GSC Sentinel-2 PDGS OCD - Emits - ESA

GSC Sentinel-2 PDGS OCD
Issue 1 Revision 2 (draft) - 25.07.2010
GMES-GSEG-EOPG-TN-09-0008
page 219 of 350
5.2.3.12 Data Access Gateway (DAG) Function
5.2.3.12.1 Function Objectives and High-Level Description
The DAG function is in charge of providing a single point of access for the download of the
PDGS imagery which is naturally scattered amongst the distributed archives.
To this end, the DAG function follows a client/server paradigm in which the server side
interacts with the relevant PDGS functions (e.g. the PDGS DAX, AI and DPC functions) and
the client side to processes the requests to the imagery by accessing the appropriate servers.
As such, the server side is deployed in each centre hosting an archive following the federated
concept together with complementary functions relevant to the data retrieval activities.
The DAG function is in charge of:
○ Encapsulating the PDGS distributed nature through a single access point for data
discovery and retrieval;
○ Requesting the required PDIs in support of the User-Products assembly and supply;
○ Prioritising the simultaneous data-access activities according to the request priority
level as supplied by the MMUS function and the available network resources;
○ Triggering the user-defined processors as requested by the MMUS function to support
the hosted-processing activities implemented through the collaboration agreements;
○ Providing a low level interface (command-line program, API, etc) to the data-access
services it supports (data download and processor triggering mechanisms) towards the
Front-End Services (i.e. the MMUS & OLIB functions) and potential legacy systems
hosted at the GSPs;
5.2.3.12.2 Design Drivers and Constraints
The following drivers and constraints are identified:
○ High availability and reliability. The DAG function nominal activities (PDI search
location, PDI transport, and product assembly) are aimed for intensive operations;
○ High performance and tight coupling with the underlying digital network for the
provision of the product data items (i.e. download of PDIs). Flexibility in integrating
various network transportation topologies and technologies is identified as a additional
key driver;
○ Clear client/server responsibilities separation and well defined protocol for the PDIs
search location, retrieval and User-Product reconstruction activities following the
defined product model (cf. [RD-07]);
○ Client side software interoperability by third-party elements in support of the different
Front-End Services (i.e. the MMUS and OLIB functions);
○ Client side software portability to different platforms (HW+OS);
5.2.3.12.3 Function Interfaces
The following function interfaces are identified for the DAG function:
ESA UNCLASSIFIED – For Official Use
© ESA
The copyright of this document is the property of ESA. It is supplied in confidence and shall not be reproduced, copied or
communicated to any third party without written permission from ESA.