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  1. Application Development
  2. Table API & SQL
  3. User-defined Sources & Sinks

User-defined Sources & Sinks

A TableSource provides access to data which is stored in external systems (database, key-value store, message queue) or files. After a TableSource is registered in a TableEnvironment it can be accessed by Table API or SQL queries.

A TableSink emits a Table to an external storage system, such as a database, key-value store, message queue, or file system (in different encodings, e.g., CSV, Parquet, or ORC).

A TableFactory allows for separating the declaration of a connection to an external system from the actual implementation. A table factory creates configured instances of table sources and sinks from normalized, string-based properties. The properties can be generated programmatically using a Descriptor or via YAML configuration files for the SQL Client.

Have a look at the common concepts and API page for details how to register a TableSource and how to emit a Table through a TableSink. See the built-in sources, sinks, and formats page for examples how to use factories.

Define a TableSource

A TableSource is a generic interface that gives Table API and SQL queries access to data stored in an external system. It provides the schema of the table and the records that are mapped to rows with the table’s schema. Depending on whether the TableSource is used in a streaming or batch query, the records are produced as a DataSet or DataStream.

If a TableSource is used in a streaming query it must implement the StreamTableSource interface, if it is used in a batch query it must implement the BatchTableSource interface. A TableSource can also implement both interfaces and be used in streaming and batch queries.

StreamTableSource and BatchTableSource extend the base interface TableSource that defines the following methods:

TableSource<T> {

  public TableSchema getTableSchema();

  public TypeInformation<T> getReturnType();

  public String explainSource();
}
TableSource[T] {

  def getTableSchema: TableSchema

  def getReturnType: TypeInformation[T]

  def explainSource: String

}

  • getTableSchema(): Returns the schema of the table, i.e., the names and types of the fields of the table. The field types are defined using Flink’s TypeInformation (see Table API types and SQL types).

  • getReturnType(): Returns the physical type of the DataStream (StreamTableSource) or DataSet (BatchTableSource) and the records that are produced by the TableSource.

  • explainSource(): Returns a String that describes the TableSource. This method is optional and used for display purposes only.

The TableSource interface separates the logical table schema from the physical type of the returned DataStream or DataSet. As a consequence, all fields of the table schema (getTableSchema()) must be mapped to a field with corresponding type of the physical return type (getReturnType()). By default, this mapping is done based on field names. For example, a TableSource that defines a table schema with two fields [name: String, size: Integer] requires a TypeInformation with at least two fields called name and size of type String and Integer, respectively. This could be a PojoTypeInfo or a RowTypeInfo that have two fields named name and size with matching types.

However, some types, such as Tuple or CaseClass types, do support custom field names. If a TableSource returns a DataStream or DataSet of a type with fixed field names, it can implement the DefinedFieldMapping interface to map field names from the table schema to field names of the physical return type.

Defining a BatchTableSource

The BatchTableSource interface extends the TableSource interface and defines one additional method:

BatchTableSource<T> implements TableSource<T> {

  public DataSet<T> getDataSet(ExecutionEnvironment execEnv);
}
BatchTableSource[T] extends TableSource[T] {

  def getDataSet(execEnv: ExecutionEnvironment): DataSet[T]
}
  • getDataSet(execEnv): Returns a DataSet with the data of the table. The type of the DataSet must be identical to the return type defined by the TableSource.getReturnType() method. The DataSet can by created using a regular data source of the DataSet API. Commonly, a BatchTableSource is implemented by wrapping a InputFormat or batch connector.

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Defining a StreamTableSource

The StreamTableSource interface extends the TableSource interface and defines one additional method:

StreamTableSource<T> implements TableSource<T> {

  public DataStream<T> getDataStream(StreamExecutionEnvironment execEnv);
}
StreamTableSource[T] extends TableSource[T] {

  def getDataStream(execEnv: StreamExecutionEnvironment): DataStream[T]
}
  • getDataStream(execEnv): Returns a DataStream with the data of the table. The type of the DataStream must be identical to the return type defined by the TableSource.getReturnType() method. The DataStream can by created using a regular data source of the DataStream API. Commonly, a StreamTableSource is implemented by wrapping a SourceFunction or a stream connector.

