Introduce actions, table feature, properties

spark/src/main/scala/org/apache/spark/sql/delta/Checkpoints.scala

@@ -458,7 +458,8 @@ object Checkpoints extends DeltaLogging {
         action
       }
       // commitInfo, cdc, remove.tags and remove.stats are not included in the checkpoint
-      .drop("commitInfo", "cdc")
+      // TODO: Add support for V2 Checkpoints here.
+      .drop("commitInfo", "cdc", "checkpointMetadata", "sidecarFile")
       .withColumn("remove", col("remove").dropFields("tags", "stats"))
 
     val chk = buildCheckpoint(base, snapshot)
@@ -684,3 +685,64 @@ object Checkpoints extends DeltaLogging {
     } else Some(struct(partitionValues: _*).as(STRUCT_PARTITIONS_COL_NAME))
   }
 }
+
+object V2Checkpoint {
+  /** Format for V2 Checkpoints */
+  sealed abstract class Format(val name: String) {
+    def fileFormat: FileFormat
+  }
+
+  def toFormat(fileName: String): Format = fileName match {
+    case _ if fileName.endsWith(Format.JSON.name) => Format.JSON
+    case _ if fileName.endsWith(Format.PARQUET.name) => Format.PARQUET
+    case _ => throw new IllegalStateException(s"Unknown v2 checkpoint file format: ${fileName}")
+  }
+
+  object Format {
+    /** json v2 checkpoint */
+    object JSON extends Format("json") {
+      override def fileFormat: FileFormat = DeltaLogFileIndex.CHECKPOINT_FILE_FORMAT_JSON
+    }
+
+    /** parquet v2 checkpoint */
+    object PARQUET extends Format("parquet") {
+      override def fileFormat: FileFormat = DeltaLogFileIndex.CHECKPOINT_FILE_FORMAT_PARQUET
+    }
+
+    /** All valid formats for the top level file of v2 checkpoints. */
+    val ALL: Set[Format] = Set(Format.JSON, Format.PARQUET)
+
+    /** The string representations of all the valid formats. */
+    val ALL_AS_STRINGS: Set[String] = ALL.map(_.name)
+  }
+}
+
+object CheckpointPolicy {
+
+  sealed abstract class Policy(val name: String) {
+    override def toString: String = name
+    def needsV2CheckpointSupport: Boolean = true
+  }
+
+  /**
+   * Write classic single file/multi-part checkpoints when this policy is enabled.
+   * Note that [[V2CheckpointTableFeature]] is not required for this checkpoint policy.
+   */
+  case object Classic extends Policy("classic") {
+    override def needsV2CheckpointSupport: Boolean = false
+  }
+
+  /**
+   * Write V2 checkpoints when this policy is enabled.
+   * This needs [[V2CheckpointTableFeature]] to be enabled on the table.
+   */
+  case object V2 extends Policy("v2")
+
+  /** ALl checkpoint policies */
+  val ALL: Seq[Policy] = Seq(Classic, V2)
+
+  /** Converts a `name` String into a [[Policy]] */
+  def fromName(name: String): Policy = ALL.find(_.name == name).getOrElse {
+    throw new IllegalStateException(s"Invalid policy $name")
+  }
+}

spark/src/main/scala/org/apache/spark/sql/delta/DeltaConfig.scala

@@ -624,6 +624,15 @@ trait DeltaConfigsBase extends DeltaLogging {
     "must be Serializable"
   )
 
+  /** Policy to decide what kind of checkpoint to write to a table. */
+  val CHECKPOINT_POLICY = buildConfig[CheckpointPolicy.Policy](
+    key = "checkpointPolicy-dev",
+    defaultValue = CheckpointPolicy.Classic.name,
+    fromString = str => CheckpointPolicy.fromName(str),
+    validationFunction = (v => CheckpointPolicy.ALL.exists(_.name == v.name)),
+    helpMessage = s"can be one of the " +
+      s"following: ${CheckpointPolicy.Classic.name}, ${CheckpointPolicy.V2.name}")
+
   /**
    * Indicates whether Row Tracking is enabled on the table. When this flag is turned on, all rows
    * are guaranteed to have Row IDs and Row Commit Versions assigned to them, and writers are

spark/src/main/scala/org/apache/spark/sql/delta/DeltaLogFileIndex.scala

@@ -89,6 +89,7 @@ object DeltaLogFileIndex {
 
   lazy val COMMIT_FILE_FORMAT = new JsonFileFormat
   lazy val CHECKPOINT_FILE_FORMAT_PARQUET = new ParquetFileFormat
+  lazy val CHECKPOINT_FILE_FORMAT_JSON = new JsonFileFormat
 
