chore: import upstream snapshot with attribution

docs/md/explanation/architecture/client_server.md

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+# Client/Server replicated
+
+<img src="./architecture.sub2.svg" />
+
+_For medium-sized, real-time, synchronized and/or editable data sets with many
+concurrent users._
+
+The dataset is instantiated in-memory with a Python or Node.js Perspective
+server, and web applications create duplicates of these tables in a local
+WebAssembly client in the browser, synchonized efficiently to the server via
+Apache Arrow. This design scales well with additional concurrent users, as
+browsers only need to download the initial data set and subsequent update
+deltas, while operations like scrolling, pivots, sorting, etc. are performed on
+the client.
+
+Python servers can make especially good use of additional threads, as
+Perspective will release the GIL for almost all operations. Interactive
+performance on the client is very good and identical to client-only
+architecture. Updates and edits are seamlessly synchonized across clients via
+their virtual server counterparts using websockets and Apache Arrow.
+
+## Python and Tornado server
+
+```python
+from perspective import Server, PerspectiveTornadoHandler
+
+server = Server()
+client = server.new_local_client()
+client.table(csv, name="my_table")
+routes = [(
+    r"/websocket",
+    perspective.handlers.tornado.PerspectiveTornadoHandler,
+    {"perspective_server": server},
+)]
+
+app = tornado.web.Application(routes)
+app.listen(8080)
+loop = tornado.ioloop.IOLoop.current()
+loop.start()
+```
+
+## Javascript client
+
+Perspective's websocket client interfaces with the Python server, then
+_replicates_ the server-side Table.
+
+```javascript
+const websocket = await perspective.websocket("ws://localhost:8080");
+const server_table = await websocket.open_table("my_table");
+const server_view = await server_table.view();
+
+const worker = await perspective.worker();
+const client_table = await worker.table(server_view);
+
+const viewer = document.createElement("perspective-viewer");
+document.body.appendChild(viewer);
+await viewer.load(client_table);
+```