Hello Samza High Level API - Code Walkthrough

This tutorial introduces the high level API by showing you how to build wikipedia application from the hello-samza high level API Yarn tutorial. Upon completion of this tutorial, you’ll know how to implement and configure a StreamApplication. Along the way, you’ll see how to use some of the basic operators as well as how to leverage key-value stores and metrics in an app.

The same hello-samza project is used for this tutorial as for many of the others. You will clone that project and by the end of the tutorial, you will have implemented a duplicate of the WikipediaApplication.

Let’s get started.

Get the Code

Check out the hello-samza project:

git clone https://git.apache.org/samza-hello-samza.git hello-samza
cd hello-samza
git checkout latest

This project already contains implementations of the wikipedia application using both the low-level task API and the high-level API. The low-level task implementations are in the samza.examples.wikipedia.task package. The high-level application implementation is in the samza.examples.wikipedia.application package.

This tutorial will provide step by step instructions to recreate the existing wikipedia application.

Introduction to Wikipedia Consumer

In order to consume events from Wikipedia, the hello-samza project includes a WikipediaSystemFactory implementation of the Samza SystemFactory that provides a WikipediaConsumer.

The WikipediaConsumer is an implementation of SystemConsumer that can consume events from Wikipedia. It is also a listener for events from the WikipediaFeed. It’s important to note that the events received in onEvent are of the type WikipediaFeedEvent, so we will expect that type for messages on our input streams. For other systems, the messages may come in the form of byte[]. In that case you may want to configure a samza serde and the application should expect the output type of that serde.

Now that we understand the Wikipedia system and the types of inputs we’ll be processing, we can proceed with creating our application.

Create the Initial Config

In the hello-samza project, configs are kept in the src/main/config/ path. This is where we will add the config for our application. Create a new file named my-wikipedia-application.properties in this location.

Core Configuration

Let’s start by adding some of the core properties to the file:

# Licensed to the Apache Software Foundation (ASF) under one
# or more contributor license agreements.  See the NOTICE file
# distributed with this work for additional information
# regarding copyright ownership.  The ASF licenses this file
# to you under the Apache License, Version 2.0 (the
# "License"); you may not use this file except in compliance
# with the License.  You may obtain a copy of the License at
#   http://www.apache.org/licenses/LICENSE-2.0
# Unless required by applicable law or agreed to in writing,
# software distributed under the License is distributed on an
# KIND, either express or implied.  See the License for the
# specific language governing permissions and limitations
# under the License.




Be sure to include the Apache header. The project will not compile without it.

Here’s a brief summary of what we configured so far.

  • app.class: the class that defines the application logic. We will create this class later.
  • app.runner.class: the runner implementation which will launch our application. Since we are using YARN, we use RemoteApplicationRunner which is required for any cluster-based deployment.
  • job.factory.class: the factory that will create the runtime instances of our jobs. Since we are using YARN, we want each job to be created as a YARN job, so we use YarnJobFactory
  • job.name: the primary identifier for the job.
  • job.default.system: the default system to use for input, output, and internal metadata streams. This can be overridden on a per-stream basis. The kafka system will be defined in the next section.
  • yarn.package.path: tells YARN where to find the job package so the Node Managers can download it.

These basic configurations are enough to launch the application on YARN but we haven’t defined any streaming systems for Samza to use, so the application would not process anything.

Next, let’s define the streaming systems with which the application will interact.

Define Systems

This Wikipedia application will consume events from Wikipedia and produce stats to a Kafka topic. We need to define those systems in config before Samza can use them. Add the following lines to the config:



The above configuration defines 2 systems; one called wikipedia and one called kafka.

A factory is required for each system, so the systems.system-name.samza.system.factory property is required for both systems. The other properties are system and use-case specific.

For the kafka system, we set the default replication factor to 1 for all streams because this application is intended for a demo deployment which utilizes a Kafka cluster with only 1 broker, so a replication factor larger than 1 is invalid. The default serde is JSON, which means by default any streams consumed or produced to the kafka system will use a json serde, which we will define in the next section.

