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Federation Design |
The Earth System Grid Federation (ESGF) is a spontaneous collaboration of groups, agencies and institutions around the world, that are dedicated to the development and operation of a long-term system for the management, access and analysis of climate data. Some of the challenges that ESGF is committed to address include:
- The enormous scale of the data holdings, moving from Peta-bytes to Exa-bytes
- Support for both model output and a wide variety of observational data
- The distributed nature of the data archives, which are geographically distributed and autonomously operated * The need to enable users to access and analyze data with a wide variety of client tools - not just web browsers, but also rich desktop clients, libraries and toolkits
- The need to harmonize and federate multiple local access policies
The ESGF architecture is based on a system of autonomous and distributed Nodes, which interoperate through common acceptance of federation protocols and trust agreements. Data is stored at multiple Nodes, and served through local data and metadata services. Nodes exchange information about their data holdings and services, trust each other for registering users and establishing access control decisions. The net result is that a user can use a web browser or rich desktop client, connect to any Node, and seamlessly find and access data throughout the federation (see ESGF Architecture for more details).
At each Node, the ESGF software stack is the result of the integration of multiple applications and servers, either developed by some of the ESGF partners, or freely available from the community. The ESGF software development methodology is based on the principle of modularity, open source and open development (see the ESGF Manifesto for more details).
Interoperability among all Nodes in the ESGF federation is based on a peer-to-peer paradigm for exchanging information about services, trusts, and metadata holdings. Specifically, the following protocols and mechanism make all the Nodes in the federation work together as a whole:
- The ESGF Registry. The ESGF Registry contains all relevant information
about each Node in the federation: its type, the URL endpoints of the
services it exposes, its public certificates, and so on. This information is
not kept in a central location, rather it is continually exchanged among all
Nodes so that each Node always has a local up-to-date copy of the state of
the whole federation. * Single-Sign-On. Because all ESGF Nodes trust
each other's certificate authorities, a user can register and authenticate at
any of the Nodes, and be granted credentials that are honored throughout the
federation. The type of credentials granted depends on how the user is
accessing the system:
- If using a web browser, the OpenID protocol is used to exchange authentication information between the site where the user authenticates, and any other site. * If using a desktop client, an X509 short term certificate is transmitted by the client to any server that requests the user to authenticate.
- Distributed Access Control. The data served from each Node may need to be protected by policies that are administered at another Node. The ESGF security infrastructure supports this model by establishing mutual trust among all the constituents Nodes, and by transmitting security information (Attribute and Authorization statements) as signed documents encoded as SAML (The Security Assertion Markup Language). * Metadata Exchange. All Nodes in the federation continually exchange search and discovery metadata about their data holdings. As a consequence, when users initiate a search at any one site, they are able to discover resources of interest through the whole federation.
Traditionally, the data and metadata services deployed throughout the ESGF system have been made available to users through a standard web browser. Increasingly though the ESGF collaboration is working towards enabling direct access to these services via rich desktop clients and toolkits, which allow scripted and more powerful access. Specifically, the following clients are being developed.
- UV-CDAT. UV-CDAT is a high-performance visualization client that allows the user to query the ESGF data catalogs via any metadata category, and either download the selected files, or create visualization plots. * Data Mover Light. Data Mover Light (DML) is a high performance desktop client that allows bulk download of data files via either HTTP or GridFTP.
- Regional Climate Model Evaluation System (RCMES) & Apache Open Climate Workbench. RCMES is a model evaluation framework bringing remote sensing data from the National Aeronautics and Space Administration (NASA) and other agencies to bear for evaluation of climate model outputs. RCMES is Powered by Apache OCW is a comprehensive toolkit providing regridding (temporal and spatial); metrics calculation (RMSE, bias, probability distribution function, etc.) and visualization to compare remote sensing data with model output. RCMES and OCW have been plugged into the ESGF Search API.