Storm Compatibility Beta

Flink streaming is compatible with Apache Storm interfaces and therefore allows reusing code that was implemented for Storm.

You can:

  • execute a whole Storm Topology in Flink.
  • use Storm Spout/Bolt as source/operator in Flink streaming programs.

This document shows how to use existing Storm code with Flink.

Project Configuration

Support for Storm is contained in the flink-storm Maven module. The code resides in the org.apache.flink.storm package.

Add the following dependency to your pom.xml if you want to execute Storm code in Flink.

<dependency>
	<groupId>org.apache.flink</groupId>
	<artifactId>flink-storm_2.10</artifactId>
	<version>1.3.0</version>
</dependency>

Please note: Do not add storm-core as a dependency. It is already included via flink-storm.

Please note: flink-storm is not part of the provided binary Flink distribution. Thus, you need to include flink-storm classes (and their dependencies) in your program jar (also called ueber-jar or fat-jar) that is submitted to Flink’s JobManager. See WordCount Storm within flink-storm-examples/pom.xml for an example how to package a jar correctly.

If you want to avoid large ueber-jars, you can manually copy storm-core-0.9.4.jar, json-simple-1.1.jar and flink-storm-1.3.0.jar into Flink’s lib/ folder of each cluster node (before the cluster is started). For this case, it is sufficient to include only your own Spout and Bolt classes (and their internal dependencies) into the program jar.

Execute Storm Topologies

Flink provides a Storm compatible API (org.apache.flink.storm.api) that offers replacements for the following classes:

  • StormSubmitter replaced by FlinkSubmitter
  • NimbusClient and Client replaced by FlinkClient
  • LocalCluster replaced by FlinkLocalCluster

In order to submit a Storm topology to Flink, it is sufficient to replace the used Storm classes with their Flink replacements in the Storm client code that assembles the topology. The actual runtime code, ie, Spouts and Bolts, can be used unmodified. If a topology is executed in a remote cluster, parameters nimbus.host and nimbus.thrift.port are used as jobmanger.rpc.address and jobmanger.rpc.port, respectively. If a parameter is not specified, the value is taken from flink-conf.yaml.

TopologyBuilder builder = new TopologyBuilder(); // the Storm topology builder

// actual topology assembling code and used Spouts/Bolts can be used as-is
builder.setSpout("source", new FileSpout(inputFilePath));
builder.setBolt("tokenizer", new BoltTokenizer()).shuffleGrouping("source");
builder.setBolt("counter", new BoltCounter()).fieldsGrouping("tokenizer", new Fields("word"));
builder.setBolt("sink", new BoltFileSink(outputFilePath)).shuffleGrouping("counter");

Config conf = new Config();
if(runLocal) { // submit to test cluster
	// replaces: LocalCluster cluster = new LocalCluster();
	FlinkLocalCluster cluster = new FlinkLocalCluster();
	cluster.submitTopology("WordCount", conf, FlinkTopology.createTopology(builder));
} else { // submit to remote cluster
	// optional
	// conf.put(Config.NIMBUS_HOST, "remoteHost");
	// conf.put(Config.NIMBUS_THRIFT_PORT, 6123);
	// replaces: StormSubmitter.submitTopology(topologyId, conf, builder.createTopology());
	FlinkSubmitter.submitTopology("WordCount", conf, FlinkTopology.createTopology(builder));
}

Embed Storm Operators in Flink Streaming Programs

As an alternative, Spouts and Bolts can be embedded into regular streaming programs. The Storm compatibility layer offers a wrapper classes for each, namely SpoutWrapper and BoltWrapper (org.apache.flink.storm.wrappers).

Per default, both wrappers convert Storm output tuples to Flink’s Tuple types (ie, Tuple0 to Tuple25 according to the number of fields of the Storm tuples). For single field output tuples a conversion to the field’s data type is also possible (eg, String instead of Tuple1<String>).

Because Flink cannot infer the output field types of Storm operators, it is required to specify the output type manually. In order to get the correct TypeInformation object, Flink’s TypeExtractor can be used.

Embed Spouts

In order to use a Spout as Flink source, use StreamExecutionEnvironment.addSource(SourceFunction, TypeInformation). The Spout object is handed to the constructor of SpoutWrapper<OUT> that serves as first argument to addSource(...). The generic type declaration OUT specifies the type of the source output stream.

StreamExecutionEnvironment env = StreamExecutionEnvironment.getExecutionEnvironment();

// stream has `raw` type (single field output streams only)
DataStream<String> rawInput = env.addSource(
	new SpoutWrapper<String>(new FileSpout(localFilePath), new String[] { Utils.DEFAULT_STREAM_ID }), // emit default output stream as raw type
	TypeExtractor.getForClass(String.class)); // output type

// process data stream
[...]

If a Spout emits a finite number of tuples, SpoutWrapper can be configures to terminate automatically by setting numberOfInvocations parameter in its constructor. This allows the Flink program to shut down automatically after all data is processed. Per default the program will run until it is canceled manually.

Embed Bolts

In order to use a Bolt as Flink operator, use DataStream.transform(String, TypeInformation, OneInputStreamOperator). The Bolt object is handed to the constructor of BoltWrapper<IN,OUT> that serves as last argument to transform(...). The generic type declarations IN and OUT specify the type of the operator’s input and output stream, respectively.

StreamExecutionEnvironment env = StreamExecutionEnvironment.getExecutionEnvironment();
DataStream<String> text = env.readTextFile(localFilePath);

DataStream<Tuple2<String, Integer>> counts = text.transform(
	"tokenizer", // operator name
	TypeExtractor.getForObject(new Tuple2<String, Integer>("", 0)), // output type
	new BoltWrapper<String, Tuple2<String, Integer>>(new BoltTokenizer())); // Bolt operator

// do further processing
[...]

