# Subscription
Fire-and-forget handlers that listen to events and react without blocking the emitter.
---
Canonical: /handbook/blocks/subscription-pattern/
Source: web/src/content/handbook-cards/blocks/subscription-pattern.mdx
Format: Markdown for agents
---
A **subscription** is a fire-and-forget handler that listens to events and reacts without blocking the emitter. Use subscriptions for cross-service communication, event-driven workflows, async side effects, or any flow where the caller does not need an immediate response.
Subscriptions live inside services alongside [commands](/handbook/blocks/command-pattern/) and [streams](/handbook/blocks/stream-pattern/). The service builder collects subscription definitions, and the event bridge routes matching events to the right handler.
Fire-and-forget event listeners
Subscriptions do not return a response to the caller. They either acknowledge the event (ack), schedule a retry, send it to a dead-letter queue, or drop it. If you need request/response, use a command instead.
## Quick reference
### Subscription lifecycle
```mermaid
flowchart LR
A[Event Received] --> B[Filter Match?]
B -->|No| C[Event Ignored]
B -->|Yes| D[Before Guards]
D --> E[Subscription Function]
E --> F[Return Outcome]
F -->|ack| G[Message Acknowledged]
F -->|retry| H[Retry Later]
F -->|deadLetter| I[Dead Letter Queue]
F -->|drop| J[Event Dropped]
F -->|stop-consumer| K[Consumer Stopped]
```
### Key principles
- Subscriptions are **fire-and-forget** — they do not return a response to the caller.
- Handler functions must return an **explicit outcome**: `{ status: 'ack' | 'retry' | 'deadLetter' | 'drop' | 'stop-consumer' }`.
- Use **`async function`**, not arrow functions, to preserve `this` context.
- **Before guards run in parallel** — any guard that throws cancels the handler.
- **After guards are handled by the Service runtime** — you define them on the builder but the service orchestrates them.
- Filter which events to process with `subscribeToEvent()`, `filterSentFrom()`, `filterReceivedBy()`, etc.
- Bridge advice (`adviceDurable`, `adviceAutoacknowledgeMessage`, etc.) hints at infrastructure behavior but the bridge decides.