Motivation

Now many watermark-related features such as the watermark alignment have been implemented on the datastream API, and it is very convenient and flexible to configure and use these features through the datastream API. However, there is currently no way to use these features through SQL.

This FLIP proposes to enhance the availability of watermark options of SQL.

Proposed Change

Current capabilities of watermark in SQL layer

The event time attribute is defined using a WATERMARK statement in CREATE table DDL. A watermark statement defines a watermark generation expression on an existing event time field, which marks the event time field as the event time attribute. Some examples:

Monotonously Increasing Timestamps:

CREATE TABLE user_actions (
  user_name STRING,
  `data` STRING,
  current_time as proctime(),
  WATERMARK FOR current_time AS current_time
) WITH (
  ...
);

Fixed Amount of Lateness:

CREATE TABLE user_actions (
  user_name STRING,
  `data` STRING,
  user_action_time TIMESTAMP(3),
  -- declare user_action_time as event time attribute and use 5 seconds delayed watermark strategy
  WATERMARK FOR user_action_time AS user_action_time - INTERVAL '5' SECOND
) WITH (
  ...
);

These are all the capabilities of watermark in SQL layer. The following table shows a more detailed comparison:

Note that although the syntax to use watermark in SQL is the same, the location of generating watermark may be different. The watermark of the source that implements the `SupportsWatermarkPushDown` interface is generated in the source operator, while the watermark of the source that does not implement the `SupportsWatermarkPushDown` interface is generated in a downstream operator named 'WatermarkAssigner'. If the watermark is generated in the downstream 'WatermarkAssigner' operator, many watermark-related features, such as watermark alignment, will can not be implemented. So the features that this flip intends to support are only for those sources that implement the `SupportsWatermarkPushDown` interface.

Watermark-related features intends to support

In SQL layer, the watermark is closely related to each source table, so we plan to extend these features in the dynamic table options and 'OPTIONS' hint.  If the user has configured these options both in the dynamic table options and in the 'OPTIONS' hint, then the options in the 'OPTIONS' hint are preferred. If the user uses 'OPTIONS' hint for the same source table in multiple places, the first hint will be used.

We provide a poc, you can check the poc[1] to look up the code details.

1. Configurable watermark emit strategy

On datastream API, we can decide whether to emit a watermark periodically or emit a watermark for each event by code logic from the implementation of the WatermarkGenerator interface:

@Public
public interface WatermarkGenerator<T> {

    /**
    * Called for every event, allows the watermark generator to examine
    * and remember the event timestamps, or to emit a watermark based on
    * the event itself.
    */
    void onEvent(T event, long eventTimestamp, WatermarkOutput output);

    /**
    * Called periodically, and might emit a new watermark, or not.
    *
    * <p>The interval in which this method is called and Watermarks
    * are generated depends on {@link ExecutionConfig#getAutoWatermarkInterval()}.
    */
    void onPeriodicEmit(WatermarkOutput output);
}

But in SQL layer, for those sources that implement the SupportsWatermarkPushDown interface, the watermark can only be emitted periodically, the default period is 200ms, which can be changed by the pipeline parameter 'pipeline.auto-watermark-interval'. Currently, there is no way to configure the strategy to emit a watermark for each event. We would like to configure the watermark emit strategy for the source which implements the `SupportsWatermarkPushDown` interface. We intend to configure the watermark emit strategy in the following way：

For SQL, the default watermark emit-strategy is 'on-periodic', which can be set manually via table options and hint :

-- configure in table options
CREATE TABLE user_actions (
  ...
  user_action_time TIMESTAMP(3),
  WATERMARK FOR user_action_time AS user_action_time - INTERVAL '5' SECOND
) WITH (
  'scan.watermark.emit.strategy'='on-periodic',
  ...
);

-- use 'OPTIONS' hint
select ... from source_table /*+ OPTIONS('scan.watermark.emit.strategy'='on-periodic') */

If the user wants to configure the 'on-event' strategy, he/she can use table options or hint like this:

-- configure in table options
CREATE TABLE user_actions (
  ...
  user_action_time TIMESTAMP(3),
  WATERMARK FOR user_action_time AS user_action_time - INTERVAL '5' SECOND
) WITH (   
  'scan.watermark.emit.strategy'='on-event'
  ...
);

-- use 'OPTIONS' hint
select ... from source_table /*+ OPTIONS('scan.watermark.emit.strategy'='on-event') */

2.Dealing with idle sources

On datastream API, We can configure idle-timeout  to handle idle sources in the following way:

WatermarkStrategy
    .<Tuple2<Long, String>>forBoundedOutOfOrderness(Duration.ofSeconds(20))
    .withIdleness(Duration.ofMinutes(1));

However, in SQL layer, we can configure a global idle-timeout value through the parameter 'table.exec.source.idle-timeout', which means the multiple sources will share the value, we would like to configure 'idle-timeout' for each source separately, and it is expected to be configured in the following way:

-- configure in table options
CREATE TABLE user_actions (
  ...
  user_action_time TIMESTAMP(3),
  WATERMARK FOR user_action_time AS user_action_time - INTERVAL '5' SECOND
) WITH (
  'scan.watermark.idle-timeout'='1min',
  ...
);

