Cache last value for each topic and only insert into DB on change

Reduces database size

default.nix

@@ -1,7 +1,7 @@
 { python3 }:
 with python3.pkgs;
 buildPythonPackage rec {
-  name = "mqtt-config";
+  name = "mqtt-tools";
   src = ./.;
   propagatedBuildInputs = [ pyyaml paho-mqtt ];
   buildInputs = [];

mqtt-collect

@@ -7,6 +7,7 @@ import paho.mqtt.client
 import sqlite3
 import datetime
 
+MAX_VALUE_CACHE_SIZE = 1024
 
 def init_db(path, topics):
     db = sqlite3.connect(path)
@@ -31,6 +32,8 @@ def main():
     def on_disconnect(client, userdata, rc):
         client.reconnect()
 
+    current_values = {}
+
     def on_message(client, userdata, msg):
         try:
             c = db.cursor()
@@ -48,11 +51,18 @@ def main():
             else:
                 value = float(msg.payload)
             print(f"{table}: {value} ")
-            c.execute(f'CREATE TABLE IF NOT EXISTS "{table}" '
-                      '(timestamp timestamp, value real)')
-            c.execute(f'INSERT INTO "{table}" VALUES (?, ?)', (ts, value))
-            #c.execute(f'SELECT * from "{table}"')
-            db.commit()
+
+            if current_values.get(table, None) != value:
+                current_values[table] = value
+                if len(current_values) > MAX_VALUE_CACHE_SIZE:
+                    first_entry = current_values.keys().next()
+                    del current_values[first_entry]
+
+                c.execute(f'CREATE TABLE IF NOT EXISTS "{table}" '
+                          '(timestamp timestamp, value real)')
+                c.execute(f'INSERT INTO "{table}" VALUES (?, ?)', (ts, value))
+                db.commit()
+
         except Exception as e:
             print(f'Error writing to database: {e}')