---
title: Parallel Scopes
description: Run independent pull requests through the merge queue simultaneously to merge faster in monorepos.
---

By default, Mergify's merge queue operates in **serial mode**: every pull request is tested on top
of the previous one, forming a single ordered pipeline. This guarantees correctness but means
unrelated changes wait for each other.

**Parallel mode** removes that constraint. When two pull requests touch different areas of the
codebase — different **scopes** — Mergify tests and merges them independently, at the same time. Pull
requests that do share a scope are still queued together so they are tested as a group, preventing
semantic conflicts.

:::tip
  Parallel mode is designed for monorepos and large repositories where pull requests frequently
  change independent parts of the codebase. If most of your pull requests touch the same files,
  serial mode with [batching](/merge-queue/batches) may be a better fit.
:::

## Serial vs. Parallel at a Glance

In **serial mode**, every batch depends on the one before it. Even if PR #3 (docs) has nothing in
common with PR #1 (api) or PR #2 (frontend), it still waits:

```dot class="graph"
strict digraph {
  fontname="sans-serif";
  rankdir="LR";
  label="Serial Mode — Single Queue";
  nodesep=0.6;
  ranksep=0.8;

  node [shape=box, style="rounded,filled", fillcolor="#347D39", fontcolor="white", fontname="sans-serif", margin="0.3,0.18"];
  edge [color="#374151", arrowhead=normal, penwidth=1.2, fontname="sans-serif"];

  PR1 [label="Batch 1\nPR #1 (api)"];
  PR2 [label="Batch 2\nPR #2 (frontend)"];
  PR3 [label="Batch 3\nPR #3 (docs)"];

  PR1 -> PR2 -> PR3;
}
```

In **parallel mode**, Mergify groups pull requests by scope. Batches that share no scope run at the
same time:

```dot class="graph"
strict digraph {
  fontname="sans-serif";
  rankdir="TB";
  label="Parallel Mode\nIndependent Scope Queues";
  nodesep=0.8;
  ranksep=0.6;

  node [shape=box, style="rounded,filled", fontcolor="white", fontname="sans-serif", margin="0.3,0.18"];
  edge [style=invis];

  subgraph cluster_running {
    style="rounded,filled";
    fillcolor="#1CB893";
    color="#1CB893";
    fontcolor="#000000";
    label="Tested simultaneously";

    PR1 [label="Batch 1\nPR #1 (api)", fillcolor="#347D39"];
    PR2 [label="Batch 2\nPR #2 (frontend)", fillcolor="#347D39"];
    PR3 [label="Batch 3\nPR #3 (docs)", fillcolor="#347D39"];
  }

  { rank=same; PR1; PR2; PR3; }
}
```

When scopes **do** overlap, Mergify preserves ordering within that scope to guarantee the changes are
tested together:

```dot class="graph"
strict digraph {
  fontname="sans-serif";
  rankdir="LR";
  label="Parallel Mode — Overlapping Scopes Create Dependencies";
  nodesep=0.6;
  ranksep=1.0;

  node [shape=box, style="rounded,filled", fontcolor="white", fontname="sans-serif", margin="0.3,0.18"];
  edge [color="#374151", arrowhead=normal, penwidth=1.2, fontname="sans-serif"];

  PR1 [label="Batch 1\nPR #1 (api)", fillcolor="#347D39"];
  PR4 [label="Batch 2\nPR #4 (api, frontend)", fillcolor="#347D39"];
  PR3 [label="Batch 3\nPR #3 (docs)", fillcolor="#347D39"];

  PR1 -> PR4 [label="same scope: api"];

  { rank=same; PR1; PR3; }
}
```

Here PR #4 touches the `api` scope, just like PR #1 — so it must wait for PR #1 to merge first.
Meanwhile PR #3 (docs) proceeds independently.

## Setting Up Parallel Mode

Parallel mode requires two things: switching the queue mode and configuring
[scopes](/merge-queue/scopes) so Mergify knows which areas of the codebase each pull request
touches.

### 1. Define scopes

Scopes can come from file patterns declared directly in `.mergify.yml`, or from an external build
system (Nx, Bazel, Turborepo, …) via the
[`gha-mergify-ci`](https://github.com/Mergifyio/gha-mergify-ci) GitHub Action.

```yaml
scopes:
  source:
    files:
      api:
        include:
          - services/api/**/*
      frontend:
        include:
          - apps/web/**/*
      docs:
        include:
          - docs/**/*
```

See [Scopes](/merge-queue/scopes) for all configuration options and build-tool integrations.

