# Sync Phases and Waves

>v1.1

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Argo CD executes a sync operation in a number of steps. At a high-level, there are three phases *pre-sync*, *sync* and *post-sync*.  

Within each phase you can have one or more waves, than allows you to ensure certain resources are healthy before subsequent resources are synced.   

## How Do I Configure Phases?

Pre-sync and post-sync can only contain hooks. Apply the hook annotation:

```yaml
metadata:
  annotations:
    argocd.argoproj.io/hook: PreSync
```

[Read more about hooks](resource_hooks.md).

## How Do I Configure Waves?

Specify the wave using the following annotation:

```yaml
metadata:
  annotations:
    argocd.argoproj.io/sync-wave: "5"
```

Hooks and resources are assigned to wave zero by default. The wave can be negative, so you can create a wave that runs before all other resources.

## How Does It Work?

When Argo CD starts a sync, it orders the resources in the following precedence:

* The phase
* The wave they are in (lower values first)
* By kind (e.g. namespaces first)
* By name 

It then determines which the number of the next wave to apply. This is the first number where any resource is out-of-sync or unhealthy.
 
It applies resources in that wave. 

It repeats this process until all phases and waves are in in-sync and healthy.

Because an application can have resources that are unhealthy in the first wave, it may be that the app can never get to healthy.