Direct Access to Remote CDM files from an HTTP Server

NetcdfFile can open remote files served over HTTP, if the HTTP server allows range requests for a resource, indicated by the Accept-Ranges header (optional), and returns a Content-Length header (required). Therefore, the server must not return the data as "chunked" transfer-coding, such as gzip. Note this must happen for both the GET and the HEAD HTTP requests.

Performance will be strongly affected by the file format and the data access pattern. This is because each request for data will incur one or more file access requests. Each file acccess request becomes an HTTP request, incurring the cost of one round-trip message to the server. Generally, the netcdf-3 format (not netcdf-4) is most likely to give good performance, especially when reading large amounts of non-record data per request. Use OPeNDAP for more consistent performance.

The THREDDS Data Server enables range requests by default. You may have to configure other HTTP servers. Following are our notes on how to do so.

Enabling Accept-Ranges response header in Apache HTTP Server

1. Make sure mod_headers is either compiled into the httpd binary (from the commandline run: $APACHE_HOME/bin/httpd -l & look for mod_headers) or loaded as a DSO module (look in httpd.conf for mod_headers).
2. Add the following directive to apply to the web content you need (e.g., it can be at the server-level, vhost-level, <Directory></Directory>, and .htaccess): Header set Accept-Ranges bytes
   
   Example:

       <Directory "/opt/local_htdocs">
          Options FollowSymLinks Indexes
          AllowOverride None
          Order allow,deny
          Allow from all
          Header set Accept-Ranges bytes
       </Directory>

3. Restart apache
4. (optional): you can test the result by using the Live Http Headers extension in firefox.

For more information see:

• http://httpd.apache.org/docs/2.0/mod/mod_headers.html

Troubleshooting

1. Add Live HTTP Headers plug-in to Firefox.

2. Try a URL in firefox:