1 What is OPeNDAP?
OPeNDAP provides a way for researchers to access scientific data anywhere on the Internet from a wide variety of new and existing programs.
The OPeNDAP architecture uses a client/server model, with a client that sends requests for data out onto the network to some server, that answers with the requested data. This is exactly the model used by the World Wide Web where client browsers submit requests to web servers for the data that make up web pages. Of course, OPeNDAP clients can do much more than browse this data. Using flexible data types suitable for many uses, including scientific data, the OPeNDAP servers deliver real data directly to the client program in the format needed by that client.
OPeNDAP clients are very specialized browsers, constructed by linking legacy data analysis programs with OPeNDAP-enabled versions of the data access APIs they use, or by modifying the programs to use one of the OPeNDAP data access APIs. In addition to providing a sophisticated set of network-compatible APIs in several languages, there are also libraries of legacy APIs available in OPeNDAP-aware versions. The popular [http://www.unidata.ucar.edu/packages/netcdf/guide.txn_toc.html NetCDF] library, for example, can read data from remote OPeNDAP data sources as easily as it reads from a local file.
To expand the universe of data available to a user, OPeNDAP incorporates a powerful data translation facility, so that data may be stored in data structures and formats defined by the data provider, but accessed by the user in a manner identical to the access of local data files on the user's own system. Though there are limitations on the types of data that may be translated (see Translation), the facility is flexible and general enough to handle many of the possible translations. There are two important results:
- A user may not need to know that data from one set are stored in a format different from data in another set. Further, it may be possible that neither data set is stored in a format readable by the original version of the data analysis and display program he or she uses.
- No segment of OPeNDAP users will be effectively cut off from accessing data because of its storage format. A scientist who wishes to make his or her data available to other OPeNDAP users may do so while keeping that data in what may actually be a highly idiosyncratic storage format. (Of course, it doesn't have to be in a highly idiosyncratic format.)
The combination of the OPeNDAP network communication model and the data translation facility make OPeNDAP a powerful tool for the retrieval, sampling, and display of large distributed datasets. Though OPeNDAP was developed by oceanographers, its application is not limited to oceanographic data. The organizing principles and algorithms may be applied to many other fields.
The uniformity with which data appears that makes the system so useful for data analysis also eases automating data transport and manipulation tasks. For example, NOAA's [http://ferret.pmel.noaa.gov/Ferret/LAS/home/ Live Access Server (LAS)] (see, for example, [http://mynasadata.larc.nasa.gov/data.html My NASA Data]) uses OPeNDAP, as do many of the real-time observing systems that make up the [http://www.ioos.gov Integrated Ocean Observing System (IOOS)], like [http://gomoos.org Gulf of Maine Ocean Observing System].
The population of people who may be interested in a system such as OPeNDAP may be divided into data consumers and data providers. Though it was an important observation to the development of OPeNDAP that the two roles are often assumed by the same scientists, the division is a useful one for the introduction of the system. The following two sections provide a broad introduction to the roles of data consumer and data provider. The remainder of this guide is organized around this distinction between classes of users.
2 The OPeNDAP Client
OPeNDAP uses a client/server model. The OPeNDAP servers are web servers equipped to interpret an OPeNDAP URL sent to them. An OPeNDAP client composes and sends messages to those servers. There are standalone OPeNDAP clients, but most clients are constructed from data analysis programs modified to get their data remotely, from the internet, rather than locally, from a file.
Without OPeNDAP, an application program that uses one of the common data access APIs such as netCDF will operate as shown in the figure below. The user makes a request for data from the application program. This program uses some data access API to read and write data. The program uses procedures defined by that API to access the data, which is typically stored on the same host machine. Some APIs are somewhat more sophisticated than this, of course, but their general operation is as simple.
The Architecture of a Data Analysis Package
The operation of an OPeNDAP client is illustrated in the figure below. Here, the same application program that was used in the figure above has been linked with an OPeNDAP version of the data access API library. Now, in addition to being able to use local data as before, the application program is able to access data from OPeNDAP servers anywhere on the Internet in exactly the same manner as the local data.
To make some analysis program into an OPeNDAP client, just re-link it with the OPeNDAP API library. (Or, if your program uses one of the supported legacy API libraries, like netCDF, you can link with the OPeNDAP version of that library.) This will create a program that accepts URLs as well as file pathnames to identify data to be read. (See Section 3.1).
The Architecture of a Data Analysis Package Using OPeNDAP
OPeNDAP also provides a data translation facility. Data from the original data file is translated by the OPeNDAP server into the OPeNDAP data model for transmission to the client. Upon receiving the data, the client translates the data into the data model it understands. Because the data transmitted from an OPeNDAP server to the client travel in the OPeNDAP format, the dataset's original storage format is completely irrelevant to the client. If the client was originally designed to read netCDF format files, the data returned by the OPeNDAP-netCDF library will appear to have been read from a netCDF file, whatever the actual format of the files from which the data were read(4). If the program expects JGOFS data, the OPeNDAP-JGOFS library will return data that seem to have come from a JGOFS dataset, and so on.
