# YAML Exec Concurrency

The `settings.functions.exec` section controls how many [`!exec`](/functions/yaml/exec) expressions can run
at the same time during a document evaluation. Existing stack syntax stays unchanged.

## Configuration

**File:** `atmos.yaml`

```yaml
settings:
  functions:
    exec:
      max_concurrency: 4
```

- **`settings.functions.exec.max_concurrency`**

  A positive integer. Defaults to `1`, preserving serial execution. Set it above `1` to allow up to that
  many `!exec` calls to overlap during one document evaluation. Values below `1` are invalid.

## Example

With a limit of `4`, these three expressions can run concurrently:

```yaml
vars:
  network: !exec ./lookup-network
  cluster: !exec ./lookup-cluster
  registry: !exec ./lookup-registry
```

Each result is assigned to its original location, regardless of completion order. Existing output decoding
still applies, including JSON objects and arrays. No new tag or grouping syntax is needed.

Atmos does not infer dependencies between shell commands. When enabling concurrency, you are responsible
for coordinating commands that share files or other external state. Set the limit to `1` when serial execution
is required.

Commands inherit Atmos standard input. Concurrent commands share that input stream; input is not broadcast
to every command, and which command reads each part is not guaranteed. Coordinate commands that read stdin,
or set the limit to `1` when they must read serially.

Cancellation stops outstanding execution. Errors identify the expression's location. Skipping function
processing or using `--skip=exec` continues to prevent the corresponding commands from running.

## Related

- [`!exec`](/functions/yaml/exec) syntax and output decoding.
- [`settings.functions`](/cli/configuration/settings/functions) configuration.