Encore Go Pub/Sub

Instructions

Pub/Sub is for asynchronous messaging between services. Producers publish events to a Topic; consumers attach Subscriptions to react. Resources must be declared as package-level variables — never inside functions.

Topics

package events

import "encore.dev/pubsub"

type OrderCreatedEvent struct {
    OrderID string `json:"order_id"`
    UserID  string `json:"user_id"`
    Total   int    `json:"total"`
}

// Package level declaration
var OrderCreated = pubsub.NewTopic[*OrderCreatedEvent]("order-created", pubsub.TopicConfig{
    DeliveryGuarantee: pubsub.AtLeastOnce,
})

Publishing

msgID, err := events.OrderCreated.Publish(ctx, &events.OrderCreatedEvent{
    OrderID: "123",
    UserID:  "user-456",
    Total:   9999,
})

Subscriptions

package notifications

import (
    "context"
    "myapp/events"
    "encore.dev/pubsub"
)

var _ = pubsub.NewSubscription(events.OrderCreated, "send-confirmation-email",
    pubsub.SubscriptionConfig[*events.OrderCreatedEvent]{
        Handler: sendConfirmationEmail,
    },
)

func sendConfirmationEmail(ctx context.Context, event *events.OrderCreatedEvent) error {
    // Send email...
    return nil
}

Topic References

Pass topic access to library code while maintaining static analysis:

// Create a reference with publish permission
ref := pubsub.TopicRef[pubsub.Publisher[*OrderCreatedEvent]](OrderCreated)

// Use the reference in library code
func publishEvent(ref pubsub.Publisher[*OrderCreatedEvent], event *OrderCreatedEvent) error {
    _, err := ref.Publish(ctx, event)
    return err
}

Delivery Guarantees

• pubsub.AtLeastOnce (default): may deliver duplicates → handlers must be idempotent.
• pubsub.ExactlyOnce: stricter, capped throughput (AWS 300 msg/s/topic, GCP 3000+ msg/s/region). Does not deduplicate on the publish side.

Guidelines

• Topics and subscriptions must be declared as package-level variables.
• Subscription handlers must be idempotent (at-least-once delivery is the default).
• Subscription handlers receive a context.Context and the event pointer; return an error to retry per the topic's retry policy.
• Don't do heavy synchronous work in Publish callers — Publish returns once the message is queued.