> 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

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/use-release-orchestration/release-orchestration-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)