Capacity-Based Stage Queuing

Control concurrent stage execution on the new Harness Delegate using MAX_STAGES and server-side queuing.

CLOSED BETA

Delegate 3.x is currently in closed beta and available only to select users. The product team determines access based on current supported use cases. See Feature Parity for the latest details.

Capacity-based stage queuing lets you limit the number of concurrent Local (Docker) Infra stages a Delegate 3.x instance can run at once. When every eligible delegate is at capacity, new stage requests are queued server-side and dispatched automatically as capacity frees up. This prevents resource exhaustion on the delegate host and ensures predictable pipeline performance.

SCOPE

Capacity-based stage queuing applies only to Local (Docker) Infra stages running on Delegate 3.x. Everything on this page assumes that context unless stated otherwise.

Configure MAX_STAGES

To limit concurrent stage executions, set the MAX_STAGES environment variable in your delegate's config.env file. The value must be a positive integer.

ACCOUNT_ID="<ACCOUNT_ID>"
TOKEN="<DELEGATE_TOKEN>"
TAGS="macos-arm64,local"
URL="https://app.harness.io"
NAME="DelegateTest"
MAX_STAGES=1

This configuration allows the delegate to execute only one Local (Docker) Infra stage at a time. Any additional stage requests are queued until the running stage completes.

If MAX_STAGES is not set, the number of concurrent stage executions is unbounded.

For full installation and configuration details, go to Delegate Configuration Reference.

Queued stage visibility

When no eligible delegate has free capacity, the stage's Initialize step displays a log message indicating the stage is queued.

KNOWN LIMITATION

A queued stage currently appears as Running in the pipeline execution view even though it is waiting for delegate capacity at the Initialize task level. A distinct Queued status will be available in a future release.

Use cases

Local (Docker) Infra stages run directly on the delegate's host machine. Steps inside these stages can be container-based or containerless. Capacity limits are especially useful in two scenarios.

Prevent resource exhaustion

Without capacity limits, a delegate accepts any number of concurrent stages. If many stages land on the same delegate simultaneously, the host machine can run out of CPU, memory, or disk I/O, causing slowness and instability across all running pipelines. Setting MAX_STAGES to an appropriate value keeps resource consumption predictable.

Avoid tool concurrency conflicts

Some build tools rely on global system-level configurations or file locks that prevent parallel use. For example, Bazel supports only a single build per workspace by default. Running two stages that both invoke Bazel in the same workspace causes one of them to fail.

Setting MAX_STAGES=1 serializes stage execution on that delegate, eliminating conflicts between concurrent stages.

All Harness-native steps run in isolated working directories and do not interfere with one another when executing concurrently. Concurrency conflicts arise only when custom or third-party tools used in a Run step share global state or workspace locks.

How stages are distributed

When a stage execution starts, Harness assigns it to an eligible delegate based on available capacity. All Initialize tasks are submitted with NORMAL priority by default.

Free capacity calculation

Free capacity for a delegate is calculated as:

Free Capacity = MAX_STAGES − (number of stages currently executing)

Assignment rules

• Capacity available (Free Capacity > 0): The stage is assigned to the eligible delegate with the highest free capacity. This distributes work evenly across delegates.

• No capacity available (Free Capacity = 0): The stage is queued and assigned to the first eligible delegate that finishes an in-progress stage.

Queue ordering

When multiple stages are queued, they are assigned to delegates in the following order:

1. Priority: Stages with HIGH priority are assigned before stages with NORMAL priority, which are assigned before stages with LOW priority.

2. Age: Among stages of equal priority, older stages are assigned first (FIFO).

---

Manage queue priority

Queue management APIs are behind the feature flag PL_STAGE_QUEUE_API. Contact Harness Support to enable this feature for your account.

Harness provides APIs to view queued stages and adjust their execution priority. These APIs help you respond to urgent situations where a critical deployment or hotfix needs to bypass the normal queue order.