   def apply(format: FileFormat, fs: FileSystem, paths: Seq[Path]): DeltaLogFileIndex = {
     DeltaLogFileIndex(format, paths.map(fs.getFileStatus).toArray)

spark/src/main/scala/org/apache/spark/sql/delta/TableFeature.scala

@@ -324,7 +324,8 @@ object TableFeature {
       ColumnMappingTableFeature,
       TimestampNTZTableFeature,
       IcebergCompatV1TableFeature,
-      DeletionVectorsTableFeature)
+      DeletionVectorsTableFeature,
+      V2CheckpointTableFeature)
     if (DeltaUtils.isTesting) {
       features ++= Set(
         TestLegacyWriterFeature,
@@ -344,6 +345,13 @@ object TableFeature {
         // Row IDs are still under development and only available in testing.
         RowTrackingFeature)
     }
+    val exposeV2Checkpoints =
+      DeltaUtils.isTesting || SparkSession.getActiveSession.map { spark =>
+        spark.conf.get(DeltaSQLConf.EXPOSE_CHECKPOINT_V2_TABLE_FEATURE_FOR_TESTING)
+      }.getOrElse(false)
+    if (exposeV2Checkpoints) {
+      features += V2CheckpointTableFeature
+    }
     val featureMap = features.map(f => f.name.toLowerCase(Locale.ROOT) -> f).toMap
     require(features.size == featureMap.size, "Lowercase feature names must not duplicate.")
     featureMap
@@ -513,6 +521,25 @@ object IcebergCompatV1TableFeature extends WriterFeature(name = "icebergCompatV1
 }
 
 
+/**
+ * V2 Checkpoint table feature is for checkpoints with sidecars and the new format and
+ * file naming scheme.
+ * This is still WIP feature.
+ * This feature will be opensource as soon as work is complete.
+ */
+object V2CheckpointTableFeature
+  extends ReaderWriterFeature(name = "v2Checkpoint-under-development")
+  with FeatureAutomaticallyEnabledByMetadata {
+
+  override def automaticallyUpdateProtocolOfExistingTables: Boolean = true
+
+  override def metadataRequiresFeatureToBeEnabled(
+      metadata: Metadata,
+      spark: SparkSession): Boolean = {
+    DeltaConfigs.CHECKPOINT_POLICY.fromMetaData(metadata).needsV2CheckpointSupport
+  }
+}
+
 /**
  * Features below are for testing only, and are being registered to the system only in the testing
  * environment. See [[TableFeature.allSupportedFeaturesMap]] for the registration.

spark/src/main/scala/org/apache/spark/sql/delta/actions/actions.scala

@@ -30,14 +30,15 @@ import org.apache.spark.sql.delta._
 import org.apache.spark.sql.delta.commands.DeletionVectorUtils
 import org.apache.spark.sql.delta.sources.DeltaSQLConf
 import org.apache.spark.sql.delta.util.{JsonUtils, Utils => DeltaUtils}
+import org.apache.spark.sql.delta.util.FileNames
 import com.fasterxml.jackson.annotation._
 import com.fasterxml.jackson.annotation.JsonInclude.Include
 import com.fasterxml.jackson.core.JsonGenerator
 import com.fasterxml.jackson.databind._
 import com.fasterxml.jackson.databind.annotation.{JsonDeserialize, JsonSerialize}
 import com.fasterxml.jackson.databind.node.ObjectNode
 
-import org.apache.hadoop.fs.Path
+import org.apache.hadoop.fs.{FileStatus, Path}
 
 import org.apache.spark.internal.Logging
 import org.apache.spark.sql.{Column, Encoder, SparkSession}
@@ -1160,6 +1161,92 @@ object CommitInfo {
 
