Add what is fastcs explanation

Author: GDYendell

docs/explanations/transports.md

@@ -80,7 +80,17 @@ for controller_api in root_controller_api.walk_api():
 
 Transports use attribute callbacks to keep their protocol-specific representations synchronized with attribute values:
 
-![Transport Data Flow](../images/attribute-dataflow.excalidraw.png)
+---
+
+<div style="text-align: center">
+
+```{raw} html
+:file: ../images/data-flow.excalidraw.svg
+```
+
+</div>
+
+---
 
 The diagram above shows the data flow between users, transports, attributes, and
 hardware. The following table gives an overview of the data flow for the transport

docs/explanations/what-is-fastcs.md

@@ -0,0 +1,98 @@
+# What is FastCS?
+
+FastCS is a Python framework for building device drivers for scientific instruments.
+It separates the logic of communicating with a device from the control system used to
+expose it, so the same driver works with EPICS (CA or PVA), Tango, REST, and GraphQL
+without modification.
+
+---
+
+<div style="text-align: center">
+
+```{raw} html
+:file: ../images/overview.excalidraw.svg
+```
+
+</div>
+
+---
+
+## Architecture
+
+A FastCS application has three layers:
+
+**Controller** - a Python class that models the device. It holds attributes and
+commands, implements connection logic, and creates periodic polling tasks. The
+controller can create `AttributeIO`s to handle `update` and `send` operations between
+attributes and the device.
+
+**Attributes and commands** - typed values (`AttrR`, `AttrW`, `AttrRW`) and callable
+actions (`@command`) declared on the controller. Attributes represent the device's
+readable and writable parameters and carry metadata such as units, limits, and alarm
+thresholds. Commands are actions that can be triggered to run.
+
+**Transport** - the protocol layer that exposes the controller's attributes and commands
+to a control system. FastCS provides transports for EPICS CA, EPICS PVA, Tango, REST,
+and GraphQL.
+
+---
+
+<div style="text-align: center">
+
+```{raw} html
+:file: ../images/data-flow.excalidraw.svg
+```
+
+</div>
+
+---
+
+Sub-controllers let you model hierarchical devices — a sub-controller's attributes are
+exposed under a prefixed path, and a set of identical sub-controllers can be grouped
+using `ControllerVector`. See [Controllers](controllers.md) for details.
+
+## Writing a driver
+
+To create a driver, subclass `Controller`, declare attributes and commands as typed
+class members, and implement lifecycle hooks (`connect`, `reconnect`, `disconnect`) and
+periodic scan tasks. The [tutorials](../tutorials.md) walk through a complete example.
+
+## Deploying as an application
+
+Passing the controller class to `launch()` generates a standard CLI with `run` and
+`schema` commands. The `run` command takes a YAML configuration file that specifies
+both the controller's settings and which transports to start. Swapping or adding
+transports requires only a config change, not a code change. See
+[Launching the framework](../how-to/launch-framework.md) for details.
+
+## Benefits
+
+**Simple API** — a driver is a plain Python class with typed class-variable attributes.
+There is no boilerplate or protocol-specific code in the driver itself.
+
+**Control-system-agnostic testing** — controllers have no dependency on EPICS, Tango,
+or any other control system and can be unit-tested as ordinary Python objects. Testing
+device logic does not require a running IOC or device server.
+
+**Multiple transports from a single driver** — the same controller can serve EPICS CA,
+EPICS PVA, Tango, REST, and GraphQL simultaneously or in any combination.
+
+**Automatic reconnection** — if a scan task raises an exception, FastCS pauses all
+tasks and calls `reconnect()` automatically, resuming once the connection is restored.
+
+**Auto-generated OPI screens** — EPICS transports generate CSS-Phoebus screen files
+from the controller's attribute metadata, with no additional effort from the driver
+author.
+
+**Interactive shell** — FastCS starts an IPython shell alongside the running driver,
+giving direct access to the live controller instance for inspection and ad-hoc commands
+without restarting.
+
+**Structured logging and per-attribute tracing** — FastCS uses
+[loguru](https://loguru.readthedocs.io/) for structured logging. The `Tracer` mixin
+adds TRACE-level logging to individual attributes, allowing fine-grained visibility
+into specific values without increasing the verbosity of the whole driver.
+
+**Graylog integration** — the generated CLI accepts a `--graylog-endpoint` option that
+forwards all log output to a Graylog instance, with support for static and
+environment-variable-sourced fields for log enrichment.