RFC 1: An Extensible Software Architecture for EOxServer¶
This RFC proposes an extensible software architecture for EOxServer that is based on the following ideas:
- Separation of instance and distribution code
- Structuring of the distribution in layers
- Extensibility through a plugin system
EOxServer development has been initiated in the course of two ESA projects that aim at providing a harmonized standard interface to access Earth Observation (EO) products, namely:
- Heterogeneous Mission Accessibility - Follow-On Open Data Access (HMA-FO ODA)
- Open-standard Online Observation Service (O3S)
The specification of a software architecture is required by these projects. From a practical point of view, EOxServer has reached a point where a common framework for a rapidly evolving project is needed.
Summarizing the requirements in a nutshell EOxServer has to integrate:
- different OGC Web Services
- different data and processing resources
- heterogeneous data and metadata formats
This leads to the conclusion that an extensible software architecture is needed. The problems to address are discussed in further detail in the Requirements section.
The proposed architecture is modular, extensible and flexible and structured in layers. The following separate components are identified:
In this architecture the Core shall provide the central logic for the extension mechanism while the layers shall contain interface definitions based on the extension model of the Core that can be implemented by extending modules and plugins.
The main sources of requirements for EOxServer at the moment of writing this RFC are:
- the HMA-FO Open Data Access Sofware Requirements Specification (SRS)
- the O3S Software System Specification (SSS)
- the feature requests posted on the EOxServer Trac
Most of the requirements are related to the features EOxServer shall implement. There is one requirement, however, in the O3S SSS that is directly related to the software architecture; O3S_QUA_004 states:
The O3S3 shall sustain maintainability and reusability by using a modular system architecture
This shall facilitate
- isolation and removal of code defects
- integration of new functionality, such as the implementation of new interface standard versions
- extension of the system functionality according to new or modified requirements
Thus, modularity as well as integration and extension of functionality are central issues in the drafting of the EOxServer software architecture. The question remains what considerations shall govern the structuring of the software into modules, what functionality it shall implement and in what way the system shall be able to be extended.
Our approach to this question is to identify different topics of concern for the EOxServer development that shall structure the requirements analysis and give a first hint on the architectural design.
The main goal of EOxServer is to furnish an implementation of OGC Web Services (OWS) intended for use within the Earth Observation (EO) domain. These services shall provide access to different kinds of resources and to processes operating on these resources. The requirements cite different backends that the software shall implement in order to allow access to local and remote content. Finally, we discuss where and how the software is going to be deployed.
The following OGC Web Services shall be implemented:
The Web Coverage Service shall be able to present Earth Observation data, e.g. optical satellite imagery, SAR data, etc. The following extensions shall be implemented:
Earth Observation Application Profile for WCS (EO-WCS) (requirement O3S_CAP_100)This application profile is intended to ease access to large collections of Earth Observation data.
Transactional WCS (WCS-T) (requirement O3S_CAP_150)This extension of WCS introduces a Transaction operation that supports transfer of data to a WCS server.
This service shall be used to give to portrayals of the coverages the system presents. The following extension shall be implemented:
WMS Profile for EO Products (EO-WMS) (requirement O3S_CAP_240)The extension allows access to portrayals of large dataset series.
This service shall be used to present vector data.
This service shall be used to make processing resources accessible online.
EOxServer shall present various processes to the public using WPS. The processes planned for implementation at the moment of writing this RFC are specific to the use cases to be handled in the course of the O3S project. The capability to publish a variety of processes on the other hand is a general requirement for EOxServer.
Being a project focussing on the EO domain EOxServer concentrates on the processing of EO coverage (raster) data. So, the considerations made for coverages regarding the variety of data and metadata formats are valid for processes as well.
EOxServer shall enable public access to different kinds of geo-spatial resources in the Earth Observation domain. These are:
- Vector Data
Coverages are defined in a very abstract way. What EOxServer focusses on are coverages dealt with by the Earth Observation Application Profile for WCS (EO-WCS) which is a draft OGC Best Practice Paper as of writing this RFC. The main categories of resources defined in that paper are:
- Datasets are the atomic components EO-WCS objects are composed of. They are coverages that are associated with EO Metadata. EO satellite mission scenes are a good example of Datasets. They can be accessed individually even when being part of a Stitched Mosaic or Dataset Series.
