模式演化是数据管理的一个非常重要的方面。 Hudi支持常见的模式演变场景，比如添加一个空字段或提升一个字段的数据类型，开箱即用。 此外，该模式可以跨引擎查询，如Presto、Hive和Spark SQL。 下表总结了与不同Hudi表类型兼容的模式更改类型。
Schema Change | COW | MOR | Remarks |
|---|---|---|---|
Add a new nullable column at root level at the end | Yes | Yes | Yes means that a write with evolved schema succeeds and a read following the write succeeds to read entire dataset. |
Add a new nullable column to inner struct (at the end) | Yes | Yes | |
Add a new complex type field with default (map and array) | Yes | Yes | |
Add a new nullable column and change the ordering of fields | No | No | Write succeeds but read fails if the write with evolved schema updated only some of the base files but not all. Currently, Hudi does not maintain a schema registry with history of changes across base files. Nevertheless, if the upsert touched all base files then the read will succeed. |
Add a custom nullable Hudi meta column, e.g. _hoodie_meta_col | Yes | Yes | |
Promote datatype from int to long for a field at root level | Yes | Yes | For other types, Hudi supports promotion as specified in Avro schema resolution. |
Promote datatype from int to long for a nested field | Yes | Yes | |
Promote datatype from int to long for a complex type (value of map or array) | Yes | Yes | |
Add a new non-nullable column at root level at the end | No | No | In case of MOR table with Spark data source, write succeeds but read fails. As a workaround, you can make the field nullable. |
Add a new non-nullable column to inner struct (at the end) | No | No | |
Change datatype from long to int for a nested field | No | No | |
Change datatype from long to int for a complex type (value of map or array) | No | No |
让我们通过一个示例来演示Hudi中的模式演化支持。 在下面的示例中，我们将添加一个新的字符串字段，并将字段的数据类型从int改为long。
代码语言：javascript复制Welcome to
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Using Scala version 2.12.10 (OpenJDK 64-Bit Server VM, Java 1.8.0_292)
Type in expressions to have them evaluated.
Type :help for more information.
scala> import org.apache.hudi.QuickstartUtils._
import org.apache.hudi.QuickstartUtils._
scala> import scala.collection.JavaConversions._
import scala.collection.JavaConversions._
scala> import org.apache.spark.sql.SaveMode._
import org.apache.spark.sql.SaveMode._
scala> import org.apache.hudi.DataSourceReadOptions._
import org.apache.hudi.DataSourceReadOptions._
scala> import org.apache.hudi.DataSourceWriteOptions._
import org.apache.hudi.DataSourceWriteOptions._
scala> import org.apache.hudi.config.HoodieWriteConfig._
import org.apache.hudi.config.HoodieWriteConfig._
scala> import org.apache.spark.sql.types._
import org.apache.spark.sql.types._
scala> import org.apache.spark.sql.Row
import org.apache.spark.sql.Row
scala> val tableName = "hudi_trips_cow"
tableName: String = hudi_trips_cow
scala> val basePath = "file:///tmp/hudi_trips_cow"
basePath: String = file:///tmp/hudi_trips_cow
scala> val schema = StructType( Array(
| StructField("rowId", StringType,true),
| StructField("partitionId", StringType,true),
| StructField("preComb", LongType,true),
| StructField("name", StringType,true),
| StructField("versionId", StringType,true),
| StructField("intToLong", IntegerType,true)
| ))
schema: org.apache.spark.sql.types.StructType = StructType(StructField(rowId,StringType,true), StructField(partitionId,StringType,true), StructField(preComb,LongType,true), StructField(name,StringType,true), StructField(versionId,StringType,true), StructField(intToLong,IntegerType,true))
scala> val data1 = Seq(Row("row_1", "part_0", 0L, "bob", "v_0", 0),
| Row("row_2", "part_0", 0L, "john", "v_0", 0),
| Row("row_3", "part_0", 0L, "tom", "v_0", 0))
data1: Seq[org.apache.spark.sql.Row] = List([row_1,part_0,0,bob,v_0,0], [row_2,part_0,0,john,v_0,0], [row_3,part_0,0,tom,v_0,0])
scala> var dfFromData1 = spark.createDataFrame(data1, schema)
scala> dfFromData1.write.format("hudi").
| options(getQuickstartWriteConfigs).
| option(PRECOMBINE_FIELD_OPT_KEY.key, "preComb").
| option(RECORDKEY_FIELD_OPT_KEY.key, "rowId").
| option(PARTITIONPATH_FIELD_OPT_KEY.key, "partitionId").
| option("hoodie.index.type","SIMPLE").
| option(TABLE_NAME.key, tableName).
| mode(Overwrite).
