📦 box
A lightweight wrapper making Go interfaces serialisable so they can be used as arguments for Temporal workflows and activities.
Usage
Suppose I have the following interface for a message request:
type MessageRequest interface {
SendMessage() error
ScheduleMessage(time.Time) error
}
With two concrete implementations:
type SlackMessageRequest struct {
RecipientID string
Content []byte
...
}
type EmailRequest struct {
To string
Subject string
CC string
Body string
...
}
As part of some Temporal workflow, I wish to send either kind of message using an activity. Ideally, I would want the activity to be type-agnostic w.r.t. the message request, and simply interact through the interface:
func (h *CommHandler) HandleMessageDelivery(ctx context.Context, msgRequest MessageRequest) error {
...
}
However, arguments to Temporal activities are serialised and de-serialised behind the scenes, and so that intuitive solution would not work: a concrete JSON embedding of one of the implementations of the interface cannot be directly de-serialised into an interface instance.
By slightly changing the activity code, Box solves this problem:
func (h *CommHandler) HandleMessageDelivery(ctx context.Context, msgRequest Box[MessageRequest]) error {
err := msgRequest.Unbox(&SlackMessageRequest{}, &EmailRequest{})
...
err = msgRequest.Data.SendMessage()
...
}
With Box, de-serialisisation of the representation of the struct implementing the interface MessageRequest is deferred until I explicitly Unbox it, providing concrete types that satisfy the interface and might be inside the Box.
After Unbox-ing, my activity can work with the interface as if it received it as an argument directly.
Creating a new Box
box, err := NewBox[MessageRequest](&SlackMessageRequest{
...
})
How a concrete type is chosen for Unbox-ing
Unbox iteratively attempts to de-serialise the raw JSON representation of the object inside the box into each of the concrete types given, stopping at the first success.
What objects can be put inside a Box
An object can be boxed if it satisfies the following interface:
type Boxable interface {
Unbox(json.RawMessage) error
}
A straight-forward way to implement this interface is to delegate to the default json.Unmarshal function.
