RAG on GCP

[sailochana]

HI Folks ,
I was looking for RAG implementation in GCP so where ,I come across  a small confusion i.e 

I found two possibilities
1: rag api
For rag api in gcp requires vertexai api ,
2: RAG implementaion on GCP
For rag flow using document ai ,google storage ,embeddings and generation models we need document ai api ,cloud storage api,vertexai api
in this case which is more feasible ,
if rag api fullfils all requirements in single vertexai api what is the use of rag flow
can anyone explain in better way and reasons for these approaches 
Thanks .
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