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Defining a TableSource with Time Attributes

Time-based operations of streaming Table API and SQL queries, such as windowed aggregations or joins, require explicitly specified time attributes.

A TableSource defines a time attribute as a field of type Types.SQL_TIMESTAMP in its table schema. In contrast to all regular fields in the schema, a time attribute must not be matched to a physical field in the return type of the table source. Instead, a TableSource defines a time attribute by implementing a certain interface.

Defining a Processing Time Attribute

Processing time attributes are commonly used in streaming queries. A processing time attribute returns the current wall-clock time of the operator that accesses it. A TableSource defines a processing time attribute by implementing the DefinedProctimeAttribute interface. The interface looks as follows:

DefinedProctimeAttribute {

  public String getProctimeAttribute();
}
DefinedProctimeAttribute {

  def getProctimeAttribute: String
}
  • getProctimeAttribute(): Returns the name of the processing time attribute. The specified attribute must be defined of type Types.SQL_TIMESTAMP in the table schema and can be used in time-based operations. A DefinedProctimeAttribute table source can define no processing time attribute by returning null.

Attention Both StreamTableSource and BatchTableSource can implement DefinedProctimeAttribute and define a processing time attribute. In case of a BatchTableSource the processing time field is initialized with the current timestamp during the table scan.

Defining a Rowtime Attribute

Rowtime attributes are attributes of type TIMESTAMP and handled in a unified way in stream and batch queries.

A table schema field of type SQL_TIMESTAMP can be declared as rowtime attribute by specifying

  • the name of the field,
  • a TimestampExtractor that computes the actual value for the attribute (usually from one or more other fields), and
  • a WatermarkStrategy that specifies how watermarks are generated for the the rowtime attribute.

A TableSource defines a rowtime attribute by implementing the DefinedRowtimeAttributes interface. The interface looks as follows:

DefinedRowtimeAttribute {

  public List<RowtimeAttributeDescriptor> getRowtimeAttributeDescriptors();
}
DefinedRowtimeAttributes {

  def getRowtimeAttributeDescriptors: util.List[RowtimeAttributeDescriptor]
}
  • getRowtimeAttributeDescriptors(): Returns a list of RowtimeAttributeDescriptor. A RowtimeAttributeDescriptor describes a rowtime attribute with the following properties:
    • attributeName: The name of the rowtime attribute in the table schema. The field must be defined with type Types.SQL_TIMESTAMP.
    • timestampExtractor: The timestamp extractor extracts the timestamp from a record with the return type. For example, it can convert a Long field into a timestamp or parse a String-encoded timestamp. Flink comes with a set of built-in TimestampExtractor implementation for common use cases. It is also possible to provide a custom implementation.
    • watermarkStrategy: The watermark strategy defines how watermarks are generated for the rowtime attribute. Flink comes with a set of built-in WatermarkStrategy implementations for common use cases. It is also possible to provide a custom implementation.

Attention Although the getRowtimeAttributeDescriptors() method returns a list of descriptors, only a single rowtime attribute is support at the moment. We plan to remove this restriction in the future and support tables with more than one rowtime attribute.

Attention Both, StreamTableSource and BatchTableSource, can implement DefinedRowtimeAttributes and define a rowtime attribute. In either case, the rowtime field is extracted using the TimestampExtractor. Hence, a TableSource that implements StreamTableSource and BatchTableSource and defines a rowtime attribute provides exactly the same data to streaming and batch queries.

Provided Timestamp Extractors

Flink provides TimestampExtractor implementations for common use cases.

The following TimestampExtractor implementations are currently available:

  • ExistingField(fieldName): Extracts the value of a rowtime attribute from an existing LONG, SQL_TIMESTAMP, or timestamp formatted STRING field. One example of such a string would be ‘2018-05-28 12:34:56.000’.
  • StreamRecordTimestamp(): Extracts the value of a rowtime attribute from the timestamp of the DataStream StreamRecord. Note, this TimestampExtractor is not available for batch table sources.

