Demo quering counts of a event stream with Apache Flink
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Apache Flink Queryable State Demo

This repository contains a short demo of Apache Flink's Queryable State feature. You can check out the Flink documentation for more details about Queryable State.

Getting Started

  1. Requirements
  • Git
  • Java 8
  • Maven 3
  1. Clone this repository.
$ git clone
  1. Run the demo job.
% ./
[info] Building demo JAR... this can take a few seconds.
[info] Done. Demo JAR created in target/queryablestatedemo-1.0-SNAPSHOT.jar.
[info] Executing EventCountJob from queryablestatedemo-1.0-SNAPSHOT.jar (exit via Control+C)
[info] Job ID: 2488a115d832013edbbd5a6599e49e45

Generating 454435 elements per second
Generating 880747 elements per second
Generating 903855 elements per second
  1. Run the demo query client.
./ 2488a115d832013edbbd5a6599e49e45
[info] Querying job with ID '2488a115d832013edbbd5a6599e49e45'
[info] Executing EventCountClient from queryablestatedemo-1.0-SNAPSHOT.jar (exit via Control+C)

Using JobManager localhost:6124
Enter a key to query.

The EventCountJob itemCounts state instance has String keys that are three characters long and alphanumeric, e.g. 'AP2' or 'LOL'.

[info] Querying key 'abc'
446 (query took 99 ms)
[info] Querying key 'abc'
604 (query took 2 ms)
[info] Querying key 'abc'
631 (query took 1 ms)
$ ele
[info] Querying key 'ele'
834 (query took 2 ms)
$ notexistingkey
[info] Querying key 'notexistingkey'
Unknown key notexistingkey (query took 11 ms)

Getting Started


The code for this demo is found in the src/main directory under the package com.dataartisans.queryablestatedemo.

The two main classes are EventCountJob and EventCountClient.


The EventCountJob is a simple job that generates random BumpEvent instances and counts how many counts happen per bumped item. The bump events model users interacting with items on a website and liking them.

public class BumpEvent {

  // ID of the user bumping the item
  private final int userId;

  // Item being bumped
  private final String itemId;


The item IDs in our demo are three character alphanumeric Strings like ABC or 1A3. The stream of BumpEvent instances is keyed by the itemId and the count is increased for each item.

// Increment the count for each event (keyed on itemId)
FoldingStateDescriptor<BumpEvent, Long> countingState = new FoldingStateDescriptor<>(
    0L,                       // Initial value is 0
    (acc, event) -> acc + 1L, // Increment for each event

bumps.keyBy(BumpEvent::getItemId).asQueryableState("itemCounts", countingState);

The call to asQueryableState(State, StateDescriptor) exposes a queryable state stream that is identified via the name itemCounts. The countingState descriptor specifies a state instance that increments the count for each incoming event. Because we use a keyed stream here (the keyBy) each count is automatically scoped to the item ID. You can find more information about Flink's state abstraction in the documentation.

As soon as the job is running, the itemCounts state instance is available for queries. As part of this demo, you can use the included script which takes care of building the job and executing it. Make sure to note down the displayed JobID as you will need it for the query client below.

Exposing State for External Queries

As of Flink 1.2, we have two options for exposing state for queries.

  1. Queryable State Stream: The above variant is a shorthand that we call a queryable state stream. The asQueryableState call is overloaded to allow different state variants like ValueState or ReducingState, allowing you to increment the state stream in different ways.

  2. StateDescripor. The other variant is to call setQueryable("itemCounts") when creating a StateDescriptor. This is more flexible as you can expose any keyed state instance you are working with for queries. The following example is achieves the same result as the queryable state stream from above:

static class EventCounter<T> extends RichFlatMapFunction<T, Void> {

  // Flink's managed state for keeping track of the counts. The state is
  // automatically scoped to the item key and updates happen by key.
  private ReducingState<Long> runningCount;
  public void open(Configuration config) throws Exception {
    // Reducing state that keeps a sum
    ReducingStateDescriptor<Long> stateDescriptor = new ReducingStateDescriptor<>(
        "itemCounts", (a, b) -> a + b, Long.class);
    // Mark state as queryable. This is how we expose the state instance
    // for external queries.
    this.runningCount = getRuntimeContext().getReducingState(stateDescriptor);

  public void flatMap(T event, Collector<Void> collector) throws Exception {
    // Increment the count for the current key


The EventCountClient of this demo is a very simple REPL that queries the itemCounts state instance.

The QueryClientHelper takes care of setting up Flink's low level QueryableStateClient that was released with Flink 1.2.0. The QueryableStateClient is a fully asynchronous client that takes care of

  1. Location lookup: communicate with the JobManager and look up the location of a queried key among the available TaskManager instances, and
  2. Network communication: submitting the query via the network to a specific TaskManager.

This means that the client takes care of all communication with the Flink application. As a user, you only have to provide the following information:

  • JobID. The ID of the job to query (displayed by the demo job or found in the web UI)
  • Types. The types of the queried keys and returned values (our keys are Strings and the returned counts are Longs, using the StringSerializer and LongSerializer respectively)

As part of this demo, the EventCountClient takes care of setting up the types and submitting the queries. You only have to provide the JobID.

You can use the included script which takes care of building the client and executing it. It has the jobId of the running job as a required argument.

If you want to have a look at how the QueryableStateClient is set up as part of this demo, check out the QueryClientHelper class.

The main query method is Optional<V> queryState(String name, K key). The name for the demo is itemCounts and the keys are three character alphanumeric Strings like AB1. If a key is not available, the returned Optional will be empty.