docs(#119): draft architecture document

docs/architecture.md

@@ -0,0 +1,54 @@
+# Architecture overview
+
+intro text
+
+## Scope of this repository
+
+This repo includes just main Vivify which aims to be very configurable and
+easily integrable. Everything else (e.g. vivify.vim) are their own projects with
+their own repos
+
+## Executables
+
+we have two executables: `viv` and `vivify-server`:
+
+- `viv` is just a small wrapper that
+  1. runs Vivify Server
+  2. forwards everything Vivify Server prints to `stdout` to its own `stdout`
+     until the server prints `STARTUP COMPLETE`
+  3. disowns the server process so it keeps running in the background while
+     `viv` can terminate
+- `vivify-server` is the actual Node.js app that does almost everything and is
+  described in the rest of this doc
+
+## Vivify Server overview
+
+- (focus on `src/app.ts`) When `vivify-server` is called, there are two
+  scenarios which we distinguish through the [`/health` endpoint](docs/api.md):
+  1. There is already another instance of Vivify Server running: We have to
+     reuse that instance and have it open the requested viewer.
+  2. There is no other instance running: We have to start the server and
+     then handle the requests ourselves.
+- Vivify Server is
+  1. an [Express](http://expressjs.com) server that serves the
+     [API](docs/api.md) and the viewers
+  2. (focus on `src/sockets.ts`) a WebSocket server that keeps 1 socket
+     connection with each client (viewer browser tab) to
+     1. be able to manipulate clients, e.g. for editor integration
+     2. know when there are no clients left so we can shut down Vivify Server
+- rendering files happens in the `src/parser` directory
+  - for Markdown we use [markdown-it](https://www.npmjs.com/package/markdown-it)
+    and most Markdown features can be handled with plugins from there
+  - Notebook rendering is implemented from scratch but by heavily relying on the
+    existing ability render Markdown
+
+## Build infrastructure overview
+
+- we use [Node SEA](https://nodejs.org/api/single-executable-applications.html)
+  which is still in beta to compile Vivify Server into a single executable
+  without needing a Node dependency
+- this is why we can't simply use native Express static router but had to come
+  up with a custom solution in `src/routes/static.ts`:
+  - at development time, we just serve whatever is in the `static/` directory
+  - at production time we access all contents of the `static/` directory through
+    an archive that is included as an asset in the SEA