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Kinesis connector shard discovery mechanism


Hi there

Flink Kinesis consumer checks shards id for a particular pattern:
"^shardId-\\d{12}"

https://github.com/apache/flink/blob/master/flink-connectors/flink-connector-kinesis/src/main/java/org/apache/flink/streaming/connectors/kinesis/model/StreamShardHandle.java#L132

While this inlines with current Kinesis streams server implementation (all
streams follows this pattern), it confronts with AWS docs:
*ShardId*
The unique identifier of the shard within the stream.
Type: String
Length Constraints: Minimum length of 1. Maximum length of 128.

*Pattern: [a-zA-Z0-9_.-]+*Required: Yes

https://docs.aws.amazon.com/kinesis/latest/APIReference/API_Shard.html

*Intention:*
We have no guarantees and can't rely on patterns other than provided in AWS
manifest.
Any custom implementation of Kinesis mock should rely on AWS manifest which
claims ShardID to be alfanums. This prevents anyone to use Flink with such
kind of mocks.

The reason behind the scene to use particular pattern "^shardId-\\d{12}" is
to create Flink's custom Shard comparator, filter already seen shards, and
pass latest shard for client.listShards only to limit the scope for RPC
call to AWS.

In the meantime, I think we can get rid of this logic at all. The current
usage in project is:
- fix Kinesalite bug (I've already opened an issue to cover this:
https://github.com/mhart/kinesalite/issues/76). We can move this logic to
test code base to keep production code clean for now
https://github.com/apache/flink/blob/50d076ab6ad325907690a2c115ee2cb1c45775c9/flink-connectors/flink-connector-kinesis/src/main/java/org/apache/flink/streaming/connectors/kinesis/proxy/KinesisProxy.java#L464

- adjust last seen shard id. We can simply omit this cause' AWS client
won't return already seen shards and we will have new ids only or nothing.
https://github.com/apache/flink/blob/50d076ab6ad325907690a2c115ee2cb1c45775c9/flink-connectors/flink-connector-kinesis/src/main/java/org/apache/flink/streaming/connectors/kinesis/internals/KinesisDataFetcher.java#L475
https://github.com/apache/flink/blob/50d076ab6ad325907690a2c115ee2cb1c45775c9/flink-connectors/flink-connector-kinesis/src/main/java/org/apache/flink/streaming/connectors/kinesis/proxy/KinesisProxy.java#L406


What do you think?

Regards,
Eugen