-- use 'OPTIONS' hint
select ... from source_table /*+ OPTIONS('scan.watermark.idle-timeout'='1min') */

3.Watermark alignment

On datastream API，We can use watermark alignment feature in the following way :

WatermarkStrategy
    .<Tuple2<Long, String>>forBoundedOutOfOrderness(Duration.ofSeconds(20))
    .withWatermarkAlignment("align-group-1", Duration.ofSeconds(20), Duration.ofSeconds(1)); 

However，the watermark alignment function is not currently supported in SQL layer. We hope the flip can support it. It is expected to be configured in the following way:

-- configure in table options
CREATE TABLE user_actions (
...
user_action_time TIMESTAMP(3),
  WATERMARK FOR user_action_time AS user_action_time - INTERVAL '5' SECOND
) WITH (
'scan.watermark.alignment.group'='alignment-group-1',
'scan.watermark.alignment.max-drift'='1min',
'scan.watermark.alignment.update-interval'='1s',
...
);

-- use 'OPTIONS' hint
select ... from source_table /*+ OPTIONS('scan.watermark.alignment.group'='alignment-group-1', 'scan.watermark.alignment.max-drift'='1min', 'scan.watermark.alignment.update-interval'='1s') */

the option 'watermark.alignment.update-interval' is not required. the default value is 1s.

Note source connectors have to implement watermark alignment of source split in order to use the watermark alignment feature since 1.17 according flip-217 [2]. If source connector does not implement flip-217, the task will run with an error, user could set 'pipeline.watermark-alignment.allow-unaligned-source-splits'= 'true' to disable watermark alignment of source split, and watermark alignment will be working properly only when your number of splits equals to the parallelism of the source operator.

All the options described above :

'scan.watermark.emit.strategy'='on-event',
'scan.watermark.idle-timeout'='1min',
'scan.watermark.alignment.group'='alignment-group-1',
'scan.watermark.alignment.max-drift'='1min',
'scan.watermark.alignment.update-interval'='1s'

Of course, users can choose some of the items to configure according to their needs.

Public Interface

In order to implement the above features, it is expected to add a wrapper class for the watermark parameters, like this:

public class WatermarkParams implements Serializable {
    private static final long serialVersionUID = 1L;

    private WatermarkEmitStrategy emitStrategy;
    private String alignGroupName;
    private Duration alignMaxDrift;
    private Duration alignUpdateInterval;
    private long sourceIdleTimeout;
}

To describe the watermark emit strategy, we need to add an enumeration class:

public enum WatermarkEmitStrategy {
    ON_EVENT("on-event"),
    ON_PERIODIC("on-periodic")
}

The biggest change probably is that we need to modify the constructor of WatermarkPushDownSpec to use the params wrapper class：

@JsonCreator
public WatermarkPushDownSpec(
    @JsonProperty(FIELD_NAME_WATERMARK_EXPR) RexNode watermarkExpr,
    @JsonProperty(FIELD_NAME_IDLE_TIMEOUT_MILLIS) long idleTimeoutMillis,
    @JsonProperty(FIELD_NAME_PRODUCED_TYPE) RowType producedType,
    @JsonProperty(FIELD_NAME_WATERMARK_PARAMS) WatermarkParams watermarkParams) {
        super(producedType);
        this.watermarkExpr = checkNotNull(watermarkExpr);
        this.idleTimeoutMillis = idleTimeoutMillis;
        this.watermarkParams = watermarkParams;
}

Same as GeneratedWatermarkGeneratorSupplier :

public GeneratedWatermarkGeneratorSupplier(
    GeneratedWatermarkGenerator generatedWatermarkGenerator,
    WatermarkParams watermarkParams) {
        this.generatedWatermarkGenerator = generatedWatermarkGenerator;
        this.watermarkParams = watermarkParams;
}

Compiled Plan

After implementing the above features, The following contents will be added to the WatermarkPushDown module in the json plan：

"watermarkParams":{
    "emitStrategy" : "ON_PERIODIC",
    "alignGroupName" : null,
    "alignMaxDrift" : "PT0S",
    "alignUpdateInterval" : "PT1S",
    "sourceIdleTimeout" : -1
}

But this doesn't cause compatibility issues, CompilePlan and ExecutePlan are backward compatible. We will add unit tests to validate the compatibility.

Migration Plan and Compatibility

This feature is biased towards adding more support for SQL, so there are no compatibility-related issues.

If some of the features are not supported, such as the watermark alignment feature on Kinesis connector, it will behave as FLIP-182 [3] and FLIP-217 [2] designed.

Rejected Alternatives

Adding watermark related options in the SQL DDL of watermark column

This idea is like extending FLIP-66[4]. However, since we already have many options for watermark related features, this would make the DDL complex and lengthy.

Adding watermark related options with a new table-scan hint named 'WATERMARK_PARAMS'

We should be cautious about adding SQL syntax, WATERMARK_PARAMS is also SQL syntax to some extent.

Reference

[1] https://github.com/yuchengxin/flink/commits/yuankui/watermark_params

[2] FLIP-217: Support watermark alignment of source splits

[3] FLIP-182: Support watermark alignment of FLIP-27 Sources 

[4] FLIP-66: Support Time Attribute in SQL DDL