A custom TimestampExtractor can be defined by implementing the corresponding interface.

Provided Watermark Strategies

Flink provides WatermarkStrategy implementations for common use cases.

The following WatermarkStrategy implementations are currently available:

  • AscendingTimestamps: A watermark strategy for ascending timestamps. Records with timestamps that are out-of-order will be considered late.
  • BoundedOutOfOrderTimestamps(delay): A watermark strategy for timestamps that are at most out-of-order by the specified delay.
  • PreserveWatermarks(): A strategy which indicates the watermarks should be preserved from the underlying DataStream.

A custom WatermarkStrategy can be defined by implementing the corresponding interface.

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Defining a TableSource with Projection Push-Down

A TableSource supports projection push-down by implementing the ProjectableTableSource interface. The interface defines a single method:

ProjectableTableSource<T> {

  public TableSource<T> projectFields(int[] fields);
}
ProjectableTableSource[T] {

  def projectFields(fields: Array[Int]): TableSource[T]
}
  • projectFields(fields): Returns a copy of the TableSource with adjusted physical return type. The fields parameter provides the indexes of the fields that must be provided by the TableSource. The indexes relate to the TypeInformation of the physical return type, not to the logical table schema. The copied TableSource must adjust its return type and the returned DataStream or DataSet. The TableSchema of the copied TableSource must not be changed, i.e, it must be the same as the original TableSource. If the TableSource implements the DefinedFieldMapping interface, the field mapping must be adjusted to the new return type.

Attention In order for Flink to distinguish a projection push-down table source from its original form, explainSource method must be override to include information regarding the projected fields.

The ProjectableTableSource adds support to project flat fields. If the TableSource defines a table with nested schema, it can implement the NestedFieldsProjectableTableSource to extend the projection to nested fields. The NestedFieldsProjectableTableSource is defined as follows:

NestedFieldsProjectableTableSource<T> {

  public TableSource<T> projectNestedFields(int[] fields, String[][] nestedFields);
}
NestedFieldsProjectableTableSource[T] {

  def projectNestedFields(fields: Array[Int], nestedFields: Array[Array[String]]): TableSource[T]
}
  • projectNestedField(fields, nestedFields): Returns a copy of the TableSource with adjusted physical return type. Fields of the physical return type may be removed or reordered but their type must not be changed. The contract of this method is essentially the same as for the ProjectableTableSource.projectFields() method. In addition, the nestedFields parameter contains for each field index in the fields list, a list of paths to all nested fields that are accessed by the query. All other nested fields do not need to be read, parsed, and set in the records that are produced by the TableSource.

Attention the types of the projected fields must not be changed but unused fields may be set to null or to a default value.

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Defining a TableSource with Filter Push-Down

The FilterableTableSource interface adds support for filter push-down to a TableSource. A TableSource extending this interface is able to filter records such that the returned DataStream or DataSet returns fewer records.

The interface looks as follows:

FilterableTableSource<T> {

  public TableSource<T> applyPredicate(List<Expression> predicates);

  public boolean isFilterPushedDown();
}
FilterableTableSource[T] {

  def applyPredicate(predicates: java.util.List[Expression]): TableSource[T]

  def isFilterPushedDown: Boolean
}
  • applyPredicate(predicates): Returns a copy of the TableSource with added predicates. The predicates parameter is a mutable list of conjunctive predicates that are “offered” to the TableSource. The TableSource accepts to evaluate a predicate by removing it from the list. Predicates that are left in the list will be evaluated by a subsequent filter operator.
  • isFilterPushedDown(): Returns true if the applyPredicate() method was called before. Hence, isFilterPushedDown() must return true for all TableSource instances returned from a applyPredicate() call.

Attention In order for Flink to distinguish a filter push-down table source from its original form, explainSource method must be override to include information regarding the push-down filters.

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Defining a TableSource for Lookups

Attention This is an experimental feature. The interface may be changed in future versions. It’s only supported in Blink planner.