OPeNDAP does not pretend to remove all the overhead of data searches. A user will still have to keep track of the URLs of interesting data in the same way a user must now keep track of the names of files. (You may run across datasets where the data consists of OPeNDAP URLs. These are the OPeNDAP file servers, and have been developed by OPeNDAP users to organize datasets consisting of large numbers of individual files.) But it does make the access to that data simpler and quicker.
The OPeNDAP group provides a whole array of client software to implement this communication standard. These range from standalone clients to libraries to link with existing software. There is more about these in OPeNDAP Client.
3 The OPeNDAP Services
The communication between an OPeNDAP client and server is specified by the Data Access Protocol (DAP). This defines the range of messages a server must understand and the kinds of replies it makes.
There are two categories of messages an OPeNDAP server can understand. Some are required by the DAP, and others are merely suggested. A server is considered to be DAP-compliant if it can respond intelligibly to the required messages.
The requests messages a server is required to understand are these:
- Data Description
- Data values come in types and sizes. An array, for example, might consist of 10 integers. The value "ten" and the type "integer" describe the array. This request returns information about data types, so that a receiving program can allocate space appropriately. See Data Description Structure (DDS).
- Data Attribute
- This is a request to provide information about data, and typically includes information like units, names of data types, reference information and so on. See Data Attribute Structure (DAS).
- The server also must be able to respond to a request for the data itself. See data response.
A server may respond to requests like these, too:
- Some servers can convert data to ASCII values on the fly. This allows users to view data using a standard web browser, assuming the data are not too large. See ASCII return.
- The info response is a formatted page containing information from the Data Attributes and Data Description responses. It's meant to be a human-readable way to show what's available in a dataset via a standard web browser. See Info response.
- OPeNDAP servers can provide their data in terms of a SOAP request and response. For more information see SOAP Request.
- The DDX is an XML version of the Data Attribute and Data Description replies. See DDX Request.
4 The OPeNDAP Server
OPeNDAP provides a definition of the communication between client and server. Servers and clients who conform to that standard can communicate with each other. In addition to the OPeNDAP communication standard itself, the OPeNDAP group also provides an implementation of that standard server protocol, called Hyrax.
The OPeNDAP data server is made up of two pieces. You can think of them as a front-end and a back-end, though a client will not be aware of the separation. They will often be run on the same machine, and even when they are not, a client will see only the front end.
The front-end server is a Tomcat servlet, and is also called the OPeNDAP Lightweight Front-End Servlet (OLFS). Its job is to receive your request for data and manage all the different forms such a request might take. For example, you might be asking for the data, an ASCII version of the data, or a reply to a SOAP message looking for data. The front-end server can also reply to THREDDS catalog requests, for information about the data, and can directly provide some information about the data, too.
The Back-End Server (BES) is more strictly about performance, and is designed to respond quickly and efficiently to requests from the OLFS. It is a pure data server, and has only one format of request and response, relying on the OLFS to convert messages to whatever format will accommodate the user. Most users won't make requests directly to the BES.
Hyrax is an alternative name for the OPeNDAP 4 Data Server.
See the OPeNDAP 4 Data Server documentation for a description of how to install and configure an OPeNDAP data server (Hyrax).
5 Administration and Centralization of Data
Under OPeNDAP, there is no central archive of data. Data under OPeNDAP is organized in a manner similar to the World Wide Web itself. That is, all one need do to make one's data available is to start up a properly configured server on an Internet node that has access to the data to be served. Each data provider is free to join and to leave the system when it is convenient, just as any proprietor of a web page is free to delete it or add to it as whimsy demands.
Of course, as can also be seen on the World Wide Web, there are some disadvantages to the lack of central authority. If no one knows about a web site, no one will visit it. Similarly, listing a dataset in a central data catalog, such as the Global Change Master Directory (http://gcmd.gsfc.nasa.gov/),can make data available to other researchers in a way that simply configuring an OPeNDAP server does not. OPeNDAP provided a facility for registering a data set with the GCMD catalog, which makes the data set known to the OPeNDAP data location service. The THREDDS catalog service is another way to make information about your data widely available.
The remainder of this book will be divided into three major sections: instructions on the building and operating of OPeNDAP clients; a tutorial and reference on running OPeNDAP servers and making data available to OPeNDAP clients; and technical documentation describing the implementation details (and the motivation behind many of the design decisions) of the OPeNDAP software.