feat: ModuleContext periodically refreshed on the controller (#1865)

fixes #1699

`GetModuleContext` will stream updated `ModuleContext` to the runner.

---------

Co-authored-by: worstell <[email protected]>
Co-authored-by: Alec Thomas <[email protected]>
Co-authored-by: Matt Toohey <[email protected]>

backend/controller/controller.go

@@ -2,6 +2,7 @@ package controller
 
 import (
 	"context"
+	"encoding/binary"
 	"errors"
 	"fmt"
 	"io"
@@ -674,11 +675,12 @@ nextModule:
 func (s *Service) GetModuleContext(ctx context.Context, req *connect.Request[ftlv1.ModuleContextRequest], resp *connect.ServerStream[ftlv1.ModuleContextResponse]) error {
 	name := req.Msg.Module
 
-	// TODO migrate to a polling implementation that only emits responses when the configuration changes
-
 	cm := cf.ConfigFromContext(ctx)
 	sm := cf.SecretsFromContext(ctx)
 
+	// Initialize checksum to -1; a zero checksum does occur when the context contains no settings
+	lastChecksum := int64(-1)
+
 	for {
 		configs, err := cm.MapForModule(ctx, name)
 		if err != nil {
@@ -693,16 +695,53 @@ func (s *Service) GetModuleContext(ctx context.Context, req *connect.Request[ftl
 			return connect.NewError(connect.CodeInternal, fmt.Errorf("could not get databases: %w", err))
 		}
 
-		response := modulecontext.NewBuilder(name).AddConfigs(configs).AddSecrets(secrets).AddDatabases(databases).Build().ToProto()
+		checksum := configurationMapChecksum(configs)
+		checksum = (checksum * 115163) + configurationMapChecksum(secrets)
+		checksum = (checksum * 454213) + configurationDatabaseChecksum(databases)
+
+		if checksum != lastChecksum {
+			response := modulecontext.NewBuilder(name).AddConfigs(configs).AddSecrets(secrets).AddDatabases(databases).Build().ToProto()
+
+			if err := resp.Send(response); err != nil {
+				return connect.NewError(connect.CodeInternal, fmt.Errorf("could not send response: %w", err))
+			}
 
-		if err := resp.Send(response); err != nil {
-			return connect.NewError(connect.CodeInternal, fmt.Errorf("could not send response: %w", err))
+			lastChecksum = checksum
 		}
 
-		time.Sleep(s.config.ModuleUpdateFrequency)
+		select {
+		case <-ctx.Done():
+			return nil
+		case <-time.After(s.config.ModuleUpdateFrequency):
+		}
 	}
 }
 
+// configurationMapChecksum computes a checksum on the map that is order invariant.
+//
+// This operation is used to detect configuration change.
+func configurationMapChecksum(m map[string][]byte) int64 {
+	sum := int64(0)
+	for k, v := range m {
+		data := sha256.Sum(append([]byte(k), v...))
+		sum += int64(binary.BigEndian.Uint64(data[0:8]))
+	}
+	return sum
+}
+
+// configurationMapChecksum computes a checksum on the database map that is order invariant.
+//
+// This operation is used to detect configuration change.
+func configurationDatabaseChecksum(m map[string]modulecontext.Database) int64 {
+	sum := int64(0)
+	for k, v := range m {
+		// currently, only the DSN is treated as mutable configuration
+		data := sha256.Sum(append([]byte(k), []byte(v.DSN)...))
+		sum += int64(binary.BigEndian.Uint64(data[0:8]))
+	}
+	return sum
+}
+
 // AcquireLease acquires a lease on behalf of a module.
 //
 // This is a bidirectional stream where each request from the client must be