This is a prerelease version.

Adding Batching to a Custom Source

To improve the throughput of your custom sources, you can add batching to reads.

Defining a Source

Here is how you create a source that reads lines of text from a file:

public class Sources {
  public static BatchSource<String> buildLineSource() {
    return SourceBuilder
    .batch("line-source", x ->
      new BufferedReader(new FileReader("lines.txt")))
    .<String>fillBufferFn((in, buf) -> {
      String line = in.readLine();
      if (line != null) {
        buf.add(line);
      } else {
        buf.close();
      }
    })
    .destroyFn(buf -> buf.close())
    .build();
  }
}

Now it is ready to be used from the pipeline, just like a built-in source:

Pipeline p = Pipeline.create();
p.readFrom(Sources.buildLineSource())
 .writeTo(Sinks.logger());

Adding Batching

While this simple source functions correctly, it’s not optimal because it reads just one line and returns. There are some overheads to repeatedly calling fillBufferFn and you can add more than one item to the buffer:

SourceBuilder
  .batch("line-source",
    x -> new BufferedReader(new FileReader("lines.txt")))
    .<String>fillBufferFn((in, buf) -> {
    for (int i = 0; i < 128; i++) {
      String line = in.readLine();
      if (line == null) {
        buf.close();
        return;
      }
      buf.add(line);
    }
  })
  .destroyFn(buf -> buf.close())
  .build();

This code adds 128 lines at a time. In your specific case you should use the rule of thumb to target about 1 millisecond spent in a single invocation of fillBufferFn.