# Customizing repository prefixes for ECR to ECR pull through cache
<a name="pull-through-cache-private-wildcards"></a>

Pull through cache rules support both the **ecr repository prefix** and the **upstream repository prefix**. The **ecr repository prefix** is the repository namespace prefix in Amazon ECR cache registry that's associated with the rule. All repositories using this prefix become pull through cache-enabled repositories for the upstream registry defined in the rule. For example, a prefix of `prod` applies to all repositories beginning with `prod/`. To apply a template to all repositories in your registry that don't have an associated pull through cache rule, use `ROOT` as the prefix.

**Important**  
There is always an assumed `/` applied to the end of the prefix. If you specify `ecr-public` as the prefix, Amazon ECR treats that as `ecr-public/`.

The **upstream repository prefix** matches the upstream repository name. By default, it's set to `ROOT`, which allows matching with any upstream repository. You can set the **upstream repository prefix** only when the Amazon ECR repository prefix has a non-`ROOT` value.

The following table shows the mapping between cache repository names and upstream repository names based on their prefix configurations in pull through cache rules.


|  Cache namespace  |  Upstream namespace  |  Mapping relationship (cache repository → upstream repository)  | 
| --- | --- | --- | 
|  ecr-public  |  ROOT (default)  |  `ecr-public/my-app/image1` → `my-app/image1` `ecr-public/my-app/image2` → `my-app/image2`  | 
|  ROOT  |  ROOT  |  `my-app/image1` → `my-app/image1`  | 
|  team-a  |  team-a  |  `team-a/myapp/image1` → `team-a/myapp/image1`  | 
|  my-app  |  upstream-app  |  `my-app/image1` → `upstream-app/image1`  |