> ## Documentation Index
> Fetch the complete documentation index at: https://docs.sndbox.app/llms.txt
> Use this file to discover all available pages before exploring further.

# Concurrency and queues

> Bound parallel node work, runner capacity, event claims, and dead-letter behavior.

Concurrency exists at several independent layers.

| Layer | Boundary |
| - | - |
| Workflow | `maxConcurrentNodes` bounds nodes eligible to run together. |
| Runner | Reported concurrency and current workload bound local claims. |
| Pool | Aggregate pool concurrency reserves assignment capacity. |
| Event queue | Visibility claims stop two workers from owning the same current delivery. |
| Scheduler | Advisory and row locking serialize occurrence emission. |

Parallel graph branches still depend on typed inputs and control edges. Do not depend on incidental canvas order when two nodes are eligible together.

## No available runner

Remote schedules and queue events apply the configured no-runner and expiry behavior. An offline, stale, draining, incompatible, disabled, or capacity-exhausted runner is visible in rejection diagnostics but cannot claim work.

## Poison events

Repeated identical failures are bounded. After three identical failures or the configured attempt limit, the event moves to dead-letter state. Replay is a new audited delivery, not deletion of the failed evidence.


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