Implementation:Apache Flink HybridSourceReader PollNext

Knowledge Sources	Apache Flink
Domains	Stream_Processing, Source_Architecture
Last Updated	2026-02-09 00:00 GMT

Overview

Concrete tool for delegating record polling to the current source reader and signaling source completion provided by the Apache Flink connector-base module.

Description

HybridSourceReader.pollNext delegates to the current underlying SourceReader.pollNext. When the current reader returns END_OF_INPUT and it is not the final source, the reader sends a SourceReaderFinishedEvent to the enumerator and returns NOTHING_AVAILABLE. For the final source, END_OF_INPUT propagates directly. When no reader is active (during switching), it returns NOTHING_AVAILABLE.

Usage

This is an internal method called by the Flink runtime during source execution.

Code Reference

Source Location

• Repository: Apache Flink
• File: flink-connectors/flink-connector-base/src/main/java/org/apache/flink/connector/base/source/hybrid/HybridSourceReader.java
• Lines: L81-106

Signature

public class HybridSourceReader<T> implements SourceReader<T, HybridSourceSplit> {

    @Override
    public InputStatus pollNext(ReaderOutput output) throws Exception {
        if (currentReader == null) {
            return InputStatus.NOTHING_AVAILABLE;
        }
        InputStatus status = currentReader.pollNext(output);
        if (status == InputStatus.END_OF_INPUT) {
            if (!isFinalSource) {
                // Signal completion to coordinator
                readerContext.sendSourceEventToCoordinator(
                        new SourceReaderFinishedEvent(currentSourceIndex));
                currentReader = null;
                return InputStatus.NOTHING_AVAILABLE;
            }
        }
        return status;
    }
}

Import

import org.apache.flink.connector.base.source.hybrid.HybridSourceReader;
// Internal class

I/O Contract

Inputs

Name	Type	Required	Description
output	ReaderOutput	Yes	Output collector for emitting records

Outputs

Name	Type	Description
status	InputStatus	MORE_AVAILABLE, NOTHING_AVAILABLE, or END_OF_INPUT
side effect	SourceEvent	SourceReaderFinishedEvent sent when current source completes

Usage Examples

Processing Flow

// HybridSourceReader.pollNext lifecycle:
// 1. If no current reader (waiting for switch) -> NOTHING_AVAILABLE
// 2. Delegate to currentReader.pollNext(output)
// 3. If current reader returns END_OF_INPUT:
//    a. If NOT final source -> send SourceReaderFinishedEvent, null reader
//    b. If final source -> propagate END_OF_INPUT
// 4. Otherwise return the status as-is

Related Pages

Implements Principle

• Principle:Apache_Flink_Hybrid_Source_Processing