Yes you are right. If Kinesis Data Streams is used as a source for Kinesis Data Firehose, then KDF starts reading from the latest position from KDS. This is documented here - https://docs.aws.amazon.com/firehose/latest/dev/writing-with-kinesis-streams.html - "Kinesis Data Firehose starts reading data from the LATEST position of your Kinesis stream."
It is also mentioned in the FAQ - https://aws.amazon.com/kinesis/data-firehose/faqs/
Q: From where does Kinesis Data Firehose read data when my Kinesis Data Stream is configured as the source of my delivery stream?
Kinesis Data Firehose starts reading data from the LATEST position of your Kinesis Data Stream when it’s configured as the source of a delivery stream.
If you want to read records in the KDS from the beginning or from a particular position, one option could be to configure a lambda function to read from the position you are interested in and then put the records to KDF using the SDK.
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