feat: DB connection pool sizing (#2779)

backend/controller/controller.go

@@ -61,6 +61,7 @@ import (
 	ftlmaps "github.com/TBD54566975/ftl/internal/maps"
 	"github.com/TBD54566975/ftl/internal/model"
 	"github.com/TBD54566975/ftl/internal/modulecontext"
+	internalobservability "github.com/TBD54566975/ftl/internal/observability"
 	ftlreflect "github.com/TBD54566975/ftl/internal/reflect"
 	"github.com/TBD54566975/ftl/internal/rpc"
 	"github.com/TBD54566975/ftl/internal/rpc/headers"
@@ -100,6 +101,8 @@ type Config struct {
 	EventLogRetention            *time.Duration      `help:"Delete call logs after this time period. 0 to disable" env:"FTL_EVENT_LOG_RETENTION" default:"24h"`
 	ArtefactChunkSize            int                 `help:"Size of each chunk streamed to the client." default:"1048576"`
 	KMSURI                       *string             `help:"URI for KMS key e.g. with fake-kms:// or aws-kms://arn:aws:kms:ap-southeast-2:12345:key/0000-1111" env:"FTL_KMS_URI"`
+	MaxOpenDBConnections         int                 `help:"Maximum number of database connections." default:"20" env:"FTL_MAX_OPEN_DB_CONNECTIONS"`
+	MaxIdleDBConnections         int                 `help:"Maximum number of idle database connections." default:"20" env:"FTL_MAX_IDLE_DB_CONNECTIONS"`
 	CommonConfig
 }
 
@@ -112,6 +115,16 @@ func (c *Config) SetDefaults() {
 	}
 }
 
+func (c *Config) OpenDBAndInstrument() (*sql.DB, error) {
+	conn, err := internalobservability.OpenDBAndInstrument(c.DSN)
+	if err != nil {
+		return nil, fmt.Errorf("failed to open DB connection: %w", err)
+	}
+	conn.SetMaxIdleConns(c.MaxIdleDBConnections)
+	conn.SetMaxOpenConns(c.MaxOpenDBConnections)
+	return conn, nil
+}
+
 // Start the Controller. Blocks until the context is cancelled.
 func Start(ctx context.Context, config Config, runnerScaling scaling.RunnerScaling, conn *sql.DB, devel bool) error {
 	config.SetDefaults()

backend/controller/sql/databasetesting/devel.go

@@ -77,6 +77,8 @@ func CreateForDevel(ctx context.Context, dsn string, recreate bool) (*stdsql.DB,
 	if err != nil {
 		return nil, fmt.Errorf("failed to open database: %w", err)
 	}
+	realConn.SetMaxIdleConns(20)
+	realConn.SetMaxOpenConns(20)
 	// Reset transient state in the database to a clean state for development purposes.
 	// This includes things like resetting the state of async calls, leases,
 	// controller/runner registration, etc. but not anything more.

cmd/ftl-controller/main.go

@@ -50,7 +50,7 @@ func main() {
 	kctx.FatalIfErrorf(err, "failed to initialize observability")
 
 	// The FTL controller currently only supports DB as a cf provider/resolver.
-	conn, err := observability.OpenDBAndInstrument(cli.ControllerConfig.DSN)
+	conn, err := cli.ControllerConfig.OpenDBAndInstrument()
 	kctx.FatalIfErrorf(err)
 
 	dal := dbleaser.NewDatabaseLeaser(conn)

frontend/cli/cmd_box_run.go

@@ -19,7 +19,6 @@ import (
 	"github.com/TBD54566975/ftl/internal/buildengine"
 	"github.com/TBD54566975/ftl/internal/log"
 	"github.com/TBD54566975/ftl/internal/model"
-	"github.com/TBD54566975/ftl/internal/observability"
 	"github.com/TBD54566975/ftl/internal/projectconfig"
 	"github.com/TBD54566975/ftl/internal/rpc"
 )
@@ -58,7 +57,7 @@ func (b *boxRunCmd) Run(ctx context.Context, projConfig projectconfig.Config) er
 	}
 
 	// Bring up the DB connection and DAL.
-	conn, err := observability.OpenDBAndInstrument(config.DSN)
+	conn, err := config.OpenDBAndInstrument()
 	if err != nil {
 		return fmt.Errorf("failed to bring up DB connection: %w", err)
 	}

frontend/cli/cmd_serve.go

@@ -148,7 +148,7 @@ func (s *serveCmd) run(ctx context.Context, projConfig projectconfig.Config, ini
 		controllerCtx = manager.ContextWithSecrets(controllerCtx, sm)
 
 		// Bring up the DB connection and DAL.
-		conn, err := observability.OpenDBAndInstrument(config.DSN)
+		conn, err := config.OpenDBAndInstrument()
 		if err != nil {
 			return fmt.Errorf("failed to bring up DB connection: %w", err)
 		}