 }
 
+/** A trait to represent actions which can only be part of Checkpoint */
+sealed trait CheckpointOnlyAction extends Action
+
+/**
+ * An [[Action]] containing the information about a sidecar file.
+ *
+ * @param path - sidecar path relative to `_delta_log/_sidecar` directory
+ * @param sizeInBytes - size in bytes for the sidecar file
+ * @param modificationTime - modification time of the sidecar file
+ * @param tags - attributes of the sidecar file, defaults to null (which is semantically same as an
+ *               empty Map). This is kept null to ensure that the field is not present in the
+ *               generated json.
+ */
+case class SidecarFile(
+    path: String,
+    sizeInBytes: Long,
+    modificationTime: Long,
+    tags: Map[String, String] = null)
+  extends Action with CheckpointOnlyAction {
+
+  override def wrap: SingleAction = SingleAction(sidecar = this)
+
+  def toFileStatus(logPath: Path): FileStatus = {
+    val partFilePath = new Path(FileNames.sidecarDirPath(logPath), path)
+    new FileStatus(sizeInBytes, false, 0, 0, modificationTime, partFilePath)
+  }
+}
+
+object SidecarFile {
+  def apply(fileStatus: SerializableFileStatus): SidecarFile = {
+    SidecarFile(fileStatus.getHadoopPath.getName, fileStatus.length, fileStatus.modificationTime)
+  }
+
+  def apply(fileStatus: FileStatus): SidecarFile = {
+    SidecarFile(fileStatus.getPath.getName, fileStatus.getLen, fileStatus.getModificationTime)
+  }
+}
+
+/**
+ * Holds information about the Delta Checkpoint. This action will only be part of checkpoints.
+ *
+ * @param version version of the checkpoint
+ * @param tags    attributes of the checkpoint, defaults to null (which is semantically same as an
+ *                empty Map). This is kept null to ensure that the field is not present in the
+ *                generated json.
+ */
+case class CheckpointMetadata(
+    version: Long,
+    tags: Map[String, String] = null)
+  extends Action with CheckpointOnlyAction {
+
+  import CheckpointMetadata.Tags
+  override def wrap: SingleAction = SingleAction(checkpointMetadata = this)
+}
+
+object CheckpointMetadata {
+
+  def apply(
+      version: Long,
+      sidecarNumActions: Long,
+      sidecarSizeInBytes: Long,
+      numOfAddFiles: Long,
+      sidecarFileSchemaOpt: Option[StructType]): CheckpointMetadata = {
+    val tagMapWithSchema = sidecarFileSchemaOpt
+      .map(schema => Map(Tags.SIDECAR_FILE_SCHEMA.name -> schema.json))
+      .getOrElse(Map.empty)
+    CheckpointMetadata(
+      version = version,
+      tags = Map(
+        Tags.SIDECAR_NUM_ACTIONS.name -> sidecarNumActions.toString,
+        Tags.SIDECAR_SIZE_IN_BYTES.name -> sidecarSizeInBytes.toString,
+        Tags.NUM_OF_ADD_FILES.name -> numOfAddFiles.toString
+      ) ++ tagMapWithSchema
+    )
+  }
+
+  object Tags {
+    sealed abstract class KeyType(val name: String)
+
+    object SIDECAR_NUM_ACTIONS extends KeyType("sidecarNumActions")
+    object SIDECAR_SIZE_IN_BYTES extends KeyType("sidecarSizeInBytes")
+    object NUM_OF_ADD_FILES extends KeyType("numOfAddFiles")
+    object SIDECAR_FILE_SCHEMA extends KeyType("sidecarFileSchema")
+  }
+}
+
 /** A serialization helper to create a common action envelope. */
 case class SingleAction(
     txn: SetTransaction = null,
@@ -1168,6 +1255,8 @@ case class SingleAction(
     metaData: Metadata = null,
     protocol: Protocol = null,
     cdc: AddCDCFile = null,
+    checkpointMetadata: CheckpointMetadata = null,
+    sidecar: SidecarFile = null,
     domainMetadata: DomainMetadata = null,
     commitInfo: CommitInfo = null) {
 
@@ -1184,6 +1273,10 @@ case class SingleAction(
       protocol
     } else if (cdc != null) {
       cdc
+    } else if (sidecar != null) {
+      sidecar
+    } else if (checkpointMetadata != null) {
+      checkpointMetadata
     } else if (domainMetadata != null) {
       domainMetadata
     } else if (commitInfo != null) {

spark/src/main/scala/org/apache/spark/sql/delta/sources/DeltaSQLConf.scala

@@ -572,6 +572,35 @@ trait DeltaSQLConfBase {
         .checkValue(_ > 0, "partSize has to be positive")
         .createOptional
 
+  ////////////////////////////////////
+  // Checkpoint V2 Specific Configs
+  ////////////////////////////////////
+
+  val CHECKPOINT_V2_TOP_LEVEL_FILE_FORMAT =
+    buildConf("checkpointV2.topLevelFileFormat")
+      .internal()
+      .doc(
+        """
+          |The file format to use for the top level checkpoint file in V2 Checkpoints.
+          | This can be set to either json or parquet. The appropriate format will be
+          | picked automatically if this config is not specified.
+          |""".stripMargin)
+      .stringConf
+      .checkValues(Set("json", "parquet"))
+      .createOptional
+
+  val EXPOSE_CHECKPOINT_V2_TABLE_FEATURE_FOR_TESTING =
+    buildConf("checkpointV2.exposeTableFeatureForTesting")
+      .internal()
+      .doc(
+        """
+          |This conf controls whether v2 checkpoints table feature is exposed or not. Note that
+          | v2 checkpoints are in development and this should config should be used only for
+          | testing/benchmarking.
+          |""".stripMargin)
+      .booleanConf
+      .createWithDefault(false)
+
   val DELTA_WRITE_CHECKSUM_ENABLED =
     buildConf("writeChecksumFile.enabled")
       .doc("Whether the checksum file can be written.")

