Matthias J. Sax created KAFKA-4125:
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Summary: Provide low-level Processor API meta data in DSL layer
Key: KAFKA-4125
URL: https://issues.apache.org/jira/browse/KAFKA-4125
Project: Kafka
Issue Type: Sub-task
Components: streams
Reporter: Matthias J. Sax
Assignee: Guozhang Wang
Priority: Minor
For Processor API, user can get meta data like record offset, timestamp etc via
the provided {{Context}} object. It might be useful to allow uses to access
this information in DSL layer, too.
The idea would be, to do it "the Flink way", ie, by providing
RichFunctions; {{mapValue()}} for example.
Is takes a {{ValueMapper<V1, V2>}} that only has method
{noformat}
V2 apply(V1 value);
{noformat}
Thus, you cannot get any meta data within apply (it's completely "blind").
We would add two more interfaces: {{RichFunction}} with a method
{{open(Context context)}} and
{noformat}
RichValueMapper<V1, V2> extends ValueMapper<V1, V2>, RichFunction
{noformat}
This way, the user can chose to implement Rich- or Standard-function and
we do not need to change existing APIs. Both can be handed into
{{KStream.mapValues()}} for example. Internally, we check if a Rich
function is provided, and if yes, hand in the {{Context}} object once, to
make it available to the user who can now access it within {{apply()}} -- or
course, the user must set a member variable in {{open()}} to hold the
reference to the Context object.
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