The wikipedia system does not need a serde because the WikipediaConsumer already produces a usable type.


Next, we need to configure the serdes we will use for streams and stores in the application.


The json serde was used for the kafka system above. The string and integer serdes will be used later.

Configure Streams

Samza identifies streams using a unique stream ID. In most cases, the stream ID is the same as the actual stream name. However, if a stream has a name that doesn’t match the pattern [A-Za-z0-9_-]+, we need to configure a separate physical.name to associate the actual stream name with a legal stream ID. The Wikipedia channels we will consume have a ‘#’ character in the names. So for each of them we must pick a legal stream ID and then configure the physical name to match the channel.

Samza uses the job.default.system for any streams that do not explicitly specify a system. In the previous sections, we defined 2 systems, wikipedia and kafka, and we configured kafka as the default. To understand why, let’s look at the streams and how Samza will use them.

For this app, Samza will:

  1. Consume from input streams
  2. Produce to an output stream and a metrics stream
  3. Both produce and consume from job-coordination, checkpoint, and changelog streams

While the wikipedia system is necessary for case 1, it does not support producers (we can’t write Samza output to Wikipedia), which are needed for cases 2-3. So it is more convenient to use kafka as the default system. We can then explicitly configure the input streams to use the wikipedia system.




The above configurations declare 3 streams with IDs, en-wikipedia, en-wiktionary, and en-wikinews. It associates each stream with the wikipedia system we defined earlier and set the physical name to the corresponding Wikipedia channel.

Since all the Kafka streams for cases 2-3 are on the default system and do not include special characters in their names, we do not need to configure them explicitly.

Create a StreamApplication

With the core configuration settled, we turn our attention to code.

Define Application Logic

Let’s create the application class we configured above. The next 8 sections walk you through writing the code for the Wikipedia application.

Create a new class named MyWikipediaApplication in the samza.examples.wikipedia.application package. The class must implement StreamApplication and should look like this:

 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *   http://www.apache.org/licenses/LICENSE-2.0
 * Unless required by applicable law or agreed to in writing,
 * software distributed under the License is distributed on an
 * KIND, either express or implied.  See the License for the
 * specific language governing permissions and limitations
 * under the License.
package samza.examples.wikipedia.application;

import org.apache.samza.application.StreamApplication;
import org.apache.samza.config.Config;
import org.apache.samza.operators.StreamGraph;

public class MyWikipediaApplication implements StreamApplication{
  public void init(StreamGraph streamGraph, Config config) {

Be sure to include the Apache header. The project will not compile without it.

The init method is where the application logic is defined. The Config argument is the runtime configuration loaded from the properties file we defined earlier. The StreamGraph argument provides methods to declare input streams. You can then invoke a number of flexible operations on those streams. The result of each operation is another stream, so you can keep chaining more operations or direct the result to an output stream.

Next, we will declare the input streams for the Wikipedia application.


The Wikipedia application consumes events from three channels. Let’s declare each of those channels as an input streams via the StreamGraph in the init method.

MessageStream<WikipediaFeedEvent> wikipediaEvents = streamGraph.getInputStream("en-wikipedia", (k, v) -> (WikipediaFeedEvent) v);
MessageStream<WikipediaFeedEvent> wiktionaryEvents = streamGraph.getInputStream("en-wiktionary", (k, v) -> (WikipediaFeedEvent) v);
MessageStream<WikipediaFeedEvent> wikiNewsEvents = streamGraph.getInputStream("en-wikinews", (k, v) -> (WikipediaFeedEvent) v);

The first argument to the getInputStream method is the stream ID. Each ID must match the corresponding stream IDs we configured earlier.

The second argument is the message builder. It converts the input key and message to the appropriate type. In this case, we don’t have a key and want to sent the events as-is, so we have a very simple builder that just forwards the input value.