The LookupableTableSource interface adds support for the table to be accessed via key column(s) in a lookup fashion. This is very useful when used to join with a dimension table to enrich some information. If you want to use the TableSource in lookup mode, you should use the source in temporal table join syntax.

The interface looks as follows:

LookupableTableSource<T> implements TableSource<T> {

  public TableFunction<T> getLookupFunction(String[] lookupkeys);

  public AsyncTableFunction<T> getAsyncLookupFunction(String[] lookupkeys);

  public boolean isAsyncEnabled();
}
LookupableTableSource[T] extends TableSource[T] {

  def getLookupFunction(lookupKeys: Array[String]): TableFunction[T]

  def getAsyncLookupFunction(lookupKeys: Array[String]): AsyncTableFunction[T]

  def isAsyncEnabled: Boolean
}
  • getLookupFunction(lookupkeys): Returns a TableFunction which used to lookup the matched row(s) via lookup keys. The lookupkeys are the field names of LookupableTableSource in the join equal conditions. The eval method parameters of the returned TableFunction’s should be in the order which lookupkeys defined. It is recommended to define the parameters in varargs (e.g. eval(Object... lookupkeys) to match all the cases). The return type of the TableFunction must be identical to the return type defined by the TableSource.getReturnType() method.
  • getAsyncLookupFunction(lookupkeys): Optional. Similar to getLookupFunction, but the AsyncLookupFunction lookups the matched row(s) asynchronously. The underlying of AsyncLookupFunction will be called via Async I/O. The first argument of the eval method of the returned AsyncTableFunction should be defined as java.util.concurrent.CompletableFuture to collect results asynchronously (e.g. eval(CompletableFuture<Collection<String>> result, Object... lookupkeys)). The implementation of this method can throw an exception if the TableSource doesn’t support asynchronously lookup.
  • isAsyncEnabled(): Returns true if async lookup is enabled. It requires getAsyncLookupFunction(lookupkeys) is implemented if isAsyncEnabled returns true.

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Define a TableSink

A TableSink specifies how to emit a Table to an external system or location. The interface is generic such that it can support different storage locations and formats. There are different table sinks for batch tables and streaming tables.

The general interface looks as follows:

TableSink<T> {

  public TypeInformation<T> getOutputType();

  public String[] getFieldNames();

  public TypeInformation[] getFieldTypes();

  public TableSink<T> configure(String[] fieldNames, TypeInformation[] fieldTypes);
}
TableSink[T] {

  def getOutputType: TypeInformation<T>

  def getFieldNames: Array[String]

  def getFieldTypes: Array[TypeInformation]

  def configure(fieldNames: Array[String], fieldTypes: Array[TypeInformation]): TableSink[T]
}

The TableSink#configure method is called to pass the schema of the Table (field names and types) to emit to the TableSink. The method must return a new instance of the TableSink which is configured to emit the provided Table schema.

BatchTableSink

Defines an external TableSink to emit a batch table.

The interface looks as follows:

BatchTableSink<T> implements TableSink<T> {

  public void emitDataSet(DataSet<T> dataSet);
}
BatchTableSink[T] extends TableSink[T] {

  def emitDataSet(dataSet: DataSet[T]): Unit
}

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AppendStreamTableSink

Defines an external TableSink to emit a streaming table with only insert changes.

The interface looks as follows:

AppendStreamTableSink<T> implements TableSink<T> {

  public void emitDataStream(DataStream<T> dataStream);
}
AppendStreamTableSink[T] extends TableSink[T] {

  def emitDataStream(dataStream: DataStream[T]): Unit
}

If the table is also modified by update or delete changes, a TableException will be thrown.

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RetractStreamTableSink

Defines an external TableSink to emit a streaming table with insert, update, and delete changes.