| save(basePath)
scala> var tripsSnapshotDF1 = spark.read.format("hudi").load(basePath "/*/*")
tripsSnapshotDF1: org.apache.spark.sql.DataFrame = [_hoodie_commit_time: string, _hoodie_commit_seqno: string ... 9 more fields]
scala> tripsSnapshotDF1.createOrReplaceTempView("hudi_trips_snapshot")
scala> spark.sql("desc hudi_trips_snapshot").show()
-------------------- --------- -------
| col_name|data_type|comment|
-------------------- --------- -------
| _hoodie_commit_time| string| null|
|_hoodie_commit_seqno| string| null|
| _hoodie_record_key| string| null|
|_hoodie_partition...| string| null|
| _hoodie_file_name| string| null|
| rowId| string| null|
| partitionId| string| null|
| preComb| bigint| null|
| name| string| null|
| versionId| string| null|
| intToLong| int| null|
-------------------- --------- -------
scala> spark.sql("select rowId, partitionId, preComb, name, versionId, intToLong from hudi_trips_snapshot").show()
----- ----------- ------- ---- --------- ---------
|rowId|partitionId|preComb|name|versionId|intToLong|
----- ----------- ------- ---- --------- ---------
|row_3| part_0| 0| tom| v_0| 0|
|row_2| part_0| 0|john| v_0| 0|
|row_1| part_0| 0| bob| v_0| 0|
----- ----------- ------- ---- --------- ---------
// In the new schema, we are going to add a String field and
// change the datatype `intToLong` field from int to long.
scala> val newSchema = StructType( Array(
| StructField("rowId", StringType,true),
| StructField("partitionId", StringType,true),
| StructField("preComb", LongType,true),
| StructField("name", StringType,true),
| StructField("versionId", StringType,true),
| StructField("intToLong", LongType,true),
| StructField("newField", StringType,true)
| ))
newSchema: org.apache.spark.sql.types.StructType = StructType(StructField(rowId,StringType,true), StructField(partitionId,StringType,true), StructField(preComb,LongType,true), StructField(name,StringType,true), StructField(versionId,StringType,true), StructField(intToLong,LongType,true), StructField(newField,StringType,true))
scala> val data2 = Seq(Row("row_2", "part_0", 5L, "john", "v_3", 3L, "newField_1"),
| Row("row_5", "part_0", 5L, "maroon", "v_2", 2L, "newField_1"),
| Row("row_9", "part_0", 5L, "michael", "v_2", 2L, "newField_1"))
data2: Seq[org.apache.spark.sql.Row] = List([row_2,part_0,5,john,v_3,3,newField_1], [row_5,part_0,5,maroon,v_2,2,newField_1], [row_9,part_0,5,michael,v_2,2,newField_1])
scala> var dfFromData2 = spark.createDataFrame(data2, newSchema)
scala> dfFromData2.write.format("hudi").
| options(getQuickstartWriteConfigs).
| option(PRECOMBINE_FIELD_OPT_KEY.key, "preComb").
| option(RECORDKEY_FIELD_OPT_KEY.key, "rowId").
| option(PARTITIONPATH_FIELD_OPT_KEY.key, "partitionId").
| option("hoodie.index.type","SIMPLE").
| option(TABLE_NAME.key, tableName).
| mode(Append).
| save(basePath)
scala> var tripsSnapshotDF2 = spark.read.format("hudi").load(basePath "/*/*")
tripsSnapshotDF2: org.apache.spark.sql.DataFrame = [_hoodie_commit_time: string, _hoodie_commit_seqno: string ... 10 more fields]
scala> tripsSnapshotDF2.createOrReplaceTempView("hudi_trips_snapshot")
scala> spark.sql("desc hudi_trips_snapshot").show()
-------------------- --------- -------
| col_name|data_type|comment|
-------------------- --------- -------
| _hoodie_commit_time| string| null|
|_hoodie_commit_seqno| string| null|
| _hoodie_record_key| string| null|
|_hoodie_partition...| string| null|
| _hoodie_file_name| string| null|
| rowId| string| null|
| partitionId| string| null|
| preComb| bigint| null|
| name| string| null|
| versionId| string| null|
| intToLong| bigint| null|
| newField| string| null|
-------------------- --------- -------
scala> spark.sql("select rowId, partitionId, preComb, name, versionId, intToLong, newField from hudi_trips_snapshot").show()
----- ----------- ------- ------- --------- --------- ----------
|rowId|partitionId|preComb| name|versionId|intToLong| newField|
----- ----------- ------- ------- --------- --------- ----------
|row_3| part_0| 0| tom| v_0| 0| null|
|row_2| part_0| 5| john| v_3| 3|newField_1|
|row_1| part_0| 0| bob| v_0| 0| null|
|row_5| part_0| 5| maroon| v_2| 2|newField_1|
|row_9| part_0| 5|michael| v_2| 2|newField_1|
----- ----------- ------- ------- --------- --------- ----------
Copy本文为从大数据到人工智能博主「xiaozhch5」的原创文章，遵循CC 4.0 BY-SA版权协议，转载请附上原文出处链接及本声明。
原文链接：https://cloud.tencent.com/developer/article/1936496
