docs: Adds architecture doc

Author: NeedleInAJayStack

Architecture.md

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+# Architecture
+
+```mermaid
+flowchart TD
+    A(TimeseriesAPI)
+    B(RecordStore)
+    C(HistoryStore)
+    D(CurrentStore)
+    E(Ingester)
+
+    A --> B
+    A --> C
+    A --> D
+    E --> A
+```
+
+## TimeseriesAPI
+This HTTP API service exposes endpoints for managing these types of data:
+
+- Records
+- Historical values
+- Current values
+
+## RecordStore
+Stores point entity data. Different drivers may be provided for different storage backends.
+
+Design goals:
+1. Records are stored persistently
+2. Querying by freeform tags is supported
+
+## HistoryStore
+Stores historical data. Different drivers may be provided for different storage backends.
+
+Design goals:
+1. Historical data is stored persistently over long time periods, where loss of data is unacceptable
+2. Historical data frequency is relatively low compared to current data (e.g. no faster than once per minute)
+3. Querying of historical data is efficient
+
+## CurrentStore
+Stores current data. Different drivers may be provided for different storage backends.
+
+Design goals:
+1. Current data storage is allowed to be volatile, where loss of data is acceptable
+2. Current data frequency is relatively high compared to historical data (e.g. often faster than once per minute)
+
+## Ingesters
+Ingesters are responsible for delivering data from external sources and setting current values in the system. They are implemented for specific data sources, such as MQTT, etc.
+
+Design goals:
+1. Ingesters may set current values or historical values
+2. They should be implemented externally to the core system, including details like scheduling, scalability, etc.