Building Applications with Apache Flink (Part 2): Writing a custom SourceFunction for the CSV Data

In the previous article we have obtained a CSV dataset, analyzed it and built the neccessary tools for parsing it. A domain model was created, which will be used for the Stream processing. What's left is how to feed a DataStream with the actual CSV data, and this is where the SourceFunction fits in.

What we are going to build

We are going to build a SourceFunction, that uses the parsers from the previous article to read the CSV data. The LocalWeatherDataConverter is used to transform the CSV objects into the domain model, which is the emitted to the Apache Flink SourceContext.

Source Code

You can find the full source code for the example in my git repository at:


The LocalWeatherDataSourceFunction implements the SourceFunction interface, parses the CSV data from the previous and emits the measurements to the Apache Flink SourceContext.

// Copyright (c) Philipp Wagner. All rights reserved.
// Licensed under the MIT license. See LICENSE file in the project root for full license information.

package stream.sources.csv;

import csv.parser.Parsers;
import org.apache.flink.streaming.api.functions.source.SourceFunction;
import stream.sources.csv.converter.LocalWeatherDataConverter;

import java.nio.charset.StandardCharsets;
import java.nio.file.FileSystems;
import java.nio.file.Path;
import java.util.Iterator;
import java.util.Map;

public class LocalWeatherDataSourceFunction implements SourceFunction<model.LocalWeatherData> {

    private volatile boolean isRunning = true;

    private String stationFilePath;
    private String localWeatherDataFilePath;

    public LocalWeatherDataSourceFunction(String stationFilePath, String localWeatherDataFilePath) {
        this.stationFilePath = stationFilePath;
        this.localWeatherDataFilePath = localWeatherDataFilePath;

    public void run(SourceFunction.SourceContext<model.LocalWeatherData> sourceContext) throws Exception {

        // The Source needs to be Serializable, so we have to construct the Paths at this point:
        final Path csvStationPath = FileSystems.getDefault().getPath(stationFilePath);
        final Path csvLocalWeatherDataPath = FileSystems.getDefault().getPath(localWeatherDataFilePath);

        // Get the Stream of LocalWeatherData Elements in the CSV File:
        try(Stream<model.LocalWeatherData> stream = getLocalWeatherData(csvStationPath, csvLocalWeatherDataPath)) {

            // We need to get an iterator, since the SourceFunction has to break out of its main loop on cancellation:
            Iterator<model.LocalWeatherData> iterator = stream.iterator();

            // Make sure to cancel, when the Source function is canceled by an external event:
            while (isRunning && iterator.hasNext()) {
                synchronized (sourceContext.getCheckpointLock()) {

    public void cancel() {
        isRunning = false;

    private Stream<model.LocalWeatherData> getLocalWeatherData(Path csvStationPath, Path csvLocalWeatherDataPath) {

        // A map between the WBAN and Station for faster Lookups:
        final Map<String, csv.model.Station> stationMap = getStationMap(csvStationPath);

        // Turns the Stream of CSV data into the Elasticsearch representation:
        return getLocalWeatherData(csvLocalWeatherDataPath)
                // Only use Measurements with a Station:
                .filter(x -> stationMap.containsKey(x.getWban()))
                // And turn the Station and LocalWeatherData into the ElasticSearch representation:
                .map(x -> {
                    // First get the matching Station:
                    csv.model.Station station = stationMap.get(x.getWban());
                    // Convert to the Elastic Representation:
                    return LocalWeatherDataConverter.convert(x, station);

    private static Stream<csv.model.LocalWeatherData> getLocalWeatherData(Path path) {
        return Parsers.LocalWeatherDataParser().readFromFile(path, StandardCharsets.US_ASCII)
                .filter(x -> x.isValid())
                .map(x -> x.getResult());

    private static Stream<csv.model.Station> getStations(Path path) {
        return Parsers.StationParser().readFromFile(path, StandardCharsets.US_ASCII)
                .filter(x -> x.isValid())
                .map(x -> x.getResult());

    private Map<String, csv.model.Station> getStationMap(Path path) {
        try (Stream<csv.model.Station> stationStream = getStations(path)) {
            return stationStream
                    .collect(Collectors.toMap(csv.model.Station::getWban, x -> x));


The next part of the series shows how to utilize the SourceFunction to serve a DataStream.

How to contribute

One of the easiest ways to contribute is to participate in discussions. You can also contribute by submitting pull requests.

General feedback and discussions?

Do you have questions or feedback on this article? Please create an issue on the GitHub issue tracker.

Something is wrong or missing?

There may be something wrong or missing in this article. If you want to help fixing it, then please make a Pull Request to this file on GitHub.