Glance has four main parts to it:
glance-apiaccepts Image API calls for image discovery, image retrieval and image storage.glance-registrystores, processes and retrieves metadata about images (size, type, etc.).A database to store the image metadata. Like Nova, you can choose your database depending on your preference (but most people use MySQL or SQlite).
A storage repository for the actual image files. In the diagram above, Swift is shown as the image repository, but this is configurable. In addition to Swift, Glance supports normal filesystems, RADOS block devices, Amazon S3 and HTTP. Be aware that some of these choices are limited to read-only usage.
There are also a number of periodic process which run on Glance to support caching. The most important of these is the replication services, which ensures consistency and availability through the cluster. Other periodic processes include auditors, updaters and reapers.
As you can see from the diagram in the Conceptual Architecture section, Glance serves a central role to the overall IaaS picture. It accepts API requests for images (or image metadata) from end users or Nova components and can store its disk files in the object storage service, Swift.
