# Parallel
URL: /reference/blocks/system-parallel
Type: reference
Description: Execute a branch once per item and then continue after join
Keywords: parallel, system/parallel, system, block, workflow builder
`system/parallel` — Execute a branch once per item and then continue after join.

Where it appears [#where-it-appears]

The **Parallel** block lives in the **Logic & control** group of the Workflow Builder.

Fields [#fields]

| Field         | Type                                   | Required | Default      | Description                                                                                                                                                                                   |
| ------------- | -------------------------------------- | -------- | ------------ | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| `items`       | text (multi-line, supports references) | Yes      | —            | The array fanned out across branches — one branch runs per element. Accepts a literal JSON array or a \<PreviousStep.field> reference to an upstream array output.                            |
| `maxBranches` | text (supports references)             | No       | —            | Hard cap on total branches. Items beyond this index are skipped — useful as a safety net when the input array could be unexpectedly large. Default: 100.                                      |
| `concurrency` | text (supports references)             | No       | —            | How many branches run at the same time. Subsequent branches start as in-flight branches finish, until all branches complete. Lower this to throttle downstream APIs. Default: 5.              |
| `joinPolicy`  | select                                 | No       | `allSuccess` | How the join after the branches behaves. All Success only continues past the join if every branch succeeded; Always continues regardless of branch failures. Options: `allSuccess`, `always`. |

Inputs and outputs [#inputs-and-outputs]

Inputs are the configurable fields above. The block produces these outputs:

**Outputs**

Reference an output downstream with `<Parallel.field>`.

| Name       | Type   | Description                 |
| ---------- | ------ | --------------------------- |
| `items`    | array  | Items iterated over         |
| `branches` | number | Number of branches executed |

Example [#example]

A minimal configuration for this block:

```json
{
  "type": "system/parallel",
  "config": {
    "items": "<PreviousStep.field>",
    "joinPolicy": "allSuccess"
  }
}
```

Limits and failure modes [#limits-and-failure-modes]

* Required field (`items`) must be set, or the block fails validation before it runs.

Related [#related]

* [All workflow blocks](/reference/blocks)
* [Workflow Builder overview](/workflow-builder-overview)