[GitHub] [beam] lukecwik commented on a diff in pull request #24025: [#24024] Stop wrapping light weight functions with Contextful as they add a lot of overhead for functions that are meant to do almost no work.

[GitBox <gi...@apache.org>]

lukecwik commented on code in PR #24025:
URL: https://github.com/apache/beam/pull/24025#discussion_r1035251386


##########
sdks/java/core/src/main/java/org/apache/beam/sdk/transforms/FlatMapElements.java:
##########
@@ -45,16 +45,13 @@
     extends PTransform<PCollection<? extends InputT>, PCollection<OutputT>> {
   private final transient @Nullable TypeDescriptor<InputT> inputType;
   private final transient @Nullable TypeDescriptor<OutputT> outputType;
-  private final transient @Nullable Object originalFnForDisplayData;
-  private final @Nullable Contextful<Fn<InputT, Iterable<OutputT>>> fn;
+  private final @Nullable Object fn;
 
   private FlatMapElements(
-      @Nullable Contextful<Fn<InputT, Iterable<OutputT>>> fn,
-      @Nullable Object originalFnForDisplayData,
+      @Nullable Object fn,

Review Comment:
   You're correct, the type system was too painful to specify `OR` between `ProcessFunction` and `Contextful`.
   
   Note that the constructor is private so this detail isn't exposed to users.



##########
sdks/java/core/src/main/java/org/apache/beam/sdk/transforms/FlatMapElements.java:
##########
@@ -137,57 +132,90 @@ public <NewInputT> FlatMapElements<NewInputT, OutputT> via(
   @Experimental(Kind.CONTEXTFUL)
   public <NewInputT> FlatMapElements<NewInputT, OutputT> via(
       Contextful<Fn<NewInputT, Iterable<OutputT>>> fn) {
-    return new FlatMapElements<>(
-        fn, fn.getClosure(), TypeDescriptors.inputOf(fn.getClosure()), outputType);
+    return new FlatMapElements<>(fn, TypeDescriptors.inputOf(fn.getClosure()), outputType);
   }
 
   @Override
   public PCollection<OutputT> expand(PCollection<? extends InputT> input) {
     checkArgument(fn != null, ".via() is required");
-    return input.apply(
-        "FlatMap",
-        ParDo.of(
-                new DoFn<InputT, OutputT>() {
-                  @ProcessElement
-                  public void processElement(ProcessContext c) throws Exception {
-                    Iterable<OutputT> res =
-                        fn.getClosure().apply(c.element(), Fn.Context.wrapProcessContext(c));
-                    for (OutputT output : res) {
-                      c.output(output);
+    if (fn instanceof Contextful) {

Review Comment:
   Yes, half the cost of these trivial functions was going to creating and destroying a Contextful that wraps a ProcessBundleDescriptor that is passed in.
   
   Also note that the branch only happens during pipeline construction so it won't show up on the execution side.



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