### 2. Enable parallel mode

Add `mode: parallel` under `merge_queue`:

```yaml
merge_queue:
  mode: parallel
  max_parallel_checks: 5

scopes:
  source:
    files:
      api:
        include:
          - services/api/**/*
      frontend:
        include:
          - apps/web/**/*
      docs:
        include:
          - docs/**/*

queue_rules:
  - name: default
    batch_size: 3
    queue_conditions:
      - check-success = ci
```

The `max_parallel_checks` setting controls how many batches Mergify tests at the same time across
all scope queues. Tune it to match your CI capacity.

## How It Works

Once parallel mode is active, the merge queue follows these steps whenever it processes pull
requests:

1. **Scope assignment.** Each pull request is tagged with the scopes it affects, either
   automatically from file patterns or via an external upload.

2. **Batch formation.** Mergify groups pull requests that share **exactly the same set of scopes**
   into batches (respecting `batch_size`). Pull requests with different scopes form separate
   batches.

3. **Dependency tracking.** Batches that share at least one scope are linked as parent → child in a
   dependency graph. A child batch cannot merge until all its parents have merged.

4. **Parallel execution.** Batches with no shared scopes — and therefore no dependency — are tested
   by CI at the same time, up to `max_parallel_checks`.

5. **Merge.** As soon as a batch's CI passes and all its parent batches are merged, Mergify merges
   the pull requests in that batch.

### What happens when a batch fails?

The failure handling works the same way as in serial mode: Mergify splits the failed batch and
retests the parts to isolate the problematic pull request. See
[Handling Batch Failures](/merge-queue/batches#handling-batch-failure-or-timeout) for details.

Because batches in parallel mode are scoped, a failure in one scope queue does **not** block
unrelated scope queues. Only batches that depend on the failed one (via shared scopes) are affected.

## Limiting Concurrency per Scope

`max_parallel_checks` caps how many speculative checks run at once across **all** scopes. Sometimes
you want to bound a **single** scope on top of that: a scope whose tests are expensive or hit a shared
resource that can't take many concurrent runs (a staging environment or a rate-limited external
service), while the rest of your scopes can use whatever capacity the global ceiling leaves them.

`scopes.capacities` maps a scope name to the number of speculative checks that scope may run at the
same time:

```yaml
merge_queue:
  mode: parallel
  max_parallel_checks: 5

scopes:
  source:
    files:
      frontend:
        include:
          - apps/web/**/*
      backend:
        include:
          - services/api/**/*
      docs:
        include:
          - docs/**/*
  capacities:
    frontend: 2
    backend: 2
```

Here `frontend` and `backend` are each limited to 2 concurrent speculative checks. `docs` is absent
from the map, so it stays uncapped: only the global ceiling applies to it.

### How capacities relate to the global ceiling

`max_parallel_checks` is the **global ceiling**: the most speculative checks a train will ever run at
once. Each `scopes.capacities` entry is a **sub-limit inside that ceiling, not an extra budget on
top of it**:

- A speculative check consumes one global slot **and** one slot in every capped scope its batch
  belongs to.

- It starts only when the global ceiling has room **and** each of its capped scopes has room.

- A scope that isn't listed in `capacities` is unlimited; it draws on the global ceiling alone.

Because every check always takes a global slot, the total running at once **never exceeds
`max_parallel_checks`**, whatever you put in `capacities`. Capacities can only ever hold a scope
below the global ceiling; they never raise the total, so adding them to an existing configuration
cannot increase your CI load.

### Worked example

Take the configuration above (`max_parallel_checks: 5`, `frontend: 2`, `backend: 2`, `docs`
uncapped) and suppose the queue is ready to test three `frontend` batches, three `backend` batches,
and two `docs` batches. The slots might fill like this:

```dot class="graph"
strict digraph {
  fontname="sans-serif";
  rankdir="TB";
  label="Per-scope capacities — ceiling 5, frontend: 2, backend: 2, docs uncapped";
  nodesep=0.5;
  ranksep=0.7;

  node [shape=box, style="rounded,filled", fontcolor="white", fontname="sans-serif", margin="0.3,0.18"];
  edge [style=invis];

  subgraph cluster_running {
    style="rounded,filled";
    fillcolor="#1CB893";
    color="#1CB893";
    fontcolor="#000000";
    label="Running now — 5 of 5 slots used";

    F1 [label="frontend #1", fillcolor="#347D39"];
    F2 [label="frontend #2", fillcolor="#347D39"];
    B1 [label="backend #1", fillcolor="#347D39"];
    B2 [label="backend #2", fillcolor="#347D39"];
    D1 [label="docs #1", fillcolor="#347D39"];

    { rank=same; F1; F2; B1; B2; D1; }
  }

  subgraph cluster_waiting {
    style="rounded";
    color="#6B7280";
    fontcolor="#6B7280";
    label="Waiting";

    F3 [label="frontend #3\nfrontend full", fillcolor="#6B7280"];
    B3 [label="backend #3\nbackend full", fillcolor="#6B7280"];
    D2 [label="docs #2\nceiling full", fillcolor="#6B7280"];

    { rank=same; F3; B3; D2; }
  }
}
```

- `frontend` and `backend` each run at most 2 batches, so each holds back its third; those wait for
  a free slot in their own scope.

- `docs` is uncapped, but only as many `docs` batches run as there is room under the ceiling of 5.
  Here that is 1, so the second `docs` batch waits, not because `docs` is capped (it isn't) but
  because the global ceiling is full.

Two things always hold: no capped scope runs more than its limit, and no more than
`max_parallel_checks` run at once. Exactly which batches fill the slots, and whether the last free
slot goes to `docs` or to a capped scope still below its limit, follows queue order, so the split
can differ from one cycle to the next. As soon as a running check finishes, its freed global slot
(and its freed scope slot, if any) go to the next waiting batch that fits both.

### Pull requests in several scopes

A batch that touches more than one capped scope must fit in **all** of them at once. A batch
carrying both `frontend` and `backend` consumes one `frontend` slot and one `backend` slot, and
starts only when both scopes, and the global ceiling, have room. This keeps every scope's limit
honored even when changes span scopes.

### Source-agnostic

`capacities` only sets the limit; it does not decide which pull requests belong to a scope.
Membership comes from your [`scopes.source`](/merge-queue/scopes), so capacities behave the same
whether scopes are derived from [file patterns](/merge-queue/scopes/file-patterns) (`source: files`)
or pushed from an external build system (`source: manual`). You declare the limit once, no matter how
membership is computed.

:::note
  Strict branch protection (*Require branches to be up to date before merging*) clamps the whole
  train to one check at a time, so each scope's effective capacity becomes 1 and `capacities` has no
  further effect. See [Require Branches to Be Up to
  Date](/merge-queue/github-rulesets#require-branches-to-be-up-to-date).
:::

## The Monorepo Trade-Off

Parallel mode is built for the reality of monorepos: most pull requests are independent, but some
do interact.

| Scenario | What happens | Benefit |
|----------|-------------|---------|
| PRs touch **different** scopes | Tested and merged in parallel | Faster merge times — no waiting for unrelated work |
| PRs touch **the same** scope | Ordered within that scope queue and tested together | Conflicts caught before merge |
| A PR touches **multiple** scopes | Linked to all relevant scope queues | Correctness preserved across scopes |

The net effect: **pull requests merge faster when their scopes don't collide**, while pull requests
that do collide are still tested in the right order to avoid semantic conflicts reaching your main
branch.

## Compatibility and Limitations

Parallel mode changes how the queue operates. Some features that rely on strict single-queue
ordering are not available:

- **Scopes are required.** You must configure `scopes.source` (either `files` or `manual`).
  Without scopes, Mergify cannot determine which pull requests are independent.

- **`fast-forward` merge is not supported.** Because batches merge independently, Mergify needs to
  rebase them. Use `merge` or `rebase` as your `merge_method`.

- **`skip_intermediate_results` is not available.** This feature depends on the strict cumulative
  ordering of serial mode.

## Next Steps

- [Scopes](/merge-queue/scopes): learn how to define and manage scopes for your repository.

- [Batches](/merge-queue/batches): understand batch formation, sizing, and failure handling.

- [Performance](/merge-queue/performance): tune your queue for the right balance of speed, cost,
  and reliability.

- [Monorepo](/merge-queue/monorepo): broader guidance on using Mergify in monorepo setups.