Define the Write client interface with the input stage

Related to work on #7291

**TODO:** Add more details to this, this is an issue place holder at the moment, we need more work on the POC to understand what we really need to do here.

Define how inputs will interact with the write client at the input stage.

Start by reviewing related issue #7291 and the existing proof of concept, since this placeholder says more work is needed to understand the requirements. Clarify how inputs should interact with the Write client at the input stage; done means the required client interface and interaction are defined.

Written by the indexing model from the issue text.