> 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/release-orchestration/3.0/handle-failures/parallel-vs-sequential-execution.md).

# Parallel vs. Sequential Execution

Learn how to model processes with parallel and sequential execution

You can model processes that contain different sequences of execution in terms of parallel and sequential execution.

### Modeling Execution Sequences <a href="#modeling-execution-sequences" id="modeling-execution-sequences"></a>

#### Phase Dependencies <a href="#phase-dependencies" id="phase-dependencies"></a>

Phases can have dependencies preconfigured. For example:

* The **test phase** has dependencies on the **build phase**
* The **validation phase** has dependencies on the **build phase**
* Phases execute in the order defined by their dependencies

#### Activity Dependencies <a href="#activity-dependencies" id="activity-dependencies"></a>

Activities within each phase can also have dependencies. For example:

* **Activity 3** (a manual pipeline or automated activity) and **Activity 3.1** (a manual activity) are dependent on **Activity 2** (an automated pipeline activity)
* Activities execute in the order defined by their dependencies

### Execution Patterns <a href="#execution-patterns" id="execution-patterns"></a>

#### Sequential Execution <a href="#sequential-execution" id="sequential-execution"></a>

When activities have dependencies, they execute sequentially:

* Activities wait for their dependencies to complete
* Execution follows the dependency chain

#### Parallel Execution <a href="#parallel-execution" id="parallel-execution"></a>

When activities don't have dependencies on each other, they can execute in parallel:

* Independent activities can run simultaneously
* Execution is optimized for speed

With this structure, it is possible to model complex release processes and execute them using a release.

### Related Topics <a href="#related-topics" id="related-topics"></a>

* [Executing a release](/release-orchestration/3.0/execution/executing-a-release.md)
* [Activity Execution Flow](/release-orchestration/3.0/execution/activity-execution-flow.md)
* [Activity Dependencies](/release-orchestration/3.0/activities/activity-dependencies.md)

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