examples:
- TwitterSampleSPout: An example spout that reads from Twitter
- Storm-Kafka: A spout that reads from Kafka
For the spout, create a file named RandomSentenceSpout.java in the src\main\java\com\microsoft\example directory and use the following Java code as the contents:
package com.microsoft.example;
import org.apache.storm.spout.SpoutOutputCollector;
import org.apache.storm.task.TopologyContext;
import org.apache.storm.topology.OutputFieldsDeclarer;
import org.apache.storm.topology.base.BaseRichSpout;
import org.apache.storm.tuple.Fields;
import org.apache.storm.tuple.Values;
import org.apache.storm.utils.Utils;
import java.util.Map;
import java.util.Random;
//This spout randomly emits sentences
public class RandomSentenceSpout extends BaseRichSpout {
//Collector used to emit output
SpoutOutputCollector _collector;
//Used to generate a random number
Random _rand;
//Open is called when an instance of the class is created
@Override
public void open(Map conf, TopologyContext context, SpoutOutputCollector collector) {
//Set the instance collector to the one passed in
_collector = collector;
//For randomness
_rand = new Random();
}
//Emit data to the stream
@Override
public void nextTuple() {
//Sleep for a bit
Utils.sleep(100);
//The sentences that are randomly emitted
String[] sentences = new String[]{ "the cow jumped over the moon", "an apple a day keeps the doctor away",
"four score and seven years ago", "snow white and the seven dwarfs", "i am at two with nature" };
//Randomly pick a sentence
String sentence = sentences[_rand.nextInt(sentences.length)];
//Emit the sentence
_collector.emit(new Values(sentence));
}
//Ack is not implemented since this is a basic example
@Override
public void ack(Object id) {
}
//Fail is not implemented since this is a basic example
@Override
public void fail(Object id) {
}
//Declare the output fields. In this case, an sentence
@Override
public void declareOutputFields(OutputFieldsDeclarer declarer) {
declarer.declare(new Fields("sentence"));
}
}
Note
Although this topology uses only one spout, others may have several that feed data from different sources into the topology.
Create the bolts
Bolts handle the data processing. This topology uses two bolts:
SplitSentence: Splits the sentences emitted by RandomSentenceSpout into individual words.
WordCount: Counts how many times each word has occurred.
Note
Bolts can do anything, for example, computation, persistence, or talking to external components.
Create two new files, SplitSentence.java and WordCount.java in the src\main\java\com\microsoft\example directory. Use the following text as the contents for the files:
SplitSentence
package com.microsoft.example;
import java.text.BreakIterator;
import org.apache.storm.topology.BasicOutputCollector;
import org.apache.storm.topology.OutputFieldsDeclarer;
import org.apache.storm.topology.base.BaseBasicBolt;
import org.apache.storm.tuple.Fields;
import org.apache.storm.tuple.Tuple;
import org.apache.storm.tuple.Values;
//There are a variety of bolt types. In this case, use BaseBasicBolt
public class SplitSentence extends BaseBasicBolt {
//Execute is called to process tuples
@Override
public void execute(Tuple tuple, BasicOutputCollector collector) {
//Get the sentence content from the tuple
String sentence = tuple.getString(0);
//An iterator to get each word
BreakIterator boundary=BreakIterator.getWordInstance();
//Give the iterator the sentence
boundary.setText(sentence);
//Find the beginning first word
int start=boundary.first();
//Iterate over each word and emit it to the output stream
for (int end=boundary.next(); end != BreakIterator.DONE; start=end, end=boundary.next()) {
//get the word
String word=sentence.substring(start,end);
//If a word is whitespace characters, replace it with empty
word=word.replaceAll("\\s+","");
//if it's an actual word, emit it
if (!word.equals("")) {
collector.emit(new Values(word));
}
}
}
//Declare that emitted tuples contain a word field
@Override
public void declareOutputFields(OutputFieldsDeclarer declarer) {
declarer.declare(new Fields("word"));
}
}
WordCount
package com.microsoft.example;
import java.util.HashMap;
import java.util.Map;
import java.util.Iterator;
import org.apache.storm.Constants;
import org.apache.storm.topology.BasicOutputCollector;
import org.apache.storm.topology.OutputFieldsDeclarer;
import org.apache.storm.topology.base.BaseBasicBolt;
import org.apache.storm.tuple.Fields;
import org.apache.storm.tuple.Tuple;
import org.apache.storm.tuple.Values;
import org.apache.storm.Config;
// For logging
import org.apache.logging.log4j.Logger;
import org.apache.logging.log4j.LogManager;
//There are a variety of bolt types. In this case, use BaseBasicBolt
public class WordCount extends BaseBasicBolt {
//Create logger for this class
private static final Logger logger = LogManager.getLogger(WordCount.class);
//For holding words and counts
Map<String, Integer> counts = new HashMap<String, Integer>();
//How often to emit a count of words
private Integer emitFrequency;
// Default constructor
public WordCount() {
emitFrequency=5; // Default to 60 seconds
}
// Constructor that sets emit frequency
public WordCount(Integer frequency) {
emitFrequency=frequency;
}
//Configure frequency of tick tuples for this bolt
//This delivers a 'tick' tuple on a specific interval,
//which is used to trigger certain actions
@Override
public Map<String, Object> getComponentConfiguration() {
Config conf = new Config();
conf.put(Config.TOPOLOGY_TICK_TUPLE_FREQ_SECS, emitFrequency);
return conf;
}
//execute is called to process tuples
@Override
public void execute(Tuple tuple, BasicOutputCollector collector) {
//If it's a tick tuple, emit all words and counts
if(tuple.getSourceComponent().equals(Constants.SYSTEM_COMPONENT_ID)
&& tuple.getSourceStreamId().equals(Constants.SYSTEM_TICK_STREAM_ID)) {
for(String word : counts.keySet()) {
Integer count = counts.get(word);
collector.emit(new Values(word, count));
logger.info("Emitting a count of " + count + " for word " + word);
}
} else {
//Get the word contents from the tuple
String word = tuple.getString(0);
//Have we counted any already?
