(DOCSP-31343): Add system diagram and info for ingest (#86)

* Add ingest system diagram

* Move diagram to README and add some info

ingest/README.md

@@ -5,6 +5,44 @@ database.
 
 Based on https://github.com/cbush/typescript-cli-template
 
+## System Overview
+
+```mermaid
+flowchart
+    B[Pages command]
+    C[Embed command]
+    B --> D(fetch pages from source)
+    D --> E(store pages in Atlas)
+
+    C --> F(fetch pages from Atlas)
+    F -- for pages marked\n 'created' or 'updated' --> G(make embeddings)
+    G --> H(store embeddings in Atlas)
+    F -- for pages marked 'deleted' --> I(delete embeddings\nfor page)
+```
+
+The ingest tool has two major commands: `pages` and `embed`. These commands
+represent the two stages of ingesting content.
+
+### Stage 1: Pages
+
+The `pages` command fetches pages from data sources and stores them in Atlas
+with a last updated timestamp. A "page" is some text with a URL. A data source
+is an arbitrary collection of pages. You can create a new data source by
+implementing `DataSource`.
+
+For each given data source, the `pages` command compares the pages with those
+already stored in the database and only updates those that are new, have
+changed, or have been deleted. The command does not actually delete documents
+from the database, but instead marks a page as "deleted", so that the next stage
+knows to delete the corresponding embeddings.
+
+### Stage 2: Embed
+
+The `embed` command creates embeddings for pages that have been updated since a
+given date. For pages that have been deleted, the command deletes any
+corresponding embeddings in the database. If a page is new or has been updated,
+the command regenerates the corresponding embeddings for that page.
+
 ## Development
 
 ### Build & Run
@@ -20,4 +58,3 @@ node .
 Add commands to `src/commands/`. The CLI automatically picks up any non-test .ts
 file that default-exports a `yargs.CommandModule`. See existing commands for
 example.
-