When to use queue prioritization

Queue prioritization is useful when you have limited delegate capacity and need to expedite specific stages ahead of others already waiting in the queue. Common scenarios include:

• A hotfix deployment needs to run immediately while multiple PR builds are queued

• A critical release pipeline must take precedence over routine CI jobs

• An urgent customer issue requires immediate testing or deployment

Without queue management, all stages execute strictly in order based on their original submission time. With queue management enabled, authorized users can promote urgent work to the front of the queue.

View queued and running stages

The List Queued Stages API shows all stages currently queued or executing across your delegates. This visibility helps you understand what work is pending and identify which stages might benefit from priority adjustment.

Endpoint:

GET /v2/stages/queue

Required parameters:

• accountIdentifier - Your Harness account ID

• x-api-key - Your API key for authentication

Optional filters:

You can scope the queue view to specific organizational boundaries or filter by delegate or status.

Parameter	Description	Example
orgIdentifier	Show only stages from this organization	default
projectIdentifier	Show only stages from this project (requires orgIdentifier)	mobile_team
status	Filter by QUEUED, RUNNING, or ALL	QUEUED
delegateName	Show only stages for this delegate	mac-runner-01
sort	Order by priority_time (default) or time	priority_time

Example request:

curl -X GET \
  'https://app.harness.io/gateway/pipeline/api/v2/stages/queue?accountIdentifier=YOUR_ACCOUNT_ID&status=QUEUED' \
  -H 'x-api-key: YOUR_API_KEY'

Example response:

Queue listing API response showing queued and running stages

{
  "status": "SUCCESS",
  "data": {
    "stages": [
      {
        "pipelineIdentifier": "build_ios_app",
        "pipelineName": "Build iOS App",
        "pipelineExecutionId": "exec_abc123def456",
        "stageIdentifier": "build_stage",
        "stageName": "Build Stage",
        "status": "QUEUED",
        "priority": "HIGH",
        "queuePosition": 1,
        "orgIdentifier": "default",
        "projectIdentifier": "mobile_team",
        "triggeredBy": {
          "name": "Jane Smith",
          "email": "[e-mail]"
        },
        "triggerType": "MANUAL",
        "eligibleDelegates": [
          {
            "name": "mac-runner-01",
            "hostName": "mac-mini-01.local"
          }
        ],
        "executingDelegate": null,
        "createdAt": 1709567890000,
        "queuedDuration": "2m 34s"
      },
      {
        "pipelineIdentifier": "pr_validation",
        "pipelineName": "PR Validation",
        "pipelineExecutionId": "exec_def789ghi012",
        "stageIdentifier": "test_stage",
        "stageName": "Test Stage",
        "status": "RUNNING",
        "priority": "NORMAL",
        "queuePosition": null,
        "orgIdentifier": "default",
        "projectIdentifier": "mobile_team",
        "executingDelegate": {
          "name": "mac-runner-01",
          "hostName": "mac-mini-01.local"
        },
        "createdAt": 1709567800000,
        "queuedDuration": null
      }
    ],
    "totalQueued": 1,
    "totalRunning": 1,
    "pageIndex": 0,
    "pageSize": 50,
    "totalItems": 2
  }
}

Key response fields:

Field	Description
pipelineExecutionId	Unique execution ID that links to the pipeline execution in Harness UI
stageIdentifier	Stage identifier from your pipeline YAML
priority	Current priority: HIGH, NORMAL, or LOW
queuePosition	Position in queue (1 = next to execute). Null for running stages
eligibleDelegates	Delegates that can run this stage (shown for queued stages)
executingDelegate	Delegate currently running this stage (shown for running stages)
queuedDuration	How long the stage has been waiting

The default sort order (priority_time) shows stages exactly as they will be picked up - highest priority first, then oldest within each priority level.

Change stage priority

The Update Stage Priority API lets you change the priority of queued stages. Increasing a stage's priority moves it ahead of other queued stages with lower priority.

Only stages in QUEUED status can be reprioritized. Running stages cannot be interrupted or reprioritized.

Endpoint:

PUT /v2/stages/queue/priority

Request body:

You identify stages using the pipeline execution ID and stage identifier returned by the List API.