Integer count = counts.get(word);
if (count == null)
count = 0;
//Increment the count and store it
count++;
counts.put(word, count);
}
}
//Declare that this emits a tuple containing two fields; word and count
@Override
public void declareOutputFields(OutputFieldsDeclarer declarer) {
declarer.declare(new Fields("word", "count"));
}
}
Define the topology
The topology ties the spouts and bolts together into a graph, which defines how data flows between the components. It also provides parallelism hints that Storm uses when creating instances of the components within the cluster.
The following image is a basic diagram of the graph of components for this topology.
To implement the topology, create a file named WordCountTopology.java in the src\main\java\com\microsoft\example directory. Use the following Java code as the contents of the file:
package com.microsoft.example;
import org.apache.storm.Config;
import org.apache.storm.LocalCluster;
import org.apache.storm.StormSubmitter;
import org.apache.storm.topology.TopologyBuilder;
import org.apache.storm.tuple.Fields;
import com.microsoft.example.RandomSentenceSpout;
public class WordCountTopology {
//Entry point for the topology
public static void main(String[] args) throws Exception {
//Used to build the topology
TopologyBuilder builder = new TopologyBuilder();
//Add the spout, with a name of 'spout'
//and parallelism hint of 5 executors
builder.setSpout("spout", new RandomSentenceSpout(), 5);
//Add the SplitSentence bolt, with a name of 'split'
//and parallelism hint of 8 executors
//shufflegrouping subscribes to the spout, and equally distributes
//tuples (sentences) across instances of the SplitSentence bolt
builder.setBolt("split", new SplitSentence(), 8).shuffleGrouping("spout");
//Add the counter, with a name of 'count'
//and parallelism hint of 12 executors
//fieldsgrouping subscribes to the split bolt, and
//ensures that the same word is sent to the same instance (group by field 'word')
builder.setBolt("count", new WordCount(), 12).fieldsGrouping("split", new Fields("word"));
//new configuration
Config conf = new Config();
//Set to false to disable debug information when
// running in production on a cluster
conf.setDebug(false);
//If there are arguments, we are running on a cluster
if (args != null && args.length > 0) {
//parallelism hint to set the number of workers
conf.setNumWorkers(3);
//submit the topology
StormSubmitter.submitTopology(args[0], conf, builder.createTopology());
}
//Otherwise, we are running locally
else {
//Cap the maximum number of executors that can be spawned
//for a component to 3
conf.setMaxTaskParallelism(3);
//LocalCluster is used to run locally
LocalCluster cluster = new LocalCluster();
//submit the topology
cluster.submitTopology("word-count", conf, builder.createTopology());
//sleep
Thread.sleep(10000);
//shut down the cluster
cluster.shutdown();
}
}
}
Configure logging
Storm uses Apache Log4j to log information. If you do not configure logging, the topology emits diagnostic information. To control what is logged, create a file named log4j2.xml in the resources directory. Use the following XML as the contents of the file.
<?xml version="1.0" encoding="UTF-8"?>
<Configuration>
<Appenders>
<Console name="STDOUT" target="SYSTEM_OUT">
<PatternLayout pattern="%d{HH:mm:ss} [%t] %-5level %logger{36} - %msg%n"/>
</Console>
</Appenders>
<Loggers>
<Logger name="com.microsoft.example" level="trace" additivity="false">
<AppenderRef ref="STDOUT"/>
</Logger>
<Root level="error">
<Appender-Ref ref="STDOUT"/>
</Root>
</Loggers>
</Configuration>
This XML configures a new logger for the com.microsoft.example class, which includes the components in this example topology. The level is set to trace for this logger, which captures any logging information emitted by components in this topology.
