3.server basics

8/9/2019 3.Server Basics

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Server-Side: The Basics

This part of the workshop contains an overviewof the two servers which OPeNDAP has

developed. One uses the Common GatewayInterface (CGI) technology while the other usesJava Servlets. This section also covers theoperations common to all DAP 2.0 compliant

servers. Finally, we also discuss varioussecurity concerns such as configuring a serverwith username/password access.


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Two Servers from OPeNDAP

Both use a web server to provide networkingand access control

One server* uses C/C++ and CGIs One server uses Java and Servlets

Each can serve several types of data

* In our documentation we often talk about 'the netCDF server' or the 'HDFserver.' In reality those are handlers or modules in the CGI-server. Similarly thDTS and DRDS are modules of the Java/Servlet-based server.


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What the Servers do

Return the four objects defined by theDataAccess Protocol, version 2

DAS: Semantic metadata DDS: Syntactic metadata

DataDDS: Data with the relevant DDS parts

Error:When a request cannot be satisfied


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Additional Server Responses

Both servers also support some 'services.'

Some service responses are made by

transforming one or more of the objectresponses.

Services:

ASCII: Get data in ASCII

Info: HTML-encoded metadata

Simple access interface: An HTML form


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More Services...

The ASCII, Info and Interface (akaHTML)services work with a dataset.

Other services return information about theserver:

Version: Text data that identifies the server

Directory: Provides a way to 'browse' data

sources

Help: How to ask the server for different

responses


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Summary of Server Responses

Four object-responses defined by DAP 2.0

DAS, DDS, DataDDS, Error

Six services:

ASCII, HTML metadata, HTML interface,

Directory

Version, Help


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How a Server Handles a Request

The URL contains the host and

protocol ('localhost' and 'http' in

this example); IP addressing

locates httpd on the correct

machine.

The server recognizes the URL

as referencing a CGI.

The CGI is passed information

from the URL.

See also: Server Installation

Guide


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The CGI Server's Architecture httpd: A web daemon

Dispatch Script: The CGI

program

A collection of 'handlers;'

each builds one of the

responses

The dispatch script chooses

which handler to run


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How the Server Handles a

Request '/opendap/nph-dods'

selects the nph-dods CGI

based on the web server's

configuration.

The '.dds' suffix tells nph-

dods this is a request for a

DDS object.

The data source name's

'.nc' suffix tells nph-dods to

use the netCDF family of

handlers


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OPeNDAP Servlet Compared to

CGI The Servlet- and CGI-based server's are

conceptually similar:

Both use a web server Both use a dispatch mechanism (CGI: nph-dods,

Servlet: DODSServlet.java)

Both delegate response generation to 'handlers'

(CGI: programs written in C++, Servlet: Javaclasses)


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How the Server Chooses a

Handler The CGI-based server uses a configuration

file named 'DODS/etc/dods.rc'.

In that file, regular expressions are used tomatch URLs to specific handlers.

The server uses 'real' regular expressionswhich can be quite complicated, but in

general a server's default dods.rc file issufficient.


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Servlet/CGI Comparison, cont.

Differences:

The CGI server uses Perl for the Dispatch, the

Servlet software uses Java The CGI server's handlers can be written in C++,

C, Python, ..., anything that can produce an Unix

orWin32 executable.

The Servlet server's handlers are written inJava.*

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Choosing a server

Are the data stored in a 'standard' format?

Are the data stored in files that need to be

aggregated? Are the data stored using an 'in-house'

format?

Other considerations: CGI- or Servlet-based?


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Standard formats

The Perl/C++ CGI-server supports:

netCDF

HDF4, HDF5*

Matlab

U. of Miami DSP

The Java Servlet-server supports:

SQL (uses JDBC drivers)

netCDF

* HDF5 has not publicly been released but is available by request. Testing is not complete, but

some testing work has been donated to the project by Bob Bane.


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Aggregation

The Java/Servlet-based Aggregation Serverworks with netCDF files and other Array data.

The CGI-based JGOFS server can aggregatesome types of point data.

The GrADS Data Server (GDS) from COLAcan also perform aggregations.


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Support for in-house formats

The Perl/C++ CGI-based server:

Data which can be described using FreeForm

Data which can be read using a JGOFS method

The Java/Servlet-based server:

Data in relational databases can be served using

the DODS Relational Database Server (DRDS)*

* A RDB has many of the characteristics of an in-house format.


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Installing and Testing a Server

CGI-based servers

Servlet-based servers

Security


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Install the CGI-Based Server

Configure the web server

Choose directories for both the server and the

data Modify the web server's configuration

accordingly

Use the installServers script to copy the CGI-

Based server's components Copy or link the data files


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Servlet-based Servers

Instructions for Tomcat; other servletengines* are similar

Configure Tomcat Copy the dods.war file into Tomcat's

webapps directory and restart Tomcat

Edit theWEB-INF/web.xml as per the server

directions

Seeser le m f r a l s f ser le e g es


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Security

Security functions are provided by the webserver and/or servlet engine

Apache provides:

HTTP over SSL (HTTPS)

Several authentication schemes (Basic, Digest)

'Realm' limits per user, group and IP address

OPeNDAP servers support all of these

OPeNDAP clients support HTTP proxies

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