From dc143a6dc7dd75fc1e01524781ae9e746dc2c960 Mon Sep 17 00:00:00 2001
From: andig <andi@evcc.io>
Date: Mon, 10 Aug 2026 21:54:37 +0200
Subject: [PATCH] MQTT: publish one forecast message per key (BC) (#32716)

---
 core/site_tariffs.go | 32 +++++++++++++++++++++++++-------
 server/mqtt_test.go  | 22 ++++++++++++++++++++++
 2 files changed, 47 insertions(+), 7 deletions(-)

diff --git a/core/site_tariffs.go b/core/site_tariffs.go
index e9790a020..f26a7ffd5 100644
--- a/core/site_tariffs.go
+++ b/core/site_tariffs.go
@@ -1,6 +1,7 @@
 package core
 
 import (
+	"encoding/json"
 	"math"
 	"time"
 
@@ -13,6 +14,17 @@ import (
 	"github.com/samber/lo"
 )
 
+// forecastSeries and solarDetails implement BytesMarshaler so MQTT publishes one
+// json message per forecast key instead of decomposing every slot into its own
+// topic (several thousand messages per update).
+type forecastSeries [][]float64
+
+var _ api.BytesMarshaler = (*forecastSeries)(nil)
+
+func (s forecastSeries) MarshalBytes() ([]byte, error) {
+	return json.Marshal(s)
+}
+
 type solarDetails struct {
 	Scale            float64      `json:"scale"`                      // scale factor yield/forecasted today, 1 if unscaled
 	Today            dailyDetails `json:"today,omitempty"`            // tomorrow
@@ -21,6 +33,12 @@ type solarDetails struct {
 	Timeseries       timeseries   `json:"timeseries,omitempty"`       // timeseries of forecasted energy
 }
 
+var _ api.BytesMarshaler = (*solarDetails)(nil)
+
+func (d solarDetails) MarshalBytes() ([]byte, error) {
+	return json.Marshal(d)
+}
+
 type dailyDetails struct {
 	Yield    float64 `json:"energy"`
 	Complete bool    `json:"complete"`
@@ -29,7 +47,7 @@ type dailyDetails struct {
 // forecastRates publishes rates as [start, end, value] with the timestamps in
 // unix seconds. The forecast is the largest payload evcc sends and RFC3339
 // timestamps are two thirds of it.
-func forecastRates(rr api.Rates) [][]float64 {
+func forecastRates(rr api.Rates) forecastSeries {
 	// keep nil for empty rates: shards are published without omitempty
 	if len(rr) == 0 {
 		return nil
@@ -116,12 +134,12 @@ func (site *Site) publishTariffs(greenShareHome float64, greenShareLoadpoints fl
 	}
 
 	fc := struct {
-		Co2         [][]float64   `json:"co2,omitempty"`
-		FeedIn      [][]float64   `json:"feedin,omitempty"`
-		Grid        [][]float64   `json:"grid,omitempty"`
-		Planner     [][]float64   `json:"planner,omitempty"`
-		Solar       *solarDetails `json:"solar,omitempty"`
-		Temperature [][]float64   `json:"temperature,omitempty"`
+		Co2         forecastSeries `json:"co2,omitempty"`
+		FeedIn      forecastSeries `json:"feedin,omitempty"`
+		Grid        forecastSeries `json:"grid,omitempty"`
+		Planner     forecastSeries `json:"planner,omitempty"`
+		Solar       *solarDetails  `json:"solar,omitempty"`
+		Temperature forecastSeries `json:"temperature,omitempty"`
 	}{
 		Co2:         forecastRates(tariff.Rates(site.GetTariff(api.TariffUsageCo2))),
 		FeedIn:      forecastRates(tariff.Rates(site.GetTariff(api.TariffUsageFeedIn))),
diff --git a/server/mqtt_test.go b/server/mqtt_test.go
index 87b05d1e0..115800433 100644
--- a/server/mqtt_test.go
+++ b/server/mqtt_test.go
@@ -1,6 +1,7 @@
 package server
 
 import (
+	"encoding/json"
 	"math"
 	"slices"
 	"strconv"
@@ -8,6 +9,7 @@ import (
 	"time"
 
 	"github.com/evcc-io/evcc/core/types"
+	"github.com/evcc-io/evcc/util"
 	"github.com/samber/lo"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/suite"
@@ -97,6 +99,26 @@ func (suite *mqttSuite) TestSlice() {
 	suite.Equal([]string{"2", "10", "20"}, suite.payloads, "payloads")
 }
 
+type jsonSeries [][]float64
+
+func (s jsonSeries) MarshalBytes() ([]byte, error) {
+	return json.Marshal(s)
+}
+
+// a sharded struct publishes one message per key, BytesMarshaler fields as json
+func (suite *mqttSuite) TestSharderBytesMarshaler() {
+	p := struct {
+		Foo jsonSeries `json:"foo,omitempty"`
+		Bar jsonSeries `json:"bar,omitempty"`
+	}{
+		Foo: jsonSeries{{1, 2, 3}, {4, 5, 6}},
+	}
+
+	suite.publish("test", false, util.NewSharder("test", p))
+	suite.Equal([]string{"test/foo", "test/bar"}, suite.topics, "topics")
+	suite.Equal([]string{"[[1,2,3],[4,5,6]]", ""}, suite.payloads, "payloads")
+}
+
 func (suite *mqttSuite) TestNilInterface() {
 	var ptr *time.Time
 	suite.publish("test", false, ptr)