The <Root level="error"> section configures the root level of logging (everything not in com.microsoft.example) to only log error information.
For more information on configuring logging for Log4j, see http://logging.apache.org/log4j/2.x/manual/configuration.html.
Note
Storm version 0.10.0 and higher use Log4j 2.x. Older versions of storm used Log4j 1.x, which used a different format for log configuration. For information on the older configuration, see http://wiki.apache.org/logging-log4j/Log4jXmlFormat.
Test the topology locally
After you save the files, use the following command to test the topology locally.
mvn compile exec:java -Dstorm.topology=com.microsoft.example.WordCountTopology
As it runs, the topology displays startup information. The following text is an example of the word count output:
17:33:27 [Thread-12-count] INFO com.microsoft.example.WordCount - Emitting a count of 56 for word snow
17:33:27 [Thread-12-count] INFO com.microsoft.example.WordCount - Emitting a count of 56 for word white
17:33:27 [Thread-12-count] INFO com.microsoft.example.WordCount - Emitting a count of 112 for word seven
17:33:27 [Thread-16-count] INFO com.microsoft.example.WordCount - Emitting a count of 195 for word the
17:33:27 [Thread-30-count] INFO com.microsoft.example.WordCount - Emitting a count of 113 for word and
17:33:27 [Thread-30-count] INFO com.microsoft.example.WordCount - Emitting a count of 57 for word dwarfs
17:33:27 [Thread-12-count] INFO com.microsoft.example.WordCount - Emitting a count of 57 for word snow
This example log indicates that the word 'and' has been emitted 113 times. The count continues to go up as long as the topology runs because the spout continuously emits the same sentences.
There is a 5-second interval between emission of words and counts. The WordCount component is configured to only emit information when a tick tuple arrives. It requests that tick tuples are only delivered every five seconds.
Convert the topology to Flux
Flux is a new framework available with Storm 0.10.0 and higher, which allows you to separate configuration from implementation. Your components are still defined in Java, but the topology is defined using a YAML file. You can package a default topology definition with your project, or use a standalone file when submitting the topology. When submitting the topology to Storm, you can use environment variables or configuration files to populate values in the YAML topology definition.
The YAML file defines the components to use for the topology and the data flow between them. You can include a YAML file as part of the jar file or you can use an external YAML file.
For more information on Flux, see Flux framework (https://storm.apache.org/releases/1.0.6/flux.html).
Warning
Due to a bug (https://issues.apache.org/jira/browse/STORM-2055) with Storm 1.0.1, you may need to install a Storm development environment to run Flux topologies locally.
Move the
WordCountTopology.javafile out of the project. Previously, this file defined the topology, but isn't needed with Flux.In the
resourcesdirectory, create a file namedtopology.yaml. Use the following text as the contents of this file.name: "wordcount" # friendly name for the topology config: # Topology configuration topology.workers: 1 # Hint for the number of workers to create spouts: # Spout definitions - id: "sentence-spout" className: "com.microsoft.example.RandomSentenceSpout" parallelism: 1 # parallelism hint bolts: # Bolt definitions - id: "splitter-bolt" className: "com.microsoft.example.SplitSentence" parallelism: 1 - id: "counter-bolt" className: "com.microsoft.example.WordCount" constructorArgs: - 10 parallelism: 1 streams: # Stream definitions - name: "Spout --> Splitter" # name isn't used (placeholder for logging, UI, etc.) from: "sentence-spout" # The stream emitter to: "splitter-bolt" # The stream consumer grouping: # Grouping type type: SHUFFLE - name: "Splitter -> Counter" from: "splitter-bolt" to: "counter-bolt" grouping: type: FIELDS args: ["word"] # field(s) to group onMake the following changes to the
pom.xmlfile.Add the following new dependency in the
<dependencies>section:<!-- Add a dependency on the Flux framework --> <dependency> <groupId>org.apache.storm</groupId> <artifactId>flux-core</artifactId> <version>${storm.version}</version> </dependency>Add the following plugin to the
<plugins>section. This plugin handles the creation of a package (jar file) for the project, and applies some transformations specific to Flux when creating the package.<!-- build an uber jar --> <plugin> <groupId>org.apache.maven.plugins</groupId> <artifactId>maven-shade-plugin</artifactId> <version>2.3</version> <configuration> <transformers> <!-- Keep us from getting a "can't overwrite file error" --> <transformer implementation="org.apache.maven.plugins.shade.resource.ApacheLicenseResourceTransformer" /> <transformer implementation="org.apache.maven.plugins.shade.resource.ServicesResourceTransformer" /> <!-- We're using Flux, so refer to it as main --> <transformer implementation="org.apache.maven.plugins.shade.resource.ManifestResourceTransformer"> <mainClass>org.apache.storm.flux.Flux</mainClass> </transformer> </transformers> <!-- Keep us from getting a bad signature error --> <filters> <filter> <artifact>*:*</artifact> <excludes> <exclude>META-INF/*.SF</exclude> <exclude>META-INF/*.DSA</exclude> <exclude>META-INF/*.RSA</exclude> </excludes> </filter> </filters> </configuration> <executions> <execution> <phase>package</phase> <goals> <goal>shade</goal> </goals> </execution> </executions> </plugin>In the exec-maven-plugin
