Merging the following example YAML trees:

```yaml
services:
  foo:
    command: ["echo", "foo"]
```

```yaml
services:
  foo:
    command: ["echo", "bar"]
```

Results in a Compose application model equivalent to the YAML tree:

```yaml
services:
  foo:
    command: ["echo", "bar"]
```

### Unique resources

Applies to the [ports](services.md#ports), [volumes](services.md#volumes), [secrets](services.md#secrets) and [configs](services.md#configs) services attributes.
While these types are modeled in a Compose file as a sequence, they have special uniqueness requirements:

| Attribute   | Unique key               |
|-------------|--------------------------|
| volumes     |  target                  |
| secrets     |  target                  |
| configs     |  target                  |
| ports       |  {ip, target, published, protocol}   |

When merging Compose files, Compose appends new entries that do not violate a uniqueness constraint and merge entries that share a unique key.

Merging the following example YAML trees:

```yaml
services:
  foo:
    volumes:
      - foo:/work
```

```yaml
services:
  foo:
    volumes:
      - bar:/work
```

Results in a Compose application model equivalent to the YAML tree:

```yaml
services:
  foo:
    volumes:
      - bar:/work
```

### Reset value

In addition to the previously described mechanism, an override Compose file can also be used to remove elements from your application model.
For this purpose, the custom [YAML tag](https://yaml.org/spec/1.2.2/#24-tags) `!reset` can be set to
override a value set by the overridden Compose file. A valid value for attribute must be provided,
but will be ignored and target attribute will be set with type's default value or `null`. 

For readability, it is recommended to explicitly set the attribute value to the null (`null`) or empty
array `[]` (with `!reset null` or `!reset []`) so that it is clear that resulting attribute will be
cleared.

A base `compose.yaml` file:

```yaml
services:
  app:
    image: myapp
    ports:
      - "8080:80" 
    environment:
      FOO: BAR           
```

And a `compose.override.yaml` file:

```yaml
services:
  app:
    image: myapp
    ports: !reset []
    environment:
      FOO: !reset null
```

Results in:

```yaml
services:
  app:
    image: myapp
```

### Replace value

{{< summary-bar feature_name="Compose replace file" >}}

While `!reset` can be used to remove a declaration from a Compose file using an override file, `!override` allows you
to fully replace an attribute, bypassing the standard merge rules. A typical example is to fully replace a resource definition, to rely on a distinct model but using the same name.

A base `compose.yaml` file:

```yaml
services:
  app:
    image: myapp
    ports:
      - "8080:80"
```

To remove the original port, but expose a new one, the following override file is used:

```yaml
services:
  app:
    ports: !override
      - "8443:443" 
```

This results in: 

```yaml
services:
  app:
    image: myapp
    ports:
      - "8443:443" 
```

If `!override` had not been used, both `8080:80` and `8443:443` would be exposed as per the [merging rules outlined above](#sequence). 

## Additional resources

For more information on how merge can be used to create a composite Compose file, see [Working with multiple Compose files](/manuals/compose/how-tos/multiple-compose-files/_index.md)