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Status

Current state:  ["Under Discussion"] Accepted

Discussion thread:  here (<- link to https://mail-archiveslists.apache.org/mod_mbox/flink-dev/)
thread/n6nsvbwhs5kwlj5kjgv24by2tk5mh9xd

VOTE thread: JIRA: here (<- link to https://issueslists.apache.org/jira/browse/FLINK-XXXX)

Released: 1.18

Please keep the discussion on the mailing list rather than commenting on the wiki (wiki discussions get unwieldy fast).

Motivation

CREATE TABLE AS SELECT(CTAS) statement has been support by FLIP-218, but it's not atomic. It will create the table first before job running. If the job execution fails, or is cancelled, the table will not be dropped.

We want Flink to support atomic CTAS, where only the table is created when the Job succeeds. 

we refer to FLIP-218: Support SELECT clause in CREATE TABLE(CTAS) , use the existing JobStatusHook mechanism and extend Catalog's new API to implement atomic CTAS capabilities.

Public Interfaces

thread/8c0wlp72kq0dhcbpy08nl1kb28q17kv3

JIRA:

Released: 1.18

Please keep the discussion on the mailing list rather than commenting on the wiki (wiki discussions get unwieldy fast).

Motivation

CREATE TABLE AS SELECT(CTAS) statement has been support by FLIP-218, but it's not atomic. It will create the table first before job running. If the job execution fails, or is cancelled, the table will not be dropped.

We want Flink to support atomic CTAS, where only the table is created when the Job succeeds. 

we refer to FLIP-218: Support SELECT clause in CREATE TABLE(CTAS) , use the existing JobStatusHook mechanism and extend Catalog's new API to implement atomic CTAS capabilities.

Public Interfaces

Introduce interface SupportsStaging , which provided applyStaging API. If DynamicTableSink implements the interface SupportsStaging, it indicates that it supports atomic operations.

/**
 * Enables different staged operations to ensure atomicity in a {@link DynamicTableSink}.
 *
 * <p>By default, if this interface is not implemented, indicating that atomic operations are not
 * supported, then a non-atomic implementation is used.
 */
@PublicEvolving
public interface SupportsStaging {

    /**
     * Provides a {@link StagedTable} that provided transaction abstraction. StagedTable will be
     * combined with {@link JobStatusHook} to achieve atomicity support in the Flink framework. Call
     * the relevant API of StagedTable when the Job state is switched.
     *
     * <p>This method will be called at the compile stage.
     *
     * @param StagingContext Tell DynamicTableSink, the operation type of this StagedTable,
     *     expandable
     * @return {@link StagedTable} that can be serialized and provides atomic operations
     */
    StagedTable applyStaging(StagingContext context);

    /**
     * The context is intended to tell DynamicTableSink the type of this operation. In this way,
     * DynamicTableSink can return the corresponding implementation of StagedTable according to the
     * specific operation. More types of operations can be extended in the future.
     */
    interface StagingContext {
        StagingPurpose getStagingPurpose();
    }

    enum StagingPurpose {
        CREATE_TABLE_AS,
        CREATE_TABLE_AS_IF_NOT_EXISTS
    }
}

Introduce StagedTable interface that support Introduce interface SupportsStaging , which provided applyStaging API. If DynamicTableSink implements the interface SupportsStaging, it indicates that it supports atomic operations.

/**
 * Enables different staged operations to ensure atomicity in a {@link DynamicTableSink}.
 *A {@link StagedTable} for atomic semantics using a two-phase commit protocol, combined with
 * <p>By default, if this interface is not implemented, indicating that atomic operations are not{@link JobStatusHook} for atomic CTAS. {@link StagedTable} will be a member variable of
 * supported,CtasJobStatusHook thenand acan non-atomic implementation is used.be serialized;
 */
@PublicEvolving
public interface* SupportsStaging<p>CtasJobStatusHook#onCreated {

will call the begin /**
method of StagedTable;
   * ProvidesCtasJobStatusHook#onFinished awill {@linkcall StagedTable}the thatcommit providedmethod transaction abstraction. StagedTable will be
     * combined with {@link JobStatusHook} to achieve atomicity support in the Flink framework. Callof StagedTable;
 * CtasJobStatusHook#onFailed and CtasJobStatusHook#onCanceled will call the abort method of StagedTable;
 */
@PublicEvolving
public interface StagedTable extends Serializable {