spark/src/main/scala/org/apache/spark/sql/delta/util/FileNames.scala

@@ -16,6 +16,8 @@
 
 package org.apache.spark.sql.delta.util
 
+import java.util.UUID
+
 import org.apache.hadoop.fs.{FileStatus, Path}
 
 /** Helper for creating file names for specific commits / checkpoints. */
@@ -146,4 +148,27 @@ object FileNames {
     val DELTA, CHECKPOINT, CHECKSUM, OTHER = Value
   }
 
+
+  /** File path for a new V2 Checkpoint Json file */
+  def newV2CheckpointJsonFile(path: Path, version: Long): Path =
+    new Path(path, f"$version%020d.checkpoint.${UUID.randomUUID.toString}.json")
+
+  /** File path for a new V2 Checkpoint Parquet file */
+  def newV2CheckpointParquetFile(path: Path, version: Long): Path =
+    new Path(path, f"$version%020d.checkpoint.${UUID.randomUUID.toString}.parquet")
+
+  /** File path for a V2 Checkpoint's Sidecar file */
+  def newV2CheckpointSidecarFile(
+      logPath: Path,
+      version: Long,
+      numParts: Int,
+      currentPart: Int): Path = {
+    val basePath = sidecarDirPath(logPath)
+    val uuid = UUID.randomUUID.toString
+    new Path(basePath, f"$version%020d.checkpoint.$currentPart%010d.$numParts%010d.$uuid.parquet")
+  }
+
+  val SIDECAR_SUBDIR = "_sidecars"
+  /** Returns path to the sidecar directory */
+  def sidecarDirPath(logPath: Path): Path = new Path(logPath, SIDECAR_SUBDIR)
 }

spark/src/test/scala/org/apache/spark/sql/delta/ActionSerializerSuite.scala

@@ -382,6 +382,45 @@ class ActionSerializerSuite extends QueryTest with SharedSparkSession with Delta
     }
   }
 
+  test("CheckpointMetadata - serialize/deserialize") {
+    val m1 = CheckpointMetadata(version = 1, tags = null) // tags are null
+    val m2 = m1.copy(tags = Map()) // tags are empty
+    val m3 = m1.copy( // tags are non empty
+      tags = Map("k1" -> "v1", "schema" -> """{"type":"struct","fields":[]}""")
+    )
+
+    assert(m1.json === """{"checkpointMetadata":{"version":1}}""")
+    assert(m2.json === """{"checkpointMetadata":{"version":1,"tags":{}}}""")
+    assert(m3.json ===
+      """{"checkpointMetadata":{"version":1,""" +
+        """"tags":{"k1":"v1","schema":"{\"type\":\"struct\",\"fields\":[]}"}}}""")
+
+    Seq(m1, m2, m3).foreach { metadata =>
+      assert(metadata === JsonUtils.fromJson[SingleAction](metadata.json).unwrap)
+    }
+  }
+
+  test("SidecarFile - serialize/deserialize") {
+    val f1 = // tags are null
+      SidecarFile(path = "/t1/p1", sizeInBytes = 1L, modificationTime = 3, tags = null)
+    val f2 = f1.copy(tags = Map()) // tags are empty
+    val f3 = f2.copy( // tags are non empty
+      tags = Map("k1" -> "v1", "schema" -> """{"type":"struct","fields":[]}""")
+    )
+
+    assert(f1.json ===
+      """{"sidecar":{"path":"/t1/p1","sizeInBytes":1,"modificationTime":3}}""")
+    assert(f2.json ===
+      """{"sidecar":{"path":"/t1/p1","sizeInBytes":1,""" +
+        """"modificationTime":3,"tags":{}}}""")
+    assert(f3.json ===
+      """{"sidecar":{"path":"/t1/p1","sizeInBytes":1,"modificationTime":3,""" +
+        """"tags":{"k1":"v1","schema":"{\"type\":\"struct\",\"fields\":[]}"}}}""".stripMargin)
+
+    Seq(f1, f2, f3).foreach { file =>
+      assert(file === JsonUtils.fromJson[SingleAction](file.json).unwrap)
+    }
+  }
 
   testActionSerDe(
     "AddCDCFile (without tags) - json serialization/deserialization",