IEEE Geoscience and Remote Sensing Magazine - December 2015 - 11

OPeNDAP enables users to access data on remote
OPeNDAP Server Cluster
servers using the World
Wide Web information system. An OPeNDAP client
can be an ordinary browser,
a graphics program or web
HTTP
Firewall
application. OPeNDAP inOPeNDAP
Call
corporates a data translation facility, so data may be
stored in data structures and
LP DAAC
Loadbalancer
LP DAAC
Public
formats defined by the data
Data
Users
provider, but are returned
Pool
to user application requests
in common, applicationfriendly formats. Advantages of using OPeNDAP include the ability to retrieve Figure 1. LP DAAC Implementation of OPeNDAP Architecture.
subsets of a file or files, and
the ability to aggregate data
rates public access to data and servers with a Web Applicafrom several files in one transfer operation.
tion Firewall (WAF) to ensure security considerations are
OPeNDAP communications use a client/server model
met. Multiple server clusters are used to serve simultaneous
similar to that of the World Wide Web, and specified by
requests, with a load balancer in the De-Militarized Zone
the Data Access Protocol (DAP) (Gallagher, Potter, Sgou(DMZ) between the WAF on the perimeter network and inros, Hankin, & Flierl, 2007). This specification defines the
ternal network firewalls to queue requests and monitor the
range of messages a server must understand and the types
performance of their completion.
of replies it generates.
To support workflows using temporal data, the timeIn addition to the OPeNDAP communication stanbased aspect of MODIS product suites were aggregated
dard itself, OPeNDAP.org also provides an implementainto a single web access point for all temporal products.
tion of a standard server protocol, called Hyrax. Hyrax
Because the data and metadata are in storage locations
is an OPeNDAP data server with a client/server model
available to web services, an initial set of software was
composed of two components: an OPenDAP Lightweight
developed to create NetCDF
Front-end Server (OLFS); and a Back-End Server (BES). The
Markup Language (NcML),
client-facing OLFS utilizes a modular design and a Java
or .ncml files, which depict
servlet mechanism to receive and manage client requests
a time-series of Hierarchifor data. The BES quickly and efficiently delivers data reOPeNDAP eNABLeS
cal Data Format (HDF) file
quests back to the front-end for data inspection and/or deuSerS TO ACCeSS DATA ON
entries for a specific product
livery. The Hyrax specification of OPeNDAP also provides
reMOTe SerVerS uSiNg THe
set. NcML is compatible with
support for Thematic Real-time Environmental DistribWOrLD WiDe WeB
HDF files used for MODIS
uted Data Services (THREDDS) catalogs and support for
iNFOrMATiON SYSTeM.
products, and most NcML
multiple source and returned data representations. Hyrax
software libraries for openfunctionality also includes dataset aggregation (e.g. time
source applications accept
series data), adding and modifying dataset content, and
HDF format files and data
numerous other features.
(UCAR, 2013). The Uniform Resource Locator (URL) synTo take advantage of these recent technology developtax for two OPeNDAP service implementations, Hyrax
ments, and provide better capabilities to exploit the con(OPeNDAP 4 Data Server) and THREDDS, is slightly diftent of its data archives, the LP DAAC is expanding its use
ferent. Therefore, a set of NcML aggregation files were deof web services to access, manipulate, modify, and deliver
signed and created for each implementation, though curdata. A set of compute clusters has been configured for raprent LP DAAC service implementations are focused on the
id access to data and metadata, and to provide a platform in
Hyrax protocol. The NcML aggregation software also loops
close proximity to the data to accept remote user requests
through all horizontal and vertical MODIS geographical
using services such as OPeNDAP for data transformations
tile formulations, to create a complete global tile-based
before delivery to the user. Figure 1 shows the LP DAAC
.ncml file set. Time aggregations of MODIS granule referOPeNDAP infrastructure deployed for providing access
ences are automatically updated overnight for all tiled and
to MODIS and other data sets in the LP DAAC Data Pool
global MODIS products via cron initiation. These .ncml
archive. The configuration of hardware and software sepadecember 2015

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Table of Contents for the Digital Edition of IEEE Geoscience and Remote Sensing Magazine - December 2015