feat(transform): Add UtilityControl Transform for Generating Ctrl Messages

build/config/substation.libsonnet

@@ -971,6 +971,14 @@
     },
     util: $.transform.utility,
     utility: {
+      control(settings={}): {
+        local default = {
+          batch: $.config.batch,
+        },
+
+        type: 'utility_control',
+        settings: std.prune(std.mergePatch(default, $.helpers.abbv(settings))),
+      },
       delay(settings={}): {
         local default = {
           duration: null,

examples/config/transform/utility/generate_ctrl/config.jsonnet

@@ -0,0 +1,13 @@
+// This example shows how to use the `utility_control` transform to
+// generate a control (ctrl) Message based on the amount of data Messages
+// received by the system. ctrl Messages overrides the settings of the
+// `aggregate_to_array` transform (and any other transform that supports).
+local sub = import '../../../../../build/config/substation.libsonnet';
+
+{
+  transforms: [
+    sub.tf.utility.control({ batch: { count: 2 } }),
+    sub.tf.aggregate.to.array({ batch: { count: 10000 } }),
+    sub.tf.send.stdout(),
+  ],
+}

examples/config/transform/utility/generate_ctrl/data.jsonl

@@ -0,0 +1,13 @@
+{"a":"b"}
+{"c":"d"}
+{"e":"f"}
+{"g":"h"}
+{"i":"j"}
+{"k":"l"}
+{"m":"n"}
+{"o":"p"}
+{"q":"r"}
+{"s":"t"}
+{"u":"v"}
+{"w":"x"}
+{"y":"z"}

transform/aggregate.go

@@ -19,6 +19,10 @@ func (c *aggregateArrayConfig) Decode(in interface{}) error {
 }
 
 func aggToArray(data [][]byte) []byte {
+	if len(data) == 0 {
+		return nil
+	}
+
 	return slices.Concat([]byte("["), bytes.Join(data, []byte(",")), []byte("]"))
 }
 
@@ -43,6 +47,10 @@ func (c *aggregateStrConfig) Validate() error {
 }
 
 func aggToStr(data [][]byte, separator []byte) []byte {
+	if len(data) == 0 {
+		return nil
+	}
+
 	return bytes.Join(data, separator)
 }
 

transform/transform.go

@@ -173,6 +173,8 @@ func New(ctx context.Context, cfg config.Config) (Transformer, error) { //nolint
 	case "time_to_unix_milli":
 		return newTimeToUnixMilli(ctx, cfg)
 	// Utility transforms.
+	case "utility_control":
+		return newUtilityControl(ctx, cfg)
 	case "utility_delay":
 		return newUtilityDelay(ctx, cfg)
 	case "utility_drop":

transform/utility_control.go

@@ -0,0 +1,81 @@
+package transform
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"sync"
+
+	"github.com/brexhq/substation/config"
+	"github.com/brexhq/substation/internal/aggregate"
+	iconfig "github.com/brexhq/substation/internal/config"
+	"github.com/brexhq/substation/message"
+)
+
+type utilityControlConfig struct {
+	Batch iconfig.Batch `json:"batch"`
+}
+
+func (c *utilityControlConfig) Decode(in interface{}) error {
+	return iconfig.Decode(in, c)
+}
+
+func newUtilityControl(_ context.Context, cfg config.Config) (*utilityControl, error) {
+	conf := utilityControlConfig{}
+	if err := conf.Decode(cfg.Settings); err != nil {
+		return nil, fmt.Errorf("transform: utility_control: %v", err)
+	}
+
+	agg, err := aggregate.New(aggregate.Config{
+		Count:    conf.Batch.Count,
+		Size:     conf.Batch.Size,
+		Duration: conf.Batch.Duration,
+	})
+	if err != nil {
+		return nil, fmt.Errorf("transform: utility_control: %v", err)
+	}
+
+	tf := utilityControl{
+		conf: conf,
+		agg:  *agg,
+	}
+
+	return &tf, nil
+}
+
+type utilityControl struct {
+	conf utilityControlConfig
+
+	mu  sync.Mutex
+	agg aggregate.Aggregate
+}
+
+func (tf *utilityControl) Transform(_ context.Context, msg *message.Message) ([]*message.Message, error) {
+	tf.mu.Lock()
+	defer tf.mu.Unlock()
+
+	if msg.IsControl() {
+		// If a control message is received, then the aggregation is reset
+		// to prevent sending duplicate control messages.
+		tf.agg.ResetAll()
+
+		return []*message.Message{msg}, nil
+	}
+
+	if ok := tf.agg.Add("", msg.Data()); ok {
+		return []*message.Message{msg}, nil
+	}
+
+	tf.agg.Reset("")
+	if ok := tf.agg.Add("", msg.Data()); !ok {
+		return nil, fmt.Errorf("transform: utility_control: %v", errSendBatchMisconfigured)
+	}
+
+	ctrl := message.New().AsControl()
+	return []*message.Message{msg, ctrl}, nil
+}
+
+func (tf *utilityControl) String() string {
+	b, _ := json.Marshal(tf.conf)
+	return string(b)
+}