- Stitched Mosaics
- Stitched Mosaics are made up from a collection of Datasets that share a common range type and grid. Other than Dataset Series they are not merely a container for Datasets but coverages themselves. The coverage values are generated from the contributing datasets. This process must follow some rule to decide what value to take into account in the areas where the contributing Datasets overlap. The most common rule is “latest-on-top”.
- Dataset Series
- Dataset Series represent collections of Datasets or Stitched Mosaics. They do not impose any constraints on the contained objects, so very heterogeneous data can be included in the same series.
A major problem for the EOxServer implementation is that raster data coverages originating from EO satellite missions are very heterogeneous. They can use a vide variety of data and metadata formats and are often associated with additional data like bitmasks, etc. that should be presented by EOxServer as well. Furthermore, the data packaging is different for every mission.
Support for Vector Data handling is required by O3S Use Case 2. In that use case road network data shall be generated from Pléiades satellite data using automated feature detection algorithms. The road network data shall be presented using WFS and WMS.
EOxServer shall implement various backends to access data it presents to the public via the OGC Web Services:
The only requirements originating from the HMA-FO ODA and O3S projects regarding deployment concern the implementation of the O3S Use Cases. Every use case requires one or more instances of EOxServer to be deployed. The instances have different purposes and thus shall present different services and different resources.
The fact that EOxServer shall be deployed many times in different configurations (possibly on the same server) calls for a strict separation of distribution and instance data.
The ability to activate or deactivate various components of the system implies not only that the architecture must be modular but also that it must be configurable to use different combinations of modules.
The conclusion of the requirements review is that the EOxServer Architecture shall be:
- flexible in the sense that it must be possible to select different combinations of modules to deploy and activate
- able to present resources using different OGC Web Services
- able to access data from different backends
- able to handle different data, metadata and packaging formats
- separating distribution and instance data
The development of the software architecture will be based on these considerations.
The software architecture development for EOxServer does not start at zero. There are already considerations made in the proposal phase of the O3S project and there is the status quo of version 0.1.0. Taking into account this preparational work and the outcomes of the requirements review, the outlines of the Proposed Architecture will be developed in the last subsection and the following sections.
The O3S draft Architectural Design Document (ADD/SDD) has already proposed a software architecture which is, however, outdated in certain aspects due to changes made in the requirements phase of O3S. Here is an overview of the O3S draft architecture:
This identifies four servers and extending modules:
- WPS Server
- WCS Server
- WCS Earth Observation Application Profile Module
- WCS-T Module
- WCPS Module (not included in the requirements any more)
- WFS Server
- WFS-T Module (not included in the requirements any more)
- WMS Server
- WMS Profile for EO Products Module
An additional integrating Data Access Layer is foreseen that shall implement storage patterns such as image pyramids and offer an API to read and write data that hides the internal details of data storage from the service and extension modules using it.
During the requirements phase of O3S and the early development of EOxServer many deviations from this original design have been made necessary. Most importantly:
- Django has been added as dependency
- GDAL has been added as dependency
- the implementation of WCPS has been postponed
- the implementation of WFS-T has been postponed
- Django has made use of different geo-spatial database backends possible
- requirements for remote storage backends have been added
Although the basic concepts of the draft architecture remain valid, an updated version is needed for EOxServer to fulfill its requirements and evolve beyond the project horizon of O3S.
Status Quo of Release 0.1.1¶
As of release 0.1.1 EOxServer is an integrated Django project including a single Django application and additional modules that support OGC Web Service (OWS) request handling and data integration.
The data model is contained in the
eoxserver.server application. So
ows view, the central entrance point for OWS requests, and
the administration client view as well as tools for automatic data
Supporting modules are gathered in the
eoxserver.lib module. These
contain the core application logic for OWS request handling, coverage
and metadata manipulation as well as utilities e.g. for XML processing.
EOxServer 0.1.1 includes an extension mechanism already which so far is
restricted to services. The
eoxserver.lib.registry module maintains
a central registry for the concrete implementations of OWS interfaces
which may be published in the
eoxserver.modules namespace. At the
moment there are implementations for WMS 1.0, 1.1 and 1.3, WCS 1.0, 1.1
and 2.0 as well as a preliminary version of EO-WCS. All these modules
use MapServer MapScript for image manipulation and part of the request
handling in the backend.
This approach fulfills some of the requirements summarized above already, but further development of the architecture and the code is necessary to be fully compliant. Most importantly:
- extensibility and flexibility have to be enhanced
- WPS must be implemented
- WFS must be implemented
- support for remote backends is necessary
The proposed architecture for EOxServer shall be based on the following principles:
- Separation of Instance and Distribution: instance applications shall be separated from EOxServer distribution code in order to facilitate deployment of multiple services on the same machine and to support flexible configurations of services
- Layered Architecture of the Distribution: The software architecture shall be structured in layers and a core that contains basic common functionality; each layer builds on the capabilities of the underlying ones to fulfill its tasks
- Extensibility: the EOxServer distribution shall be extensible by additional modules and plugins; the distribution core shall provide functionality to enable dynamic binding to extending modules
The identification of different layers is performed based on the structuring of the system components underlying the requirements analysis.
The implementation of EOxServer shall use the following dependencies:
- Python: Python shall serve as the
implementation language; it has been chosen because
- it facilitates rapid development
- the geospatial libraries used all have Python bindings
- Django: Django has been
selected as development framework because
- it provides an object-relational mapper that supports various database backends
- it supports geospatial databases and integrates vector data handling functionality in the GeoDjango extension
- it allows for rapid web application development
- Spatial Database Backend: using GeoDjango, EOxServer shall support at least the SpatiaLite and PostGIS geospatially enabled RDBMS backends.
- MapServer: EOxServer shall build on MapServer MapScript in order to facilitate OGC Web Service handling
- GDAL/OGR: For image processing tasks and vector data manipulation the Python binding of the GDAL/OGR libraries shall be used
Concerning the software architecture, the use of Django enforces a Model-View-Controller (MVC) substructure of the distribution layers of EOxServer.
Distribution Core and Layers¶
The breakdown of the distribution into core and layers is as follows:
- The Core shall contain modules for common use throughout the different components of EOxServer. This includes the global configuration data model, the implementation of the extension mechanism as well as the basic functionality for the EOxServer administration client
- Service Layer
- This layer contains the core request handling logic as well as the implementation of services and service extensions
- Processing Layer
- This layer contains the processing models used internally by EOxServer as well as the data model and the basic handling routines for processes to be published using WPS
- Data Integration Layer
- This layer shall provide data models for resources as well as an abstraction layer for different data formats and data packaging formats
- Data Access Layer
- This layer shall provide backends for local and remote data access
Each of the four layers shall be sub-structured in:
- data model
- for public access (if applicable)
- for the administration client
- core handling logic
- interface definitions for extensions
- modules implementing the interface definitions
Structure of the Architecture Specification¶
The further specification of the proposed architecture is subdivided into several sections and separate RFCs. This RFC 1 contains a description of the different architectural layers and of EOxServer instances:
The following RFCs discuss different aspects of the architecture in further detail:
The Core shall act as a “glue” for EOxServer that links the different parts of the software together and provides functionality used throughout the EOxServer project.
It defines the core of the configuration data model which is extended by the layers and implementing modules. The configuration is partly stored in the database and partly in files. Both parts need to be easily modifiable and extensible.
Therefore the Core also includes an administration client that can be used by system operators to edit the part of the configuration stored in the database. The basic functionality of the administrator, the entry view and its extension mechanisms shall be part of the Core.
The Core includes modules for common use, for instance utilities for the handling of spatio-temporal metadata as well as for decoding and encoding of XML documents.
Most importantly, the Core contains the central logic that enables the dynamic extension of system functionality. The layers shall provide interface definitions based on the extension model of the Core that can be implemented by extending modules and plugins. For more details see RFC 2: Extension Mechanism for EOxServer.
