hvanhovell commented on code in PR #38468:
URL: https://github.com/apache/spark/pull/38468#discussion_r1019036963


##########
connector/connect/src/main/scala/org/apache/spark/sql/connect/service/SparkConnectStreamHandler.scala:
##########
@@ -117,10 +129,91 @@ class SparkConnectStreamHandler(responseObserver: 
StreamObserver[Response]) exte
       responseObserver.onNext(response.build())
     }
 
-    responseObserver.onNext(sendMetricsToResponse(clientId, rows))
+    responseObserver.onNext(sendMetricsToResponse(clientId, dataframe))
     responseObserver.onCompleted()
   }
 
+  def processRowsAsArrowBatches(clientId: String, dataframe: DataFrame): Unit 
= {
+    val spark = dataframe.sparkSession
+    val schema = dataframe.schema
+    // TODO: control the batch size instead of max records
+    val maxRecordsPerBatch = spark.sessionState.conf.arrowMaxRecordsPerBatch
+    val timeZoneId = spark.sessionState.conf.sessionLocalTimeZone
+
+    SQLExecution.withNewExecutionId(dataframe.queryExecution, 
Some("collectArrow")) {
+      val pool = 
ThreadUtils.newDaemonSingleThreadExecutor("connect-collect-arrow")
+      val tasks = collection.mutable.ArrayBuffer.empty[Future[_]]
+      val rows = dataframe.queryExecution.executedPlan.execute()
+
+      if (rows.getNumPartitions > 0) {
+        val batches = rows.mapPartitionsInternal { iter =>
+          ArrowConverters
+            .toArrowBatchIterator(iter, schema, maxRecordsPerBatch, timeZoneId)
+        }
+
+        val processPartition = (iter: Iterator[(Array[Byte], Long, Long)]) => 
iter.toArray
+
+        val resultHandler = (partitionId: Int, taskResult: Array[(Array[Byte], 
Long, Long)]) => {
+          if (taskResult.exists(_._1.nonEmpty)) {
+            // only send non-empty partitions
+            val task = pool.submit(new Runnable {
+              override def run(): Unit = {
+                var batchId = partitionId.toLong << 33
+                taskResult.foreach { case (bytes, count, size) =>
+                  val response = 
proto.Response.newBuilder().setClientId(clientId)
+                  val batch = proto.Response.ArrowBatch
+                    .newBuilder()
+                    .setBatchId(batchId)
+                    .setRowCount(count)
+                    .setUncompressedBytes(size)
+                    .setCompressedBytes(bytes.length)
+                    .setData(ByteString.copyFrom(bytes))
+                    .build()
+                  response.setArrowBatch(batch)
+                  responseObserver.onNext(response.build())

Review Comment:
   You could do that yes. If we get the format right we could just serve the 
block directly to the client (to avoid deserialization).
   
   Alternatively you could also write the data to some persistent storage on 
the executors, and just pass the file paths to the client. That will be the 
most efficient from the POV of the driver. This requires you have some garbage 
collection in place to clean-up results.



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