Optimize paginated getChildren: return children names instead of Path…

…WithStat

zookeeper-server/src/main/java/org/apache/zookeeper/ChildrenBatchIterator.java

@@ -21,24 +21,18 @@
 import java.util.LinkedList;
 import java.util.List;
 import java.util.NoSuchElementException;
+import org.apache.zookeeper.data.PathWithStat;
 
 /**
  * Iterator over children nodes of a given path.
- * <p>
- * Note: the final collection of children may not be strongly consistent with the server.
- * If there are concurrent writes to the children during iteration, the final collection could
- * miss some children or contain some duplicate children.
- *
- * @see ZooKeeper#getAllChildrenPaginated(String, boolean)
  */
-class ChildrenBatchIterator implements RemoteIterator<String> {
+class ChildrenBatchIterator implements RemoteIterator<PathWithStat> {
 
     private final ZooKeeper zooKeeper;
     private final String path;
     private final Watcher watcher;
     private final int batchSize;
-    private final LinkedList<String> childrenQueue;
-    private final PaginationNextPage nextPage;
+    private final LinkedList<PathWithStat> childrenQueue;
     private long nextBatchMinZxid;
     private int nextBatchZxidOffset;
 
@@ -53,44 +47,55 @@ class ChildrenBatchIterator implements RemoteIterator<String> {
         this.nextBatchMinZxid = minZxid;
 
         this.childrenQueue = new LinkedList<>();
-        this.nextPage = new PaginationNextPage();
 
-        batchGetChildren();
+        List<PathWithStat> firstChildrenBatch = zooKeeper.getChildren(path, watcher, batchSize, nextBatchMinZxid, nextBatchZxidOffset);
+        childrenQueue.addAll(firstChildrenBatch);
+
+        updateOffsetsForNextBatch(firstChildrenBatch);
     }
 
     @Override
     public boolean hasNext() {
+
         // next() never lets childrenQueue empty unless we iterated over all children
         return !childrenQueue.isEmpty();
     }
 
     @Override
-    public String next() throws KeeperException, InterruptedException {
+    public PathWithStat next() throws KeeperException, InterruptedException, NoSuchElementException {
+
         if (!hasNext()) {
             throw new NoSuchElementException("No more children");
         }
 
         // If we're down to the last element, backfill before returning it
-        if (childrenQueue.size() == 1 && nextBatchMinZxid != ZooDefs.GetChildrenPaginated.lastPageMinCzxid
-                && nextBatchZxidOffset != ZooDefs.GetChildrenPaginated.lastPageMinCzxidOffset) {
-            batchGetChildren();
+        if (childrenQueue.size() == 1) {
+
+            List<PathWithStat> childrenBatch = zooKeeper.getChildren(path, watcher, batchSize, nextBatchMinZxid, nextBatchZxidOffset);
+            childrenQueue.addAll(childrenBatch);
+
+            updateOffsetsForNextBatch(childrenBatch);
         }
 
-        return childrenQueue.pop();
+        PathWithStat returnChildren = childrenQueue.pop();
+
+        return returnChildren;
     }
 
     /**
-     * Prepare minZxid and zxidOffset for the next batch request
+     * Prepare minZxid and zkidOffset for the next batch request based on the children returned in the current
      */
-    private void updateOffsetsForNextBatch() {
-        nextBatchMinZxid = nextPage.getMinCzxid();
-        nextBatchZxidOffset = nextPage.getMinCzxidOffset();
-    }
+    private void updateOffsetsForNextBatch(List<PathWithStat> children) {
 
-    private void batchGetChildren() throws KeeperException, InterruptedException {
-        List<String> childrenBatch =
-                zooKeeper.getChildren(path, watcher, batchSize, nextBatchMinZxid, nextBatchZxidOffset, nextPage);
-        childrenQueue.addAll(childrenBatch);
-        updateOffsetsForNextBatch();
+        for (PathWithStat child : children) {
+            long childZxid = child.getStat().getCzxid();
+
+            if (nextBatchMinZxid == childZxid) {
+                ++nextBatchZxidOffset;
+            } else {
+                nextBatchZxidOffset = 1;
+                nextBatchMinZxid = childZxid;
+            }
+        }
     }
 }

zookeeper-server/src/main/java/org/apache/zookeeper/RemoteIterator.java

@@ -27,18 +27,16 @@ public interface RemoteIterator<E> {
 
     /**
      * Returns true if the iterator has more elements.
-     *
      * @return true if the iterator has more elements, false otherwise.
      */
     boolean hasNext();
 
     /**
      * Returns the next element in the iteration.
-     *
      * @return the next element in the iteration.
      * @throws InterruptedException if the thread is interrupted
      * @throws KeeperException if an error is encountered server-side
      * @throws NoSuchElementException if the iteration has no more elements
      */
-    E next() throws InterruptedException, KeeperException;
+    E next() throws InterruptedException, KeeperException, NoSuchElementException;
 }