Basic architecture

OpenStack Glance has a client-server architecture that provides a REST API to the user through which requests to the server can be performed.

A Glance Domain Controller manages the internal server operations that is divided into layers. Specific tasks are implemented by each layer.

All the file (Image data) operations are performed using glance_store library, which is responsible for interaction with external storage back ends and (or) local filesystem(s). The glance_store library provides a uniform interface to access the backend stores.

Glance uses a central database (Glance DB) that is shared amongst all the components in the system and is sql-based by default. Other types of database backends are somewhat supported and used by operators but are not extensively tested upstream.

Image 1. OpenStack Glance Architecture

Following components are present in the Glance architecture:

• A client - any application that makes use of a Glance server.

• REST API - Glance functionalities are exposed via REST.

• Database Abstraction Layer (DAL) - an application programming interface

  (API) that unifies the communication between Glance and databases.

• Glance Domain Controller - middleware that implements the main

  Glance functionalities such as authorization, notifications, policies, database connections.

• Glance Store - used to organize interactions between Glance and various

  data stores.

• Registry Layer - optional layer that is used to organise secure

  communication between the domain and the DAL by using a separate service.