Named Attribute Access for Embedded Bolts

Bolts can accesses input tuple fields via name (additionally to access via index). To use this feature with embedded Bolts, you need to have either a

  1. POJO type input stream or
  2. Tuple type input stream and specify the input schema (i.e. name-to-index-mapping)

For POJO input types, Flink accesses the fields via reflection. For this case, Flink expects either a corresponding public member variable or public getter method. For example, if a Bolt accesses a field via name sentence (eg, String s = input.getStringByField("sentence");), the input POJO class must have a member variable public String sentence; or method public String getSentence() { ... }; (pay attention to camel-case naming).

For Tuple input types, it is required to specify the input schema using Storm’s Fields class. For this case, the constructor of BoltWrapper takes an additional argument: new BoltWrapper<Tuple1<String>, ...>(..., new Fields("sentence")). The input type is Tuple1<String> and Fields("sentence") specify that input.getStringByField("sentence") is equivalent to input.getString(0).

See BoltTokenizerWordCountPojo and BoltTokenizerWordCountWithNames for examples.

Configuring Spouts and Bolts

In Storm, Spouts and Bolts can be configured with a globally distributed Map object that is given to submitTopology(...) method of LocalCluster or StormSubmitter. This Map is provided by the user next to the topology and gets forwarded as a parameter to the calls Spout.open(...) and Bolt.prepare(...). If a whole topology is executed in Flink using FlinkTopologyBuilder etc., there is no special attention required – it works as in regular Storm.

For embedded usage, Flink’s configuration mechanism must be used. A global configuration can be set in a StreamExecutionEnvironment via .getConfig().setGlobalJobParameters(...). Flink’s regular Configuration class can be used to configure Spouts and Bolts. However, Configuration does not support arbitrary key data types as Storm does (only String keys are allowed). Thus, Flink additionally provides StormConfig class that can be used like a raw Map to provide full compatibility to Storm.

StreamExecutionEnvironment env = StreamExecutionEnvironment.getExecutionEnvironment();

StormConfig config = new StormConfig();
// set config values
[...]

// set global Storm configuration
env.getConfig().setGlobalJobParameters(config);

// assemble program with embedded Spouts and/or Bolts
[...]

Multiple Output Streams

Flink can also handle the declaration of multiple output streams for Spouts and Bolts. If a whole topology is executed in Flink using FlinkTopologyBuilder etc., there is no special attention required – it works as in regular Storm.

For embedded usage, the output stream will be of data type SplitStreamType<T> and must be split by using DataStream.split(...) and SplitStream.select(...). Flink provides the predefined output selector StormStreamSelector<T> for .split(...) already. Furthermore, the wrapper type SplitStreamTuple<T> can be removed using SplitStreamMapper<T>.

[...]

// get DataStream from Spout or Bolt which declares two output streams s1 and s2 with output type SomeType
DataStream<SplitStreamType<SomeType>> multiStream = ...

SplitStream<SplitStreamType<SomeType>> splitStream = multiStream.split(new StormStreamSelector<SomeType>());

// remove SplitStreamType using SplitStreamMapper to get data stream of type SomeType
DataStream<SomeType> s1 = splitStream.select("s1").map(new SplitStreamMapper<SomeType>()).returns(SomeType.class);
DataStream<SomeType> s2 = splitStream.select("s2").map(new SplitStreamMapper<SomeType>()).returns(SomeType.class);

// do further processing on s1 and s2
[...]

See SpoutSplitExample.java for a full example.

Flink Extensions

Finite Spouts

In Flink, streaming sources can be finite, ie, emit a finite number of records and stop after emitting the last record. However, Spouts usually emit infinite streams. The bridge between the two approaches is the FiniteSpout interface which, in addition to IRichSpout, contains a reachedEnd() method, where the user can specify a stopping-condition. The user can create a finite Spout by implementing this interface instead of (or additionally to) IRichSpout, and implementing the reachedEnd() method in addition. In contrast to a SpoutWrapper that is configured to emit a finite number of tuples, FiniteSpout interface allows to implement more complex termination criteria.

Although finite Spouts are not necessary to embed Spouts into a Flink streaming program or to submit a whole Storm topology to Flink, there are cases where they may come in handy:

  • to achieve that a native Spout behaves the same way as a finite Flink source with minimal modifications
  • the user wants to process a stream only for some time; after that, the Spout can stop automatically
  • reading a file into a stream
  • for testing purposes

An example of a finite Spout that emits records for 10 seconds only:

public class TimedFiniteSpout extends BaseRichSpout implements FiniteSpout {
	[...] // implement open(), nextTuple(), ...

	private long starttime = System.currentTimeMillis();

	public boolean reachedEnd() {
		return System.currentTimeMillis() - starttime > 10000l;
	}
}

Storm Compatibility Examples

You can find more examples in Maven module flink-storm-examples. For the different versions of WordCount, see README.md. To run the examples, you need to assemble a correct jar file. flink-storm-examples-1.3.0.jar is no valid jar file for job execution (it is only a standard maven artifact).

There are example jars for embedded Spout and Bolt, namely WordCount-SpoutSource.jar and WordCount-BoltTokenizer.jar, respectively. Compare pom.xml to see how both jars are built. Furthermore, there is one example for whole Storm topologies (WordCount-StormTopology.jar).

You can run each of those examples via bin/flink run <jarname>.jar. The correct entry point class is contained in each jar’s manifest file.