docs: simple diagram on architecture

README.md

@@ -11,6 +11,73 @@ check out the documentation at [docs.aura.radio](https://docs.aura.radio/)
 
 To learn more about Automated Radio go to [aura.radio](https://aura.radio).
 
+## Overview
+
+The following diagram gives an simplified overview on the architecture.
+
+```mermaid
+flowchart LR
+
+    api-- GET calendar -->steering[Steering API]
+    api-- GET playlist -->tank[Tank API]
+    ea[Engine API]
+    mail[Mailserver]
+    events-- on_boot\non_sick\non_resurrect -->monitor[Monitor]
+    events-- on_play\non_fallback_active -->clock[Clock]
+    events-- on_fallback_active\non_sick\non_resurrect -->mailer[Mailer]
+    clock-- POST clock_data -->ea
+    monitor-- PING heartbeat -->monserver[Monitoring Server]
+    monitor-- POST health_data -->ea
+    mailer-- send mail -->mail
+    mixer-- UNIX SOCKET -->engine_core
+
+    subgraph external
+        ea
+        mail
+        monserver
+        engine_core
+        steering
+        tank
+        db[(PostgreSQL DB)]
+    end
+    subgraph engine
+        engine_instance[Engine]-.-oplayer & events & scheduler
+
+        scheduler-- control\ncommand\ntimer -->player[Player]
+
+        player-- control action -->mixer
+        events[Event Dispatcher *]
+
+        subgraph scheduling
+            api[Fetch & Cache API]
+            timetable<-->api
+            timetable<-->db
+            scheduler<-->timetable[Timetable Management ***]
+            scheduler-- cycle -->scheduler
+        end
+        subgraph core[core]
+            mixer[Mixer]
+        end
+        subgraph plugins
+            clock
+            monitor
+            mailer
+        end
+    end
+
+    style external fill:#FFF
+    style ea fill:#FFF,stroke:#333,stroke-width:1px
+    style steering fill:#FFF,stroke:#333,stroke-width:1px
+    style tank fill:#FFF,stroke:#333,stroke-width:1px
+    style monserver fill:#FFF,stroke:#333,stroke-width:1px
+    style engine_core fill:#FFF,stroke:#333,stroke-width:1px
+    style mail fill:#FFF,stroke:#333,stroke-width:1px
+```
+
+- **Event Dispatcher**: Events are issued and consumed in multiple locations of Engine. The diagram shows only a few ones, in order to support the overview.
+- **Mailer**: This module notifies on incidents and fallbacks via mail. This is not a sophisticated monitoring solution and will be [replaced](https://gitlab.servus.at/aura/engine/-/issues/109) by [Prometheus Metrics](https://gitlab.servus.at/aura/aura/-/issues/98) and a dedicated [Notification Service for Programme Coordinators](https://gitlab.servus.at/aura/aura/-/issues/139).
+- **Timetable Management**: Currently a database is used for caching and high-availability purposes. This is [subject to changes](https://gitlab.servus.at/aura/engine/-/issues/100) in subsequent releases.
+
 ## Prerequisites
 
 Before you begin, ensure you have met the following requirements: