glance/doc/source/architecture.rst

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      Licensed under the Apache License, Version 2.0 (the "License"); you may
      not use this file except in compliance with the License. You may obtain
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==================
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.

.. figure:: /images/architecture.png
   :figwidth: 100%
   :align: center
   :alt: OpenStack Glance Architecture

.. centered:: 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.