Instances are virtual machines that run inside the cloud.
Before you can launch an instance, gather the following parameters:
The instance source. This can be an image, a snapshot, or a block storage volume that contains an image or snapshot.
A name for your instance.
The flavor for your instance, which defines the compute, memory, and storage capacity of nova computing instances. A flavor is an available hardware configuration for a server. It defines the "size" of a virtual server that can be launched.
Any user data files: A user data file is a special key in the metadata service that holds a file that cloud-aware applications in the guest instance can access. For example, one application that uses user data is the cloud-init system, which is an open-source package from Ubuntu that is available on various Linux distributions and which handles early initialization of a cloud instance.
Access and security credentials, which include one or both of the following credentials:
A key pair for your instance, which are SSH credentials that are injected into images when they are launched. For the key pair to be successfully injected, the image must contain the
cloud-initpackage. Create at least one key pair for each project. If you already have generated a key pair with an external tool, you can import it into OpenStack. You can use the key pair for multiple instances that belong to that project.A security group, which defines which incoming network traffic is forwarded to instances. Security groups hold a set of firewall policies, known as security group rules.
If needed, you can assign a floating (public) IP address to a running instance.
You can also attach a block storage device, or volume, for persistent storage.
![[Note]](../common/images/admon/note.png) | Note |
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Instances that use the default security group cannot, by default, be accessed from any IP address outside of the cloud. If you want those IP addresses to access the instances, you must modify the rules for the default security group. You can also assign a floating IP address to a running instance to make it accessible from outside the cloud. See the section called “Manage IP addresses”. |
After you gather the parameters that you need to launch an instance, you can launch it from an image or a volume. You can launch an instance directly from one of the available OpenStack images or from an image that you have copied to a persistent volume. The OpenStack Image Service provides a pool of images that are accessible to members of different projects.
