- **原始碼示例**
StreamingJobGraphGenerator.java
```java
for (StreamEdge outEdge : currentNode.getOutEdges()) {
				if (isChainable(outEdge, streamGraph)) {
					chainableOutputs.add(outEdge);
				} else {
					nonChainableOutputs.add(outEdge);
				}
			}
```

- **isChainable()**
```java
	public static boolean isChainable(StreamEdge edge, StreamGraph streamGraph) {
		StreamNode downStreamVertex = streamGraph.getTargetVertex(edge);
		return downStreamVertex.getInEdges().size() == 1
				&& isChainableInput(edge, streamGraph);
	}
```

- **isChainableInput()**
```java
	private static boolean isChainableInput(StreamEdge edge, StreamGraph streamGraph) {
		StreamNode upStreamVertex = streamGraph.getSourceVertex(edge);
		StreamNode downStreamVertex = streamGraph.getTargetVertex(edge);
		if (!(upStreamVertex.isSameSlotSharingGroup(downStreamVertex)
			&& areOperatorsChainable(upStreamVertex, downStreamVertex, streamGraph)
			&& (edge.getPartitioner() instanceof ForwardPartitioner)
			&& edge.getShuffleMode() != ShuffleMode.BATCH
			&& upStreamVertex.getParallelism() == downStreamVertex.getParallelism()
			&& streamGraph.isChainingEnabled())) {
			return false;
		}
		// check that we do not have a union operation, because unions currently only work
		// through the network/byte-channel stack.
		// we check that by testing that each "type" (which means input position) is used only once
		for (StreamEdge inEdge : downStreamVertex.getInEdges()) {
			if (inEdge != edge && inEdge.getTypeNumber() == edge.getTypeNumber()) {
				return false;
			}
		}
		return true;
	}
```

Flink 運算元鏈可以合併的原始碼分析

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