Elsevier

Environmental Modelling & Software

Volume 60, October 2014, Pages 346-356
Environmental Modelling & Software

A protocol for machine-readable cache policies in OGC web services: Application to the EuroGeoSource information system

https://doi.org/10.1016/j.envsoft.2014.06.026Get rights and content

Highlights

  • Caches are used to improve efficiency in the access to certain standard geoservices.

  • Service owners may express cache policies in natural language or enforce them by DRM.

  • We propose a protocol based on machine-readable cache policies without DRM.

  • Our proposal can be fully automated and used as a first step towards DRM if needed.

  • This approach has been tested in a European project on minerals and energy resources.

Abstract

An efficient access to the contents provided through OGC web services, widely used in environmental information systems, is usually achieved by means of caching strategies. Service-owners may be interested in expressing the conditions required to allow for this. If these conditions are expressed in a machine-readable way, automatic harvesters can be programmed to follow them.

This paper proposes a protocol to specify and follow cache policies for OGC web services expressed in a machine-readable language. A preliminary implementation of this protocol has been tested in the EuroGeoSource project, where a number of Web Feature Services providing mineral deposits and energy resources are periodically cached to improve the efficiency and availability of several applications. The protocol addresses a nowadays common case, and can possibly be extended to allow for more detailed policies. Further work will help to determine how it could be integrated into a full Digital Rights Management system.

Introduction

Geospatial services are widely used in environmental modeling software, as they provide a flexible, reusable alternative to monolithic and closed applications, and provide support for next-generation integrated environmental modeling systems (Granell et al., 2013, Laniak et al., 2013). Services following ISO/OGC (International Organization for Standardization/Open Geospatial Consortium) standards (we will refer to them as OGC web services) are being used to implement different computational models (e.g. alpine runoff events (Granell et al., 2010)), to share environmental data (e.g. historical landslides and floods (Salvati et al., 2009)), in environmental e-government applications (Latre et al., 2013), and also in combination with other services (Gebhardt et al., 2010, Peckham and Goodall, 2013).

Many types of OGC web services, such as Web Map Services (WMS), Web Map Tile Services (WMTS), Web Feature Services (WFS), Web Coverage Services (WCS) and Catalog Services for the Web (CSW), are “geographic model/information management services” according to the ISO 19119:2005 geographic services taxonomy (ISO, 2005). To access this kind of services with good performance, or to improve its availability, caching their contents is a good strategy. The most common practices are CSW metadata harvesting (Li et al., 2011, Deng and Wu, 2010) and WMS tiling (Liu and Nie, 2010, García et al., 2013). Tiling can be used for raster data (Lia et al., 2011), so it can be adapted to WCS. Caching is also starting to be applied to WFS vector data (Pla and Lleopart, 2010).

Caching is a technique often used in environmental software when performance is required, specially in distributed web-based systems and, more recently, in cloud based systems. For example, in a virtual database for ecological data, Frehner and Brändli (2006) use caching in their integration layer, on top of several WFSs. In another web services-based system, in this case for hydrologic data, Ames et al. (2012) use caches for keeping local copies of observational data series.

In the context of geospatial services, a cache often means a temporary storage for some of the contents offered by these services. In general terms, a cache improves the performance perceived by the users of a service because it allows to pre-generate, and reuse, the results produced by certain time-consuming operations offered by this service. The provider of the service may choose to establish a cache, but the users of this service may also choose to do so on their own side, i.e. in their own desktop computer or in a local server, if they need to, what is the more relevant case for this paper.

Caching a service can be a heavy load for it. For instance, tiling a WMS means making many thousands, even millions, of requests which must be added to those made by its regular users. Besides this, the cached contents, e.g. the map tiles, are stored for an undefined amount of time beyond the control of the service rights-holder. These are good reasons that may lead to establish and express certain conditions to cache a service contents. For instance, the Spain Cadastre WMS1 can be used freely, but includes in its capabilities the prohibition to make tiled requests and massive downloads. The UK Ordnance Survey Open Space developer agreement2 grants permission to the automatic, immediate and temporary storing (caching) of data. In France Géoportail, caching is prohibited unless an explicit license is obtained.3

As the contents provided through OGC web services change, caching those contents needs to be done periodically to keep them updated. Since the conditions to cache those services can change too, it would be useful to express them in a machine-readable way, so that a cooperating, automatic cache updater could react to those changes. However, this is not the current situation as the natural language licenses in the examples of the previous paragraph show.