Update priority API request payload

{
  "accountIdentifier": "YOUR_ACCOUNT_ID",
  "stages": [
    {
      "pipelineExecutionId": "exec_def789ghi012",
      "stageIdentifier": "test_stage"
    }
  ],
  "priority": "HIGH"
}

Request fields:

Field	Required	Description
accountIdentifier	Yes	Your Harness account ID
stages	Yes	Array of stages to update (maximum 10 per request)
stages[].pipelineExecutionId	Yes	Pipeline execution ID from the List API
stages[].stageIdentifier	Yes	Stage identifier from the List API
priority	Yes	New priority: HIGH, NORMAL, or LOW

Example request:

curl -X PUT \
  'https://app.harness.io/gateway/pipeline/api/v2/stages/queue/priority' \
  -H 'Content-Type: application/json' \
  -H 'x-api-key: YOUR_API_KEY' \
  -d '{
    "accountIdentifier": "YOUR_ACCOUNT_ID",
    "stages": [
      {
        "pipelineExecutionId": "exec_def789ghi012",
        "stageIdentifier": "test_stage"
      }
    ],
    "priority": "HIGH"
  }'

Example response:

Priority update confirmation response

{
  "status": "SUCCESS",
  "data": {
    "updated": [
      {
        "pipelineExecutionId": "exec_def789ghi012",
        "stageIdentifier": "test_stage",
        "previousPriority": "NORMAL",
        "newPriority": "HIGH",
        "newQueuePosition": 1
      }
    ],
    "failed": []
  }
}

The response confirms which stages were successfully updated and shows their new position in the queue.

Common errors:

Error	Cause	Solution
Stage is currently RUNNING and cannot be reprioritized	The stage already started executing	Wait for stage to complete or abort the execution
No queued stage found for the given execution and stage identifier	Stage not found or already completed	Verify the execution ID and stage identifier from the List API
Invalid priority. Must be one of: HIGH, NORMAL, LOW	Invalid priority value in request	Use only HIGH, NORMAL, or LOW
User does not have permission to modify queue priority	Missing queue management permission	Contact your account administrator to grant the Queue Admin role

Priority levels

Harness supports three priority levels that determine queue order:

Priority	Behavior	Typical Use
HIGH	Executes ahead of NORMAL and LOW stages	Hotfixes, production incidents, critical releases
NORMAL	Default priority for all stages	Regular CI builds, PR validation, scheduled pipelines
LOW	Executes after NORMAL and HIGH stages	Non-urgent batch jobs, cleanup tasks, nightly builds

When a delegate finishes a stage and has free capacity, it picks the next queued stage using this order:

1. All HIGH priority stages (oldest first)

2. All NORMAL priority stages (oldest first)

3. All LOW priority stages (oldest first)

Changing a stage's priority does not interrupt already-running stages. The new priority takes effect only when determining which queued stage executes next.

Access control

Queue management operations require specific permissions controlled through Harness RBAC.

Permissions:

Permission	Grants Access To	Default Roles
queue_view	List Queued Stages API	All users with pipeline execution access
queue_manage	Update Stage Priority API	Custom Queue Admin role only

To grant queue management permissions, create a custom role in Harness with the queue_manage permission and assign it to users who need the ability to reprioritize queued stages.

Go to Manage access control for instructions on creating custom roles and assigning permissions via Resource Groups.

Scope and filtering

The List API respects organizational boundaries based on the parameters you provide:

• Account scope (accountIdentifier only): Returns all queued and running stages across every organization and project in your account

• Organization scope (accountIdentifier + orgIdentifier): Returns stages from all projects within the specified organization

• Project scope (accountIdentifier + orgIdentifier + projectIdentifier): Returns stages from the specified project only

Users only see stages from projects they have access to based on their RBAC configuration. Stages from inaccessible projects are filtered out automatically.

Next steps

• Install Harness Delegate: Set up Delegate 3.x and configure MAX_STAGES.

• Delegate overview: Learn about the transactional execution model and architecture.

• Feature parity: Compare supported features between the new and legacy delegates.