# Machine
See also: [How to manage machines](https://documentation.ubuntu.com/juju/4.0/howto/manage-machines.md#manage-machines)
In Juju, a **machine** is a [compute resource](https://documentation.ubuntu.com/juju/4.0/reference/resource-compute.md#resource-compute) requested implicitly (e.g., through [juju deploy](https://documentation.ubuntu.com/juju/4.0/reference/juju-cli/list-of-juju-cli-commands/deploy.md#command-juju-deploy), [juju add-unit](https://documentation.ubuntu.com/juju/4.0/reference/juju-cli/list-of-juju-cli-commands/add-unit.md#command-juju-add-unit), etc.) or explicitly (e.g., through [juju add-machine](https://documentation.ubuntu.com/juju/4.0/reference/juju-cli/list-of-juju-cli-commands/add-machine.md#command-juju-add-machine)) from a machine [cloud](https://documentation.ubuntu.com/juju/4.0/reference/cloud.md#cloud).
#### IMPORTANT
This definition suggests that, regardless of whether the cloud is a bare metal cloud, a virtual machine cloud, or a LXD container- or VM-based cloud, and regardless of whether the command targets a regular instance or rather a LXD container on a regular instance (all the commands above can apply to both), what’s being provisioned is always, from the point of view of Juju, a ‘machine’.
From the point of view of an end user, this is absolutely true, with one small caveat – even though listed in `juju` outputs under ‘Machines’, and in general handled via the same CLI commands as a machine, a LXD container provisioned on top of a regular cloud instance will be named after its host machine; e.g., `0/lxd/5` = LXD container `5` on machine `0`.
## Machines and units
When you deploy an [application](https://documentation.ubuntu.com/juju/4.0/reference/application.md#application) on a machine, there is usually one [unit](https://documentation.ubuntu.com/juju/4.0/reference/unit.md#unit) per machine. However, it is usually possible to optimise resources by deploying multiple units of the same or of different applications to the same machine.
## Machines and system (LXD) containers
In Juju, they are both essentially the same – ‘machines’. For example, most `juju` CLI commands that target machines can actually target system containers in the exact same way.
### Example
E.g., `juju add-machine lxd` starts a LXD container on a new machine and adds *both* as ‘machines’ – the only difference being that the container ‘machine’ is prefixed with the ID of its host machine and the annotation `lxd`:
```text
$ juju add-machine lxd
created container 1/lxd/0
$ juju machines
Machine State Address Inst id Base AZ Message
0 started 10.154.118.110 juju-dadfb7-0 ubuntu@22.04 Running
1 pending pending ubuntu@22.04 Creating container
1/lxd/0 pending pending ubuntu@22.04
```
And if you then deploy an application to a LXD container (without specifying any particular container), that will again provision two machines:
```text
$ juju deploy postgresql --to lxd
Located charm "postgresql" in charm-hub, revision 288
Deploying "postgresql" from charm-hub charm "postgresql", revision 288 in channel 14/stable on ubuntu@22.04/stable
ubuntu@charm-dev:~/.local/share/juju$ juju machines
Machine State Address Inst id Base AZ Message
0 started 10.154.118.110 juju-dadfb7-0 ubuntu@22.04 Running
1 started 10.154.118.72 juju-dadfb7-1 ubuntu@22.04 Running
1/lxd/0 pending juju-dadfb7-1-lxd-0 ubuntu@22.04 Container started
2 started 10.154.118.209 juju-dadfb7-2 ubuntu@22.04 Running
2/lxd/0 pending pending ubuntu@22.04 acquiring LXD image
```
## Machine designations
In Juju, many different commands have a machine argument. The shape of this argument depends on whether the machine is existing vs. new and a regular cloud instance vs. a LXD container on top of a regular cloud instance. The argument can also contain combinations, in comma-separated format. The examples below illustrate all the various cases:
| shape of the machine argument | meaning |
|---------------------------------|--------------------------------------------------------------------------------------------|
| | a new machine |
| `0` | machine 0 |
| `0,4` | machines 0 and 4 |
| `lxd` | a new LXD container or (if specified with `virt-type=virtual-machine`) VM on a new machine |
| `lxd:25` | a new LXD container or (if specified with `virt-type=virtual-machine`) VM on machine 25 |
| `0/lxd/4` | LXD container `4` on machine `0` |
| `3,0/lxd/2,lxd:5` | machine 3, LXD container 2 on machine 0, and a new LXD container on machine 5 |
## Machine customisation
A machine’s specific hardware can be customised via [constraints](https://documentation.ubuntu.com/juju/4.0/reference/constraint.md#constraint).
## Machine base
#### Versionadded
Added in version 3.1.0.
In Juju, a **base** is a way to identify a particular operating system (OS) image for a Juju [machine](#machine).
This can be done via the name of the OS followed by the `@` symbol and the channel of the OS that you want to target, specified in terms of `