The interface looks as follows:

RetractStreamTableSink<T> implements TableSink<Tuple2<Boolean, T>> {

  public TypeInformation<T> getRecordType();

  public void emitDataStream(DataStream<Tuple2<Boolean, T>> dataStream);
}
RetractStreamTableSink[T] extends TableSink[Tuple2[Boolean, T]] {

  def getRecordType: TypeInformation[T]

  def emitDataStream(dataStream: DataStream[Tuple2[Boolean, T]]): Unit
}

The table will be converted into a stream of accumulate and retraction messages which are encoded as Java Tuple2. The first field is a boolean flag to indicate the message type (true indicates insert, false indicates delete). The second field holds the record of the requested type T.

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UpsertStreamTableSink

Defines an external TableSink to emit a streaming table with insert, update, and delete changes.

The interface looks as follows:

UpsertStreamTableSink<T> implements TableSink<Tuple2<Boolean, T>> {

  public void setKeyFields(String[] keys);

  public void setIsAppendOnly(boolean isAppendOnly);

  public TypeInformation<T> getRecordType();

  public void emitDataStream(DataStream<Tuple2<Boolean, T>> dataStream);
}
UpsertStreamTableSink[T] extends TableSink[Tuple2[Boolean, T]] {

  def setKeyFields(keys: Array[String]): Unit

  def setIsAppendOnly(isAppendOnly: Boolean): Unit

  def getRecordType: TypeInformation[T]

  def emitDataStream(dataStream: DataStream[Tuple2[Boolean, T]]): Unit
}

The table must be have unique key fields (atomic or composite) or be append-only. If the table does not have a unique key and is not append-only, a TableException will be thrown. The unique key of the table is configured by the UpsertStreamTableSink#setKeyFields() method.

The table will be converted into a stream of upsert and delete messages which are encoded as a Java Tuple2. The first field is a boolean flag to indicate the message type. The second field holds the record of the requested type T.

A message with true boolean field is an upsert message for the configured key. A message with false flag is a delete message for the configured key. If the table is append-only, all messages will have a true flag and must be interpreted as insertions.

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Define a TableFactory

A TableFactory allows to create different table-related instances from string-based properties. All available factories are called for matching to the given set of properties and a corresponding factory class.

Factories leverage Java’s Service Provider Interfaces (SPI) for discovering. This means that every dependency and JAR file should contain a file org.apache.flink.table.factories.TableFactory in the META_INF/services resource directory that lists all available table factories that it provides.

Every table factory needs to implement the following interface:

package org.apache.flink.table.factories;

interface TableFactory {

  Map<String, String> requiredContext();

  List<String> supportedProperties();
}
package org.apache.flink.table.factories

trait TableFactory {

  def requiredContext(): util.Map[String, String]

  def supportedProperties(): util.List[String]
}
  • requiredContext(): Specifies the context that this factory has been implemented for. The framework guarantees to only match for this factory if the specified set of properties and values are met. Typical properties might be connector.type, format.type, or update-mode. Property keys such as connector.property-version and format.property-version are reserved for future backwards compatibility cases.
  • supportedProperties: List of property keys that this factory can handle. This method will be used for validation. If a property is passed that this factory cannot handle, an exception will be thrown. The list must not contain the keys that are specified by the context.

In order to create a specific instance, a factory class can implement one or more interfaces provided in org.apache.flink.table.factories:

  • BatchTableSourceFactory: Creates a batch table source.
  • BatchTableSinkFactory: Creates a batch table sink.
  • StreamTableSourceFactory: Creates a stream table source.
  • StreamTableSinkFactory: Creates a stream table sink.
  • DeserializationSchemaFactory: Creates a deserialization schema format.
  • SerializationSchemaFactory: Creates a serialization schema format.

The discovery of a factory happens in multiple stages:

  • Discover all available factories.
  • Filter by factory class (e.g., StreamTableSourceFactory).
  • Filter by matching context.
  • Filter by supported properties.
  • Verify that exactly one factory matches, otherwise throw an AmbiguousTableFactoryException or NoMatchingTableFactoryException.

