ACA README - added architecture components description

Author: dantelmomsft

docs/aca/README-ACA.md

@@ -2,6 +2,12 @@
 
 ![Microservice RAG Architecture](aca-hla.png)
 
+* The API app is implemented as springboot 2.7.x app using the Microsoft JDK. It provides the ask and chat apis which are used by the chat web app. It's responsible for implementing the RAG pattern orchestrating the interaction between the LLM model (Open AI - ChatGPT) and the retriever (Azure AI Search).
+* The Chat Web App is built in React and deployed as a static web app on nginx. Furthermore Nginx act as reverse proxy for api calls to the API app. This also solves the CORS issue.
+* The indexer APP is implemented as springboot 2.7.x app using the Microsoft JDK. It is responsible for indexing the data into Azure Cognitive Search and it's triggered by new BlobUploaded messages from serviceBus. The indexer is also responsible for chunking the documents into smaller pieces, embed them and store them in the index. Azure Document Intelligence is used to extract text from PDF documents (including tables and images)
+* Azure AI Search is used as RAG retrieval system. Different search options are available: you have traditional full text (with semantic search) search, or vector based search and finally you can opt for hybrid search which brings together the best of the previous ones.
+* EventGrid System topic is used to implement a real time mechanism to trigger the indexer app when a new document is uploaded to the blob storage. It's responsible to read BlobUploaded notification from azure storage container and push a message to the service bus queue containing the blob url.
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 ## TL;DR
 
 Open the project: