> For the complete documentation index, see [llms.txt](https://developer.harness.io/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://developer.harness.io/continuous-delivery/use-continuous-delivery/manage-deployments/controlling-deployments-with-barriers-resource-constraints-and-queue-steps.md).

# Controlling resource usage with Barriers, Resource Constraints, and Queue steps

Harness has a number of ways to control deployments and resource usage to protect resources and their capacity limits. This topic describes their differences and provides best practices.

### Control Option Definitions <a href="#control-option-definitions" id="control-option-definitions"></a>

The following options help you control how deployments use your resources.

#### Barriers <a href="#barriers" id="barriers"></a>

Barriers synchronize stages/[step groups](/continuous-delivery/use-continuous-delivery/cd-building-blocks/cd-steps/step-groups.md) of a Pipeline that are executed in parallel. Barriers ensure that one stage/step group does not proceed until another stage/step group has reached a specific point.

See [Synchronize Parallel Stages and Step Groups using Barriers](/continuous-delivery/use-continuous-delivery/manage-deployments/synchronize-deployments-using-barriers.md).

**Barriers Scope**

Barriers are scoped to the **Pipeline**.

You can only use the same Barrier within the same Pipeline. If you add a **Barrier Name** from one Pipeline into the Barrier step in another Pipeline, it will not work.

#### Resource Constraints <a href="#resource-constraints" id="resource-constraints"></a>

Resource Constraints protect resource capacity limits by preventing simultaneous deployments to the same Service + Infrastructure combination. The Harness service + infrastructure combination acts as a fixed key called the Infrastructure Key. The Infrastructure Key (the unique key used to restrict concurrent deployments) is formed using a combination of Harness service Id + environment Id + connector Id + infrastructure Id.

By default, this key is scoped at the Harness account level. This means if the infrastructure is reused in different Harness projects with the same combination of service Id + environment Id + connector Id + infrastructure Id, it can result in queuing of the infrastructure. Consequently, your pipelines will build up in a waiting state.

Resource Constraints are added to every Stage by default, but it can be disabled in a Stage's **Infrastructure** settings by enabling the **Allow simultaneous deployments on the same infrastructure** option.

See [Pipeline Resource Constraints](/continuous-delivery/use-continuous-delivery/manage-deployments/deployment-resource-constraints.md).

The automatic **Resource Constraints** setting does not apply to [Custom stages](/harness-ai/use-harness-platform/pipelines/add-a-stage.md#add-a-custom-stage). **Resource Constraints** apply to a combination of Service + Infrastructure, and Custom Stages have no Services or Infrastructures. You can use Barriers and Queue steps in any stage types.

{% hint style="info" %}
**NOTE**

When an environment is added to a Custom Stage, resource constraints do apply, and jobs may get queued based on the environment and infrastructure combination.
{% endhint %}

#### Resource constraint scoping at the project level <a href="#resource-constraint-scoping-at-the-project-level" id="resource-constraint-scoping-at-the-project-level"></a>

{% hint style="info" %}
Currently, the dynamic provisioning documented in this topic is behind the feature flag `CDS_PROJECT_SCOPED_RESOURCE_CONSTRAINT_QUEUE`. Contact [Harness Support](mailto:support@harness.io) to enable the feature.
{% endhint %}

By default resource constraints are scoped at a account level. This means if the infrastructure is reused in different Harness projects with the same combination of service Id + environment Id + connector Id + infrastructure Id, it can result in queuing of the infrastructure. Consequently, your pipelines will build up in a waiting state.

With Resource constraint scoping at the project level, users can now scope the resource constraint down to the individual project.

This scoping ensures that the resource constraint is scoped to a particular project and doesn't impact other projects using the exact same combination of service Id + environment Id + connector Id + infrastructure Id.

With project-level scoping, the Infrastructure key (the unique key used to restrict concurrent deployments) is now formed with the Harness account Id + org Id + project Id + service Id + environment Id + connector Id + infrastructure Id.

This scoping ensures that there are unique resources across the various projects and prevents a queue because of other project teams deploying into the same infrastructure.

#### Queue Steps <a href="#queue-steps" id="queue-steps"></a>

Queue Steps synchronize Pipeline executions, queuing Pipelines using a custom key.

Multiple deployments are queued so that one deployment completes before other deployments with the same key can proceed.

See [Control Resource Usage with Queue Steps](/continuous-delivery/use-continuous-delivery/manage-deployments/control-resource-usage-with-queue-steps.md).

**Queue Step Scope**

Queue Step are scoped to your entire Harness **account**.

If there are Queue steps with the same Resource Key in Pipelines in Projects A and B and the Pipeline in Project A is running, the Pipeline in Project B will be queued until the Pipeline in Project A is completed.

### Best Practices <a href="#best-practices" id="best-practices"></a>

Here are some best practices to help you decide which resource control option to use:

* **Barriers:** use when you want to coordinate the timing of different components’ deployments with stages/step groups executed in parallel in the same Pipeline.\
  For example, your Pipeline executes Stages A and B in parallel, but you want a database migration in Stage A to complete before a deployment in Stage B. You place a Barrier step after the migration in Stage A and before the deployment step in Stage B.
* **Resource Constraints:** use when you want to prevent simultaneous deployments to the same Service + Infrastructure combination. This feature is enabled by default.
* **Queue steps:** use when you want to control the **sequence** of multiple Pipeline executions. This can be used on different Pipelines or even multiple executions of the same Pipeline.