Explain Umbrella project organization in root README

README.md

@@ -33,6 +33,19 @@ Now you can visit [`localhost:4000`](http://localhost:4000) from your browser.
 
 You can also run IEx (Interactive Elixir): `iex -S mix phx.server`
 
+#### Umbrella Project Organization
+
+This repository is an [umbrella project](https://elixir-lang.org/getting-started/mix-otp/dependencies-and-umbrella-projects.html): each directory under `apps/` is a separate [Mix](https://hexdocs.pm/mix/Mix.html) project and [OTP application](https://hexdocs.pm/elixir/Application.html), but the projects can use each other as a dependency in their `mix.exs`.
+
+Each OTP application has a restricted domain
+
+| Directory               | OTP Application     | Namespace         | Purpose                                                                                                                                                                                                                                                                                                                                                                         |
+|:------------------------|:--------------------|:------------------|:--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
+| `apps/ethereum_jsonrpc` | `:ethereum_jsonrpc` | `EthereumJSONRPC` | Ethereum JSONRPC client.  It is allowed to know `Explorer`'s param format, but it cannot directly depend on `:explorer`                                                                                                                                                                                                                                                         |
+| `apps/explorer`         | `:explorer`         | `Explorer`        | Storage for the indexed chain.  Can read and write to the backing storage.  MUST be able to boot in a read-only mode when run independently from `:indexer`, so cannot depend on `:indexer` as that would start `:indexer` indexing.                                                                                                                                            |
+| `apps/explorer_web`     | `:explorer_web`     | `ExplorerWeb`     | Phoenix interface to `:explorer`.  The minimum interface to allow web access should go in `:explorer_web`.  Any business rules or interface that is not tied directly to `Phoenix` or `Plug` should go in `:explorer`. MUST be able to boot in a read-only mode when run independently from `:indexer`, so cannot depend on `:indexer` as that would start `:indexer` indexing. |
+| `apps/indexer`          | `:indexer`          | `Indexer`         | Uses `:ethereum_jsonrpc` to index chain and batch import data into `:explorer`.  Any process, `Task`, or `GenServer` that automatically reads from the chain and writes to `:explorer` should be in `:indexer`, so that automatic writes are restricted to `:indexer` and read-only mode can be achieved by not running `:indexer`.                                             |
+
 ### CircleCI Updates
 
 Configure your local CCMenu with the following url: [`https://circleci.com/gh/poanetwork/poa-explorer.cc.xml?circle-token=f8823a3d0090407c11f87028c73015a331dbf604`](https://circleci.com/gh/poanetwork/poa-explorer.cc.xml?circle-token=f8823a3d0090407c11f87028c73015a331dbf604)