    /**
     * theThis relevantmethod APIwill ofbe StagedTablecalled when the Jobjob state is switchedstarted.
 Similar to what  *it means to open a
     * transaction in <p>Thisa methodrelational willdatabase; beIn calledFlink's atatomic theCTAS compile stage.
     *scenario, it is used to do some
     * @paraminitialization StagingContextwork; TellFor DynamicTableSinkexample, initializing the operationclient typeof ofthe thisunderlying StagedTableservice, the tmp
     * path of the underlying storage, expandable
or even call the start *transaction @returnAPI {@linkof StagedTable}the thatunderlying
 can be serialized and provides* atomicservice, operationsetc.
     */
    StagedTablevoid applyStagingbegin(StagingContext context);

    /**
     * The context This method will be called when the job is intendedsucceeds. Similar to tell DynamicTableSink what it means to commit the type of this operation. In this way,
     * transaction in a relational database; In Flink's atomic CTAS scenario, it is used to do some
     * DynamicTableSinkdata visibility canrelated returnwork; theFor correspondingexample, implementationmoving ofthe StagedTableunderlying accordingdata to the target
     * directory, writing specificbuffer operation.data Moreto typesthe ofunderlying operationsstorage canservice, beor extendedeven incall the future.commit
     */
 transaction API of the interfaceunderlying StagingContextservice, {etc.
     */
    StagingPurposevoid getStagingPurposecommit();

    }/**

     enum* StagingPurposeThis {
method will be called when the job  CREATE_TABLE_AS,
        CREATE_TABLE_AS_IF_NOT_EXISTSis failed or canceled. Similar to what it means to
    }
}

Introduce StagedTable interface that support atomic operations.

/**
 * Arollback {@linkthe StagedTable} for atomic semantics using a two-phase commit protocol, combined with
 * {@link JobStatusHook} fortransaction in a relational database; In Flink's atomic CTAS. {@link StagedTable} will be a member variable of
 * CtasJobStatusHook and can be serialized;
 *
 * <p>CtasJobStatusHook#onCreated will call the begin method of StagedTable;
 * CtasJobStatusHook#onFinished will call the commit method of StagedTable;
 * CtasJobStatusHook#onFailed and CtasJobStatusHook#onCanceled will call the abort method of StagedTable;
 */
@PublicEvolving
public interface StagedTable extends Serializable {

    /**
     * This method will be called when the job is started. Similar to what it means to open a
     * transaction in a relational database; In Flink's atomic CTAS scenario, it is used to do some
     * initialization work; For example, initializing the client of the underlying service, the tmp
     * path of the underlying storage, or even call the start transaction API of the underlying
     * service, etc.
     */
    void begin();

    /**
     * This method will be called when the job is succeeds. Similar to what it means to commit the
     * transaction in a relational database; In Flink's atomic CTAS scenario, it is used to do some
     * data visibility related work; For example, moving the underlying data to the target
     * directory, writing buffer data to the underlying storage service, or even call the commit
     * transaction API of the underlying service, etc.
     */
    void commit();

    /**
     * This method will be called when the job is failed or canceled. Similar to what it means to
     * rollback the transaction in a relational database; In Flink's atomic CTAS scenario, it is
     * used to do some data cleaning; For example, delete the data in tmp directory, delete the
     * temporary data in the underlying storage service, or even call the rollback transaction API
     * of the underlying service, etc.
     */
    void abort(scenario, it is
     * used to do some data cleaning; For example, delete the data in tmp directory, delete the
     * temporary data in the underlying storage service, or even call the rollback transaction API
     * of the underlying service, etc.
     */
    void abort();
}

TableConfigOptions

Add table.ctas.atomicity-enabled option to allow users to enable atomicity when using create table as select syntax.

@PublicEvolving
public class TableConfigOptions {
    @Documentation.TableOption(execMode = Documentation.ExecMode.BATCH_STREAMING)
    public static final ConfigOption<Boolean> TABLE_CTAS_ATOMICITY_ENABLED =
            key("table.ctas.atomicity-enabled")
                    .booleanType()
                    .defaultValue(false)
                    .withDescription(
                            "Specifies if the create table as select operation is executed atomically. "
                                  + "By default, the operation is non-atomic. The target table is created in Client side, and it will not be dropped even though the job fails or is cancelled. "
                                  + "If set this option to true and DynamicTableSink implements the SupportsStaging interface, the create table as select operation is expected to be executed atomically, "
                                  + "the behavior of which depends on the actual DynamicTableSink.");
}

Proposed Changes

First we need to have a Table interface that can be combined with the abstract transaction capability, so we introduce StagedTable, which can perform start transaction, commit transaction, and abort transaction operations.

...

for example, HiveTableSink needs to access the Hive Metastore and write to HDFS(OSS etc); JDBCTableSink needs to access the back-end database;

Therefore, we introduce the interface SupportsStaging, which, if implemented by DynamicTableSink, indicates that it supports atomic operations, otherwise it does not support atomic operations.