The Service Layer contains the OWS request handling logic as well as the implementation of services and service extensions.
It defines a configuration data model for OGC Web Services and for their metadata. The model includes:
- service metadata to be published in the GetCapabilities response
- options to enable or disable a specific service or service extension for a given data source
- options to configure the services themselves, e.g. enabling or disabling certain non-mandatory features
The Service Layer provides views for public access, namely the central entrance point for OWS requests. It also contains views for the administration client that allow to configure services and service metadata.
The core handling logic for OGC Web Services is part of the Service Layer. It implements the behaviour defined by OWS Common and defines a structured approach to request handling that discerns different levels:
- service versions
- service extensions
- service operations
The way services and service extensions interact is described in further detail in RFC 3: OGC Service Extensions.
The Service Layer defines request handler interfaces for each of these levels that are implemented by modules for:
The Processing Layer contains the processing models used internally by EOxServer as well as the data model and the basic handling routines for processes to be published using WPS.
In its data model it defines the configuration options and metadata for processes. The model shall also support processing chains as described in further detail in RFC 5: Processing Chains. The Processing Layer publishes administration client views to support the configuration of processes and processing chains.
The Processing Layer defines interfaces for processes. It also contains implementations of the processes used internally by EOxServer; these include:
- coverage tiling
- coverage mosaicking
Further processes as required e.g. by the O3S Use Cases will be added as plugins based on the data model and interface definitions of the Processing Layer.
Data Integration Layer¶
The Data Integration Layer shall provide data models for resources as well as an abstraction layer for different data formats and data packaging formats.
Data packaging formats are explained in greater detail in RFC 4: Data Packaging. Roughly speaking, they represent the way data and metadata for an EO product or derived product are packaged. They shall abstract from the actual substructure of the packaging format in directories and files so these resources can be handled transparently by EOxServer.
Its data model shall include items common to all types of data as well as individual models for:
- vector data
Just as the other layers the Data Integration Layer shall publish administration client views that support adding, modifying and removal of resources and their respective metadata.
The interface definitions of the Data Integration Layer shall provide an abstraction layer for:
- various data formats
- various metadata formats
- various data packaging formats
The modules implementing these interfaces shall support:
Data Access Layer¶
The Data Access Layer shall provide transparent access to local and remote data using different backends. It constitutes an abstraction layer for data sources.
Its data model therefore provides configuration options for the backends. It contains views for the administration client to configure different data sources.
The Data Access Layer is built around the interface definitions of backends and data sources stored by them. The following backends need to be implemented:
- local backends:
- file system
- rasdaman backend
- remote backends:
- using HTTP/HTTPS
- using FTP
- using WCS
EOxServer instances are Django projects that import different EOxServer modules as Django applications.
Like every Django project they contain a settings file that governs the Django configuration and in addition the most basic parts of EOxServer configuration. Specifically:
- the connection details for the database containing the EOxServer configuration is defined in the settings file
- the Django
INSTALLED_APPSsetting must be used to define the parts of the EOxServer data model that shall be loaded
- some EOxServer configuration settings that are needed in the startup phase will be appended to the Django settings file
Apart from the settings, every Django project has an “urlconf” that defines which URLs shall point to the different views of the project. For using the full EOxServer functionality there have to be URLs pointing to the Service Layer OWS entrance point and the administration client entrance point defined by the EOxServer core.
Furthermore the instance contains the Django configuration files whose content is defined by the configuration data model of the Core and the layers.
Optionally, the instance directory may include subdirectories for the data (if stored locally) and the database (if using the file-based SpatiaLite spatial database backend).
Finally, in a production setting, it shall contain the modules needed to deploy the instance. The favourite deployment method is WSGI (see PEP 333). These must be configured as well to include the path to the instance.
The Django project may or may not contain applications itself, which may or may not use EOxServer functionality. Writing an own application is not necessary to use EOxServer, though; placing links to EOxServer views in the urlconf is sufficient.
Moved to ACCEPTED by unanimous consent without a formal vote on July 20th, 2011.