Event acknowledgement

• Start Date: 2019-09-04
• Target Version: (0.14 / 0.15)
• Reference Issues: (fill in existing related issues, if any)
• Implementation PR: (leave this empty)

Summary

Implement acknowledgement for events.

Basic example

In caller side

await this.broker.emit("user.created", { user }, { ack: true });

In event handler side

// activation.service.js
module.exports = {
    name: "activation",

    events: {
        async "user.created"(ctx) {
            try {
            await this.activateUser(ctx.params.user);
            
            // Acknowledges the received event.
            ctx.ack();
            } catch(err) {
                // Negative acknowledges the received event.
                ctx.nack();
            }
        }
    }
};

Motivation

TODO

Detailed design

TODO

Drawbacks

TODO

Alternatives

TODO

Adoption strategy

TODO

Unresolved questions

• What happens with broadcast messages. Sending to all services?
• What happens if mixins register the same event listener, so the service has multiple handlers for the same event?
• How the caller side can check the acknowledgement result that it was successful or not?
• What happens, if one listener was successful and other was failed?

Start with the caller-side broker.emit example and the event-handler example in this issue. Define the acknowledgement behavior for broadcast messages, multiple listeners, and mixed success or failure, then specify how the caller observes the result; the issue is done when those unresolved questions have a decided design.