This paper proposes a protocol to specify cache policies for OGC web services in a machine-readable Rights Expression Language (REL) that can be followed by cooperative harvesters. We will be using the term harvesters for automatic processes that cache the contents of geographic model/information management services. We need those harvesters to be “cooperative” because the protocol is not a full rights management system so it does not enforce the cache policies. The protocol is applicable to a nowadays common situation, and can also be a first step towards a Digital Rights Management (DRM) framework for those interested.

A preliminary version of this protocol has been tested in the EuroGeoSource project, see Section 4, where a number of Web Feature Services (WFS) providing data on mineral deposits and energy resources are periodically harvested and cached in a central node to improve the efficiency and availability of several applications. The data provided through these services can be used as an input in environmental models like those proposed by Côte et al. (2010) or González et al. (2011).

The rest of this paper is organized as follows. The next section reviews work related to RELs and licensing in the geospatial web. Section 3 details a protocol for OGC web services cache policies, based on ODRL 2.0. This protocol also establishes how to embed these policies in OGC web services (Section 3.3), and proposes an algorithm for cooperating harvesters to follow the policies (Section 3.4). Section 4 describes the EuroGeoSource project and how a first version of the protocol proposed in this paper was implemented there. Section 5 discusses the rationale behind some of the most significant decisions taken. To finish this paper, Section 6 summarizes the main conclusions and proposes some future lines of work.

Section snippets

Related work

Explicit license terms are necessary for geospatial assets (e.g. data and services) if the rights and obligations of their users must be clear. For instance, Spatial Data Infrastructures (SDIs) deal with this issue by defining more or less formal “Access Policies” (Béjar et al., 2012, p. 267) for their shared assets. Even open data and content4 are not really open unless their license terms, or the legal conditions under which they are available, are well-known.

In

A protocol for OGC web service cache policies

OGC web services allow for expressing some information about use “fees” and “access constraints” in their capabilities. However, neither the meaning nor the syntax of these elements is defined, as they are free text fields. This section describes a protocol to declare ISO/OGC service policies in a machine-readable format. This protocol is designed to regulate the behavior of harvesters which access data and metadata-providing OGC web services in order to download and cache those data and

Application to the EuroGeoSource information system

The EuroGeoSource project,13 co-funded by the Competitiveness and Innovation Framework Programme (CIP), under the Policy Support Programme (PSP), Geographic Information Theme, of the European Union, has developed an Internet pilot information system14 which provides access to geographical information on geo-energy and mineral resources on ten European countries.

The system has been developed following an SDI architecture based on INSPIRE

Rationale and discussion

During this work, a number of decisions have been taken. The rationale behind them is discussed in this section. These decisions have been guided by our interest to provide a novel solution to a real problem, which is both easy to adopt and extensible if needed.

An acceptable balance between simplicity (both for the service owners and for the service harvesters) and functionality has been difficult to achieve. The first issue was the granularity of the assets. From the whole service, to

Conclusions and future work

This paper proposes a protocol to specify cache policies for OGC web services in ODRL 2.0 that can be followed by cooperative harvesters. We have used ODRL 2.0 because our specific interest of using a REL not related to any particular DRM system. This separation is a recommended practice even when the final objective is the adoption of a DRM framework (Jamkhedkar et al., 2006) or the use of access control technologies (The UK Location Architecture Interoperability Board – Business

Acknowledgments

This work has been partially funded through the EuroGeoSource project (project number 250532), from the European Union's ICT Policy Support Programme as part of the Competitiveness and Innovation Framework Programme, by the Spanish Government (project TIN2012-37826-C02-01), by the Government of Aragon (project INNOVA-A1-038-13) and by the National Geographic Institute (IGN) of Spain. We also want to thank the members of the EuroGeoSource consortium their help, suggestions and hard work, and the

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