Google Cloud Pub/Sub

Google Cloud Pub/Sub provides managed messaging for event sourcing on GCP with automatic scaling.

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Why Pub/Sub

Strength	Benefit
Fully managed	No infrastructure to operate
Global	Multi-region by default
Automatic scaling	Handles traffic spikes
GCP integration	Dataflow, BigQuery, Cloud Functions
At-least-once	Guaranteed delivery

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Trade-offs

Concern	Consideration
GCP lock-in	Not portable to other clouds
Cost	Pay per message/data
Ordering	Requires ordering keys

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Configuration

[bus]
backend = "pubsub"

[bus.pubsub]
project_id = "my-gcp-project"
topic_prefix = "angzarr"

Environment Variables

export PUBSUB_PROJECT_ID="my-gcp-project"
export PUBSUB_TOPIC_PREFIX="angzarr"
export BUS_BACKEND="pubsub"
export GOOGLE_APPLICATION_CREDENTIALS="/path/to/service-account.json"

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Topic/Subscription Layout

flowchart LR
    TP["Topic: angzarr-events-player"] --> PP[Subscription: player-projector]
    TP --> OP1[Subscription: output-projector]
    TH["Topic: angzarr-events-hand"] --> HS[Subscription: hand-saga]
    TH --> OP2[Subscription: output-projector]

Message Ordering

Enable ordering with ordering keys:

// Publish with ordering key (aggregate root)
publisher.publish_with_ordering_key(
    event_bytes,
    &format!("{}#{}", domain, root),
).await?;

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Subscription Configuration

# Terraform/gcloud configuration
subscription:
  name: player-projector
  topic: angzarr-events-player
  ack_deadline_seconds: 60
  message_retention_duration: 604800s  # 7 days
  enable_message_ordering: true
  dead_letter_policy:
    dead_letter_topic: angzarr-dlq
    max_delivery_attempts: 5

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Dead Letter Handling

Failed messages route to a dead letter topic:

flowchart LR
    T["angzarr-dlq (topic)"] --> S["dlq-monitor (subscription)"]

Monitor via Cloud Console or subscribe programmatically.

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Setup

# Create topics
gcloud pubsub topics create angzarr-events-player
gcloud pubsub topics create angzarr-events-hand
gcloud pubsub topics create angzarr-dlq

# Create subscriptions
gcloud pubsub subscriptions create player-projector \
  --topic=angzarr-events-player \
  --enable-message-ordering \
  --dead-letter-topic=angzarr-dlq \
  --max-delivery-attempts=5

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Helm Deployment

values.yaml

bus:
  backend: pubsub

pubsub:
  enabled: true
  projectId: my-gcp-project
  credentials:
    secretName: pubsub-credentials
    keyFile: service-account.json

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When to Use Pub/Sub

• GCP native — Already on Google Cloud
• Serverless — Cloud Functions/Run integration
• Global reach — Multi-region by default
• Managed — No operational overhead

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Next Steps

• SNS/SQS — AWS equivalent
• Kafka — Self-managed alternative