Flink framework can determine whether DynamicTableSink supports atomicity CTAS by whether it implements the interface SupportsStaging, and if it does, get the StagedTable object through the applyStaging API, otherwise use the non-atomic CTAS implementation.

Identification of atomic CTAS

OSS etc); JDBCTableSink needs to access the back-end database;

Therefore, we introduce the interface SupportsStaging, which, if implemented by DynamicTableSink, indicates that it supports atomic operations, otherwise it does not support atomic operations.

Flink framework can determine whether DynamicTableSink supports atomicity CTAS by whether it implements the interface SupportsStaging, and if it does, get the StagedTable object through the applyStaging API, otherwise use the non-atomic CTAS implementation.

Identification of atomic CTAS

Normally, in stream mode, we consider the job to be LONG RUNNING, and even if it fails, it needs to resume afterwards, so atomic CTAS semantics are usually not needed.

In addition, there are probably many flink jobs that already use non-atomic CTAS functions, especially Stream jobs, in order to ensure the consistency of flink behavior, and to give the user maximum flexibility, in time DynamicTableSink implements the SupportsStaging interface, users can still choose non-atomic implementation according to business needs.

So, we can infer in the TableEnvironmentImpl whether atomic CTAS is used based on whether the user has enabled it and whether DynamicTableSink implements the SupportsStaging interfaceWe can identify whether a DynamicTableSink supports atomic Ctas by determining its type in TableEnvironmentImpl, like the following:

boolean isAtomicCtas = tableConfig.get(TableConfigOptions.TABLE_CTAS_ATOMICITY_ENABLED) && dynamicTableSink instanceof SupportsStaging;

...

Optional<DynamicTableSink> dynamicTableSinkOptional =
        getDynamicTableSink(
                catalogTable,
                tableIdentifier,
                createTableOperation.isTemporary(),
                catalogManager.getCatalog(catalogName)); 
if (tableConfig.get(TableConfigOptions.TABLE_CTAS_ATOMICITY_ENABLED)
        && dynamicTableSinkOptional.isPresent()
        && dynamicTableSinkOptional.get() instanceof SupportsStaging) {
     DynamicTableSink DynamicTableSink dynamicTableSink = dynamicTableSinkOptional.get();
    StagedTable stagedTable =
            ((SupportsStaging) dynamicTableSink)
                    .applyStaging(
                            new SupportsStaging.StagingContext() {
                                @Override
                                public SupportsStaging.StagingPurpose
                                        getStagingPurpose() {
                                    if (createTableOperation
                                            .isIgnoreIfExists()) {
                                        return SupportsStaging.StagingPurpose
                                                .CREATE_TABLE_AS_IF_NOT_EXISTS;
                                    }
                                    return SupportsStaging.StagingPurpose
                                            .CREATE_TABLE_AS;
                                }
                            });
    CtasJobStatusHook ctasJobStatusHook = new CtasJobStatusHook(stagedTable);
    mapOperations.add(
            ctasOperation.toStagedSinkModifyOperation(
                    createTableOperation.getTableIdentifier(),
                    catalogTable,
                    ctasCatalog,
                    dynamicTableSink));
    jobStatusHookList.add(ctasJobStatusHook);
} else {
    // execute CREATE TABLE first for non-atomic CTAS statements
    executeInternalexecuteInternal(ctasOperation.getCreateTableOperation());
    mapOperations.add(ctasOperation.getCreateTableOperationtoSinkModifyOperation());
    mapOperations.add(ctasOperation.toSinkModifyOperation(catalogManager));
}catalogManager));
}

To avoid secondary generation of DynamicTableSink, we need to construct a StagedSinkModifyOperation that inherits from SinkModifyOperation and then add the DynamicTableSink member variable.

Current non-atomic CTAS implementations

Current Flink supports non-atomic CTAS operations, when it is CreateTableASOperation, we will create the target table first, and then compile and execute the insert operation.

The current program has the following shortcomings:

First: If the insert operation fails, whether it is a compile failure or a job execution failure, flink will not drop the created target table;

Second: Before the job is executed, because the target table already exists, but no data can be readTo avoid secondary generation of DynamicTableSink, we need to construct a StagedSinkModifyOperation that inherits from SinkModifyOperation and then add the DynamicTableSink member variable.

Atomic CTAS demo

Then implementation of the atomic CTAS operation requires only two steps :

1: DynamicTableSink implements the interface SupportsStaging;

2: Introduce the implementation class of the StagedTable interface.

Hive demo

HiveTableSink implements the applyStaging API:

...