Note the streams are all MessageStreams of type WikipediaFeedEvent. MessageStream is the in-memory representation of a stream in Samza. It uses generics to ensure type safety across the streams and operations. We knew the WikipediaFeedEvent type by inspecting the WikipediaConsumer above and we made it explicit with the cast on the output of the MessageBuilder. If our inputs used a serde, we would know the type based on which serde is configured for the input streams.


We’d like to use the same processing logic for all three input streams, so we will use the mergeAll operator to merge them together. Note: this is not the same as a join because we are not associating events by key. We are simply combining three streams into one, like a union.

Add the following snippet to the init method. It merges all the input streams into a new one called allWikipediaEvents

MessageStream<WikipediaFeed.WikipediaFeedEvent> allWikipediaEvents = MessageStream.mergeAll(ImmutableList.of(wikipediaEvents, wiktionaryEvents, wikiNewsEvents));

Note there is a merge operator instance method on MessageStream, but the static mergeAll method is a more convenient alternative if you need to merge many streams.


The next step is to parse the events and extract some information. We will use the pre-existing `WikipediaParser.parseEvent()‘ method to do this. The parser extracts some flags we want to monitor as well as some metadata about the event. Inspect the method signature. The input is a WikipediaFeedEvents and the output is a Map. These types will be reflected in the types of the streams before and after the operation.

In the init method, invoke the map operation on allWikipediaEvents, passing the WikipediaParser::parseEvent method reference as follows:



Now that we have the relevant information extracted, let’s perform some aggregations over a 10-second window.

First, we need a container class for statistics we want to track. Add the following static class after the init method.

private static class WikipediaStats {
  int edits = 0;
  int byteDiff = 0;
  Set<String> titles = new HashSet<String>();
  Map<String, Integer> counts = new HashMap<String, Integer>();

Now we need to define the logic to aggregate the stats over the duration of the window. To do this, we implement FoldLeftFunction by adding the following class after the WikipediaStats class:

private class WikipediaStatsAggregator implements FoldLeftFunction<Map<String, Object>, WikipediaStats> {

  public WikipediaStats apply(Map<String, Object> edit, WikipediaStats stats) {
    // Update window stats
    stats.byteDiff += (Integer) edit.get("diff-bytes");
    stats.titles.add((String) edit.get("title"));

    Map<String, Boolean> flags = (Map<String, Boolean>) edit.get("flags");
    for (Map.Entry<String, Boolean> flag : flags.entrySet()) {
      if (Boolean.TRUE.equals(flag.getValue())) {
        stats.counts.compute(flag.getKey(), (k, v) -> v == null ? 0 : v + 1);

    return stats;

Note: the type parameters for FoldLeftFunction reflect the upstream data type and the window value type, respectively. In our case, the upstream type is the output of the parser and the window value is our WikipediaStats class.

Finally, we can define our window back in the init method by chaining the result of the parser:

        .window(Windows.tumblingWindow(Duration.ofSeconds(10), WikipediaStats::new, new WikipediaStatsAggregator()));

This defines an unkeyed tumbling window that spans 10s, which instantiates a new WikipediaStats object at the beginning of each window and aggregates the stats using WikipediaStatsAggregator.

The output of the window is a WindowPane with a key and value. Since we used an unkeyed tumbling window, the key is Void. The value is our WikipediaStats object.


We want to use a JSON serializer to output the window values to Kafka, so we will do one more map to format the output.

First, let’s define the method to format the stats as a Map<String, String> so the json serde can handle it. Paste the following after the aggregator class:

private Map<String, Integer> formatOutput(WindowPane<Void, WikipediaStats> statsWindowPane) {
  WikipediaStats stats = statsWindowPane.getMessage();

  Map<String, Integer> counts = new HashMap<String, Integer>(stats.counts);
  counts.put("edits", stats.edits);
  counts.put("bytes-added", stats.byteDiff);
  counts.put("unique-titles", stats.titles.size());

  return counts;

Now, we can invoke the method by adding another map operation to the chain in init. The operator chain should now look like this:

        .window(Windows.tumblingWindow(Duration.ofSeconds(10), WikipediaStats::new, new WikipediaStatsAggregator()))

Next we need to get the output stream to which we will send the stats. Insert the following line below the creation of the 3 input streams:

OutputStream<Void, Map<String, Integer>, Map<String, Integer>>
        wikipediaStats = streamGraph.getOutputStream("wikipedia-stats", m -> null, m -> m);

The OutputStream is parameterized by 3 types; the key type for the output, the value type for the output, and upstream type.

