unit

type Unit interface {
	// Init initializes the dm process unit
	// every unit does base initialization in `Init`, and this must pass before start running the sub task
	// other setups can be done in `Process`, but this should be treated carefully, let it's compatible with Pause / Resume
	// if initialing successfully, the outer caller should call `Close` when the unit (or the task) finished, stopped or canceled (because other units Init fail).
	// if initialing fail, Init itself should release resources it acquired before (rolling itself back).
	Init(ctx context.Context) error
	// Process processes sub task
	// When ctx.Done, stops the process and returns
	// When not in processing, call Process to continue or resume the process
	Process(ctx context.Context, pr chan pb.ProcessResult)
	// Close shuts down the process and closes the unit, after that can not call Process to resume
	Close()
	// Pause pauses the process, it can be resumed later
	Pause()
	// Resume resumes the paused process
	Resume(ctx context.Context, pr chan pb.ProcessResult)
	// Update updates the configuration
	Update(cfg *config.SubTaskConfig) error

	// Status returns the unit's current status
	Status() interface{}
	// Error returns the unit's error information
	Error() interface{}
	// Type returns the unit's type
	Type() pb.UnitType
	// IsFreshTask return whether is a fresh task (not processed before)
	// it will be used to decide where the task should become restoring
	IsFreshTask(ctx context.Context) (bool, error)
}