<configuration>section, change the value for<mainClass>toorg.apache.storm.flux.Flux. This setting allows Flux to handle running the topology locally in development.In the
<resources>section, add the following to the<includes>. This XML includes the YAML file that defines the topology as part of the project.<include>topology.yaml</include>
Test the flux topology locally
Use the following to compile and execute the Flux topology using Maven:
mvn compile exec:java -Dexec.args="--local -R /topology.yaml"If you are using PowerShell, use the following command:
mvn compile exec:java "-Dexec.args=--local -R /topology.yaml"Warning
If your topology uses Storm 1.0.1 bits, this command fails. This failure is caused by https://issues.apache.org/jira/browse/STORM-2055. Instead, install Storm in your development environment and use the following steps:
If you have installed Storm in your development environment, you can use the following commands instead:
mvn compile package storm jar target/WordCount-1.0-SNAPSHOT.jar org.apache.storm.flux.Flux --local -R /topology.yamlThe
--localparameter runs the topology in local mode on your development environment. The-R /topology.yamlparameter uses thetopology.yamlfile resource from the jar file to define the topology.As it runs, the topology displays startup information. The following text is an example of the output:
17:33:27 [Thread-12-count] INFO com.microsoft.example.WordCount - Emitting a count of 56 for word snow 17:33:27 [Thread-12-count] INFO com.microsoft.example.WordCount - Emitting a count of 56 for word white 17:33:27 [Thread-12-count] INFO com.microsoft.example.WordCount - Emitting a count of 112 for word seven 17:33:27 [Thread-16-count] INFO com.microsoft.example.WordCount - Emitting a count of 195 for word the 17:33:27 [Thread-30-count] INFO com.microsoft.example.WordCount - Emitting a count of 113 for word and 17:33:27 [Thread-30-count] INFO com.microsoft.example.WordCount - Emitting a count of 57 for word dwarfsThere is a 10-second delay between batches of logged information.
Make a copy of the
topology.yamlfile from the project. Name the new filenewtopology.yaml. In thenewtopology.yamlfile, find the following section and change the value of10to5. This modification changes the interval between emitting batches of word counts from 10 seconds to 5.- id: "counter-bolt" className: "com.microsoft.example.WordCount" constructorArgs: - 5 parallelism: 1 ```yamlTo run the topology, use the following command:
mvn exec:java -Dexec.args="--local /path/to/newtopology.yaml"Or, if you have Storm on your development environment:
storm jar target/WordCount-1.0-SNAPSHOT.jar org.apache.storm.flux.Flux --local /path/to/newtopology.yamlChange the
/path/to/newtopology.yamlto the path to the newtopology.yaml file you created in the previous step. This command uses the newtopology.yaml as the topology definition. Since we didn't include thecompileparameter, Maven uses the version of the project built in previous steps.Once the topology starts, you should notice that the time between emitted batches has changed to reflect the value in newtopology.yaml. So you can see that you can change your configuration through a YAML file without having to recompile the topology.
For more information on these and other features of the Flux framework, see Flux (https://storm.apache.org/releases/1.0.6/flux.html).
Trident
Trident is a high-level abstraction that is provided by Storm. It supports stateful processing. The primary advantage of Trident is that it can guarantee that every message that enters the topology is processed only once. Without using Trident, your topology can only guarantee that messages are processed at least once. There are also other differences, such as built-in components that can be used instead of creating bolts. In fact, bolts are replaced by less-generic components, such as filters, projections, and functions.
Trident applications can be created by using Maven projects. You use the same basic steps as presented earlier in this article—only the code is different. Trident also cannot (currently) be used with the Flux framework.
For more information about Trident, see the Trident API Overview.
Next Steps
You have learned how to create a Storm topology by using Java. Now learn how to:
You can find more example Storm topologies by visiting Example topologies for Storm on HDInsight.