Added BlockRDD and a first-cut version of checkpoint() to RDD class.

core/src/main/scala/spark/BlockRDD.scala

+package spark
+
+import scala.collection.mutable.HashMap
+
+class BlockRDDSplit(val blockId: String, idx: Int) extends Split {
+  val index = idx
+}
+
+
+class BlockRDD[T: ClassManifest](sc: SparkContext, blockIds: Array[String]) extends RDD[T](sc) {
+
+  @transient
+  val splits_ = (0 until blockIds.size).map(i => {
+    new BlockRDDSplit(blockIds(i), i).asInstanceOf[Split]
+  }).toArray 
+  
+  @transient 
+  lazy val locations_  = {
+    val blockManager = SparkEnv.get.blockManager 
+    /*val locations = blockIds.map(id => blockManager.getLocations(id))*/
+    val locations = blockManager.getLocations(blockIds) 
+    HashMap(blockIds.zip(locations):_*)
+  }
+
+  override def splits = splits_
+
+  override def compute(split: Split): Iterator[T] = {
+    val blockManager = SparkEnv.get.blockManager 
+    val blockId = split.asInstanceOf[BlockRDDSplit].blockId
+    blockManager.get(blockId) match {
+      case Some(block) => block.asInstanceOf[Iterator[T]]
+      case None => 
+        throw new Exception("Could not compute split, block " + blockId + " not found")
+    }
+  }
+
+  override def preferredLocations(split: Split) = 
+    locations_(split.asInstanceOf[BlockRDDSplit].blockId)
+
+  override val dependencies: List[Dependency[_]] = Nil
+}
+

core/src/main/scala/spark/RDD.scala

@@ -94,6 +94,20 @@ abstract class RDD[T: ClassManifest](@transient sc: SparkContext) extends Serial
 
   def getStorageLevel = storageLevel
   
+  def checkpoint(level: StorageLevel = StorageLevel.DISK_AND_MEMORY_DESER): RDD[T] = {
+    if (!level.useDisk && level.replication < 2) {
+      throw new Exception("Cannot checkpoint without using disk or replication (level requested was " + level + ")")
+    } 
+    
+    // This is a hack. Ideally this should re-use the code used by the CacheTracker
+    // to generate the key.
+    def getSplitKey(split: Split) = "rdd:%d:%d".format(this.id, split.index)
+    
+    persist(level)
+    sc.runJob(this, (iter: Iterator[T]) => {} )
+    new BlockRDD[T](sc, splits.map(getSplitKey).toArray)
+  }
+  
   // Read this RDD; will read from cache if applicable, or otherwise compute
   final def iterator(split: Split): Iterator[T] = {
     if (storageLevel != StorageLevel.NONE) {

core/src/test/scala/spark/RDDSuite.scala

@@ -42,4 +42,11 @@ class RDDSuite extends FunSuite {
     assert(result.toSet === Set(("a", 6), ("b", 2), ("c", 5)))
     sc.stop()
   }
+
+  test("checkpointing") {
+    val sc = new SparkContext("local", "test")
+    val rdd = sc.makeRDD(Array(1, 2, 3, 4), 2).flatMap(x => 1 to x).checkpoint()
+    assert(rdd.collect().toList === List(1, 1, 2, 1, 2, 3, 1, 2, 3, 4))
+    sc.stop()
+  }
 }