The first parameter of getOutputStream is the output stream ID. We will use wikipedia-stats and since it contains no special characters, we won’t bother configuring a physical name so Samza will use the stream ID as the topic name.

The second and third parameters are the key extractor and the message extractor, respectively. We have no key, so the key extractor simply produces null. The message extractor simply passes the message because it’s already the correct type for the json serde. Note: we could have skipped the previous map operator and invoked our formatter here, but we kept them separate for pedagogical purposes.

Finally, we can send our output to the output stream using the sendTo operator:

        .window(Windows.tumblingWindow(Duration.ofSeconds(10), WikipediaStats::new, new WikipediaStatsAggregator()))

Tip: Because the MessageStream type information is preserved in the operator chain, it is often easier to define the OutputStream inline with the sendTo operator and then refactor it for readability. That way you don’t have to hunt down the types.


We now have an operational Wikipedia application which provides stats aggregated over a 10 second interval. One of those stats is a count of the number of edits within the 10s window. But what if we want to keep an additional durable counter of the total edits?

We will do this by keeping a separate count outside the window and persisting it in a KeyValueStore.

We start by defining the store in the config file:


These properties declare a RocksDB key-value store named “wikipedia-stats”. The store is replicated to a changelog stream called “wikipedia-stats-changelog” on the kafka system for durability. It uses the string and integer serdes you defined earlier for keys and values respectively.

Next, we add a total count member variable to the WikipediaStats class:

int totalEdits = 0;

To use the store in the application, we need to get it from the TaskContext. Also, since we want to emit the total edit count along with the window edit count, it’s easiest to update both of them in our aggregator. Declare the store as a member variable of the WikipediaStatsAggregator class:

private KeyValueStore<String, Integer> store;

Then override the init method in WikipediaStatsAggregator to initialize the store.

public void init(Config config, TaskContext context) {
  store = (KeyValueStore<String, Integer>) context.getStore("wikipedia-stats");

Update and persist the counter in the apply method.

Integer editsAllTime = store.get("count-edits-all-time");
if (editsAllTime == null) editsAllTime = 0;
store.put("count-edits-all-time", editsAllTime);
stats.totalEdits = editsAllTime;

Finally, update the MyWikipediaApplication#formatOutput method to include the total counter.

counts.put("edits-all-time", stats.totalEdits);


Lastly, let’s add a metric to the application which counts the number of repeat edits each topic within the window interval.

As with the key-value store, we must first define the metrics reporters in the config file.


The above properties define 2 metrics reporters. The first emits metrics to a metrics topic on the kafka system. The second reporter emits metrics to JMX.

In the WikipediaStatsAggregator, declare a counter member variable.

private Counter repeatEdits;

Then add the following to the WikipediaStatsAggregator#init method to initialize the counter.

repeatEdits = context.getMetricsRegistry().newCounter("edit-counters", "repeat-edits");

Update and persist the counter from the apply method.

boolean newTitle = stats.titles.add((String) edit.get("title"));

if (!newTitle) {
  log.info("Frequent edits for title: {}", edit.get("title"));

Run and View Plan

You can set up the grid and run the application using the same instructions from the hello samza high level API Yarn tutorial. The only difference is to replace the wikipedia-application.properties config file in the config-path command line parameter with my-wikipedia-application.properties


Congratulations! You have built and executed a Wikipedia stream application on Samza using the high level API. The final application should be directly comparable to the pre-existing WikipediaApplication in the project.

You can provide feedback on this tutorial in the dev mailing list.