...

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.

...

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

...

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

...

-- 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',
  'scan.watermark.emit.on-event.gap'='10000',
  ...
);

-- use 'OPTIONS' hint
select ... from source_table /*+ OPTIONS('scan.watermark.emit.strategy'='on-event', 'scan.watermark.emit.on-event.gap'='10000') */

Note that the option 'scan.watermark.emit.on-event.gap' which is used to configure how many events to emit a watermark only works for 'on-event' strategy,  This option is not required, the default value is 1.

2.Dealing with idle sources

...

-- 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

...

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.emit.on-event.gap'='10000',
'scan.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

...

If some of the features are not supported, such as the watermark alignment feature on Kinesis connector, it will behave as FLIP-182 [23] and FLIP-217 [32] designed.

Rejected Alternatives

...

This idea is like extending FLIP-66[14]. However, since we already have many options for watermark related features, this would make the DDL complex and lengthy.

...

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

Reference

[1] https://cwiki.apache.org/confluence/display/FLINK/FLIP-66%3A+Support+Time+Attribute+in+SQL+DDL

[2] https://cwiki.apache.org/confluence/display/FLINK/FLIP-182%3A+Support+watermark+alignment+of+FLIP-27+Sources 

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[3] https://cwiki.apache.org/confluence/display/FLINK/FLIP-217%3A+Support+watermark+alignment+of+source+splits