review

kernel/kernel-api/src/main/java/io/delta/kernel/exceptions/KernelEngineException.java

@@ -21,10 +21,10 @@
  * Throws when the {@link Engine} encountered an error while executing an operation.
  */
 public class KernelEngineException extends RuntimeException {
-    private static final String msgT = "Encountered an error from the underlying engine " +
+    private static final String msgTemplate = "Encountered an error from the underlying engine " +
             "implementation while trying to %s: %s";
 
     public KernelEngineException(String attemptedOperation, Throwable cause) {
-        super(String.format(msgT, attemptedOperation, cause.getMessage()), cause);
+        super(String.format(msgTemplate, attemptedOperation, cause.getMessage()), cause);
     }
 }

kernel/kernel-api/src/main/java/io/delta/kernel/internal/checkpoints/Checkpointer.java

@@ -262,8 +262,8 @@ private Optional<CheckpointMetaData> loadMetadataFromFile(Engine engine, int tri
             } else if (cause instanceof Exception) {
                 String msg = String.format(
                         "Failed to load checkpoint metadata from file %s. " +
-                                "It must be in the process of being writing. " +
-                                "Retrying after 1sec. (current attempt = %s)",
+                                "It must be in the process of being written. " +
+                                "Retrying after 1sec. (current attempt of %s (max 3)",
                         lastCheckpointFilePath, tries);
                 logger.warn(msg, cause);
                 // we can retry until max tries are exhausted. It saves latency as the alternative

kernel/kernel-api/src/main/java/io/delta/kernel/utils/CloseableIterator.java

@@ -65,7 +65,9 @@ public interface CloseableIterator<T> extends Iterator<T>, Closeable {
      *                                {@link IOException} thrown while trying to read from a Delta
      *                                log file. It will be wrapped in this exception as cause.
      * @throws KernelException        When encountered an operation or state that is invalid or
-     *                                unsupported.
+     *                                unsupported in Kernel. For example, trying to read from a
+     *                                Delta table that has advanced features which are not yet
+     *                                supported by Kernel.
      */
     @Override
     T next();

kernel/kernel-defaults/src/test/scala/io/delta/kernel/defaults/LogReplayMetricsSuite.scala

@@ -326,6 +326,8 @@ class LogReplayMetricsSuite extends QueryTest
           expParquetVersionsRead = Seq(10),
           // we read the checkpoint twice: once for the P &M and once for the scan files
           expParquetReadSetSizes = Seq(1, 1),
+          // We try to read `_last_checkpoint` once. If it doesn't exist, we don't try reading
+          // again. If it exists, we succeed reading in the first time
           expLastCheckpointReadCalls = Some(1)
         )
       }