PORT: 8080
  TZ: utc
image: busybox
```

Items under `blkio_config.device_read_bps`, `blkio_config.device_read_iops`,
`blkio_config.device_write_bps`, `blkio_config.device_write_iops`, `devices` and
`volumes` are also treated as mappings where key is the target path inside the
container.

For example, the following input:

```yaml
services:
  common:
    image: busybox
    volumes:
      - common-volume:/var/lib/backup/data:rw
  cli:
    extends:
      service: common
    volumes:
      - cli-volume:/var/lib/backup/data:ro
```

Produces the following configuration for the `cli` service. Note that the mounted path
now points to the new volume name and `ro` flag was applied.

```yaml
image: busybox
volumes:
- cli-volume:/var/lib/backup/data:ro
```

If the referenced service definition contains `extends` mapping, the items under it
are simply copied into the new merged definition. The merging process is then kicked
off again until no `extends` keys are remaining.

For example, the following input:

```yaml
services:
  base:
    image: busybox
    user: root
  common:
    image: busybox
    extends:
      service: base
  cli:
    extends:
      service: common
```

Produces the following configuration for the `cli` service. Here, `cli` services
gets `user` key from `common` service, which in turn gets this key from `base`
service.

```yaml
image: busybox
user: root
```

##### Sequences

The following keys should be treated as sequences: `cap_add`, `cap_drop`, `configs`,
`deploy.placement.constraints`, `deploy.placement.preferences`,
`deploy.reservations.generic_resources`, `device_cgroup_rules`, `expose`,
`external_links`, `ports`, `secrets`, `security_opt`.
Any duplicates resulting from the merge are removed so that the sequence only
contains unique elements.

For example, the following input:

```yaml
services:
  common:
    image: busybox
    security_opt:
      - label=role:ROLE
  cli:
    extends:
      service: common
    security_opt:
      - label=user:USER
```

Produces the following configuration for the `cli` service.

```yaml
image: busybox
security_opt:
- label=role:ROLE
- label=user:USER
```

In case list syntax is used, the following keys should also be treated as sequences:
`dns`, `dns_search`, `env_file`, `tmpfs`. Unlike sequence fields mentioned previously,
duplicates resulting from the merge are not removed.

##### Scalars

Any other allowed keys in the service definition should be treated as scalars.

### `external_links`

`external_links` link service containers to services managed outside of your Compose application.
`external_links` define the name of an existing service to retrieve using the platform lookup mechanism.
An alias of the form `SERVICE:ALIAS` can be specified.

```yml
external_links:
  - redis
  - database:mysql
  - database:postgresql
```

### `extra_hosts`

`extra_hosts` adds hostname mappings to the container network interface configuration (`/etc/hosts` for Linux).

#### Short syntax

Short syntax uses plain strings in a list. Values must set hostname and IP address for additional hosts in the form of `HOSTNAME=IP`.

```yml
extra_hosts:
  - "somehost=162.242.195.82"
  - "otherhost=50.31.209.229"
  - "myhostv6=::1"
```

IPv6 addresses can be enclosed in square brackets, for example:

```yml
extra_hosts:
  - "myhostv6=[::1]"
```

The separator `=` is preferred, but `:` can also be used. Introduced in Docker Compose version [2.24.1](/manuals/compose/releases/release-notes.md#2241). For example:

```yml
extra_hosts:
  - "somehost:162.242.195.82"
  - "myhostv6:::1"
```

#### Long syntax

Alternatively, `extra_hosts` can be set as a mapping between hostname(s) and IP(s)

```yml
extra_hosts:
  somehost: "162.242.195.82"
  otherhost: "50.31.209.229"
  myhostv6: "::1"
```

Compose creates a matching entry with the IP address and hostname in the container's network
configuration, which means for Linux `/etc/hosts` get extra lines:

```console
162.242.195.82  somehost
50.31.209.229   otherhost
::1             myhostv6