johnyangk commented on a change in pull request #104: [NEMO-183] DAG-centric translation from Beam pipeline to IR DAG URL: https://github.com/apache/incubator-nemo/pull/104#discussion_r210535793
########## File path: compiler/frontend/beam/src/main/java/edu/snu/nemo/compiler/frontend/beam/PipelineVisitor.java ########## @@ -0,0 +1,291 @@ +/* + * Copyright (C) 2018 Seoul National University + * + * Licensed 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 "AS IS" BASIS, + * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. + * See the License for the specific language governing permissions and + * limitations under the License. + */ +package edu.snu.nemo.compiler.frontend.beam; + +import edu.snu.nemo.common.dag.DAG; +import edu.snu.nemo.common.dag.DAGBuilder; +import edu.snu.nemo.common.dag.Edge; +import edu.snu.nemo.common.dag.Vertex; +import org.apache.beam.sdk.Pipeline; +import org.apache.beam.sdk.runners.TransformHierarchy; +import org.apache.beam.sdk.transforms.ParDo; +import org.apache.beam.sdk.transforms.View; +import org.apache.beam.sdk.values.PValue; + +import java.util.*; + +/** + * Traverses through the given Beam pipeline to construct a DAG of Beam Transform, + * while preserving hierarchy of CompositeTransforms. + * This DAG will be later translated by {@link PipelineTranslator} into Nemo IR DAG. + */ +public final class PipelineVisitor extends Pipeline.PipelineVisitor.Defaults { + + private static final String TRANSFORM = "Transform-"; + private static final String DATAFLOW = "Dataflow-"; + + private final Stack<CompositeTransformVertex> compositeTransformVertexStack = new Stack<>(); + private CompositeTransformVertex rootVertex = null; + private int nextIdx = 0; + + @Override + public void visitPrimitiveTransform(final TransformHierarchy.Node node) { + final PrimitiveTransformVertex vertex = new PrimitiveTransformVertex(node, compositeTransformVertexStack.peek()); + compositeTransformVertexStack.peek().addVertex(vertex); + vertex.getPValuesConsumed() + .forEach(pValue -> { + final TransformVertex dst = getDestinationOfDataFlowEdge(vertex, pValue); + dst.parent.addDataFlow(new DataFlowEdge(dst.parent.getProducerOf(pValue), dst)); + }); + } + + @Override + public CompositeBehavior enterCompositeTransform(final TransformHierarchy.Node node) { + final CompositeTransformVertex vertex; + if (compositeTransformVertexStack.isEmpty()) { + // There is always a top-level CompositeTransform that encompasses the entire Beam pipeline. + vertex = new CompositeTransformVertex(node, null); + } else { + vertex = new CompositeTransformVertex(node, compositeTransformVertexStack.peek()); + } + compositeTransformVertexStack.push(vertex); + return CompositeBehavior.ENTER_TRANSFORM; + } + + @Override + public void leaveCompositeTransform(final TransformHierarchy.Node node) { + final CompositeTransformVertex vertex = compositeTransformVertexStack.pop(); + vertex.build(); + if (compositeTransformVertexStack.isEmpty()) { + // The vertex is the root. + if (rootVertex != null) { + throw new RuntimeException("The visitor already have traversed a Beam pipeline. " + + "Re-using a visitor is not allowed."); + } + rootVertex = vertex; + } else { + // The CompositeTransformVertex is ready; adding it to its parent vertex. + compositeTransformVertexStack.peek().addVertex(vertex); + } + } + + /** + * @return A vertex representing the top-level CompositeTransform. + */ + public CompositeTransformVertex getConvertedPipeline() { + if (rootVertex == null) { + throw new RuntimeException("The visitor have not fully traversed through a Beam pipeline."); + } + return rootVertex; + } + + /** + * Represents a {@link org.apache.beam.sdk.transforms.PTransform} as a vertex in DAG. + */ + public abstract class TransformVertex extends Vertex { Review comment: Understood that a composite vertex can have a composite vertex as a child, and so on like an onion. For example: (layer1) root composite - (layer2) child composite - (layer3) grandchild composite - (layer4) primitives My understanding of your comment is that with the proposed lazy execution we're able to do cross-layer optimizations in the Beam frontend (although I believe that's not being done at the moment). This is not possible with directly using Beam's `TransformHierarchy.Node`, which does not provide access to `Node#parts` that represent the children nodes of a composite node. Hence the rationale for our own intermediary abstraction: `TransformVertex(Primitive/Composite)` that corresponds to Beam's `TransformHierarchy`. At the expense of cross-layer optimizations, my personal preference would be handle things one layer at a time during "`enterCompositeTransform` -> `leaveCompositeTransform`". This may be implemented using a map from a Beam node(composite or primitive) to a Nemo IR partition (an IRVertex DAG or a single IRVertex), without our own `TransformVertex(Primitive/Composite)`/stack/rootVertex data structures. For example, we do optimizations in layer3 exploiting the semantics of the composite transform. When we pop out of that layer and come back to layer2, we similarly optimize layer2 while just considering the layer 3 composite transform as a single transform. (although this composite transform may contain many, complex primitive transforms) I am also fine with the current PR, if you want to go ahead with it. In that case, it'd be nice if you can add class-level comments in `PipelineVisitor` that the data structures used here correspond to those in `TransformHierarchy` (e.g., x is a parent of y when x is an 'enclosing vertex' of y), and should not be confused with the usual DAG relationship (e.g., x is a parent of y when y consumes data output by x). ---------------------------------------------------------------- This is an automated message from the Apache Git Service. To respond to the message, please log on GitHub and use the URL above to go to the specific comment. For queries about this service, please contact Infrastructure at: us...@infra.apache.org With regards, Apache Git Services