The following example shows how to provide a custom streaming source with an additional connector.debug property flag for parameterization.

import org.apache.flink.table.sources.StreamTableSource;
import org.apache.flink.types.Row;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

class MySystemTableSourceFactory implements StreamTableSourceFactory<Row> {

  @Override
  public Map<String, String> requiredContext() {
    Map<String, String> context = new HashMap<>();
    context.put("update-mode", "append");
    context.put("connector.type", "my-system");
    return context;
  }

  @Override
  public List<String> supportedProperties() {
    List<String> list = new ArrayList<>();
    list.add("connector.debug");
    return list;
  }

  @Override
  public StreamTableSource<Row> createStreamTableSource(Map<String, String> properties) {
    boolean isDebug = Boolean.valueOf(properties.get("connector.debug"));

    # additional validation of the passed properties can also happen here

    return new MySystemAppendTableSource(isDebug);
  }
}
import java.util
import org.apache.flink.table.sources.StreamTableSource
import org.apache.flink.types.Row

class MySystemTableSourceFactory extends StreamTableSourceFactory[Row] {

  override def requiredContext(): util.Map[String, String] = {
    val context = new util.HashMap[String, String]()
    context.put("update-mode", "append")
    context.put("connector.type", "my-system")
    context
  }

  override def supportedProperties(): util.List[String] = {
    val properties = new util.ArrayList[String]()
    properties.add("connector.debug")
    properties
  }

  override def createStreamTableSource(properties: util.Map[String, String]): StreamTableSource[Row] = {
    val isDebug = java.lang.Boolean.valueOf(properties.get("connector.debug"))

    # additional validation of the passed properties can also happen here

    new MySystemAppendTableSource(isDebug)
  }
}

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Use a TableFactory in the SQL Client

In a SQL Client environment file, the previously presented factory could be declared as:

tables:
 - name: MySystemTable
   type: source
   update-mode: append
   connector:
     type: my-system
     debug: true

The YAML file is translated into flattened string properties and a table factory is called with those properties that describe the connection to the external system:

update-mode=append
connector.type=my-system
connector.debug=true

Attention Properties such as tables.#.name or tables.#.type are SQL Client specifics and are not passed to any factory. The type property decides, depending on the execution environment, whether a BatchTableSourceFactory/StreamTableSourceFactory (for source), a BatchTableSinkFactory/StreamTableSinkFactory (for sink), or both (for both) need to discovered.

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Use a TableFactory in the Table & SQL API

For a type-safe, programmatic approach with explanatory Scaladoc/Javadoc, the Table & SQL API offers descriptors in org.apache.flink.table.descriptors that translate into string-based properties. See the built-in descriptors for sources, sinks, and formats as a reference.

A connector for MySystem in our example can extend ConnectorDescriptor as shown below:

import org.apache.flink.table.descriptors.ConnectorDescriptor;
import java.util.HashMap;
import java.util.Map;

/**
  * Connector to MySystem with debug mode.
  */
public class MySystemConnector extends ConnectorDescriptor {

  public final boolean isDebug;

  public MySystemConnector(boolean isDebug) {
    super("my-system", 1, false);
    this.isDebug = isDebug;
  }

  @Override
  protected Map<String, String> toConnectorProperties() {
    Map<String, String> properties = new HashMap<>();
    properties.put("connector.debug", Boolean.toString(isDebug));
    return properties;
  }
}
import org.apache.flink.table.descriptors.ConnectorDescriptor
import java.util.HashMap
import java.util.Map

/**
  * Connector to MySystem with debug mode.
  */
class MySystemConnector(isDebug: Boolean) extends ConnectorDescriptor("my-system", 1, false) {
  
  override protected def toConnectorProperties(): Map[String, String] = {
    val properties = new HashMap[String, String]
    properties.put("connector.debug", isDebug.toString)
    properties
  }
}

The descriptor can then be used in the API as follows:

StreamTableEnvironment tableEnv = // ...

tableEnv
  .connect(new MySystemConnector(true))
  .inAppendMode()
  .registerTableSource("MySystemTable");
val tableEnv: StreamTableEnvironment = // ...

tableEnv
  .connect(new MySystemConnector(isDebug = true))
  .inAppendMode()
  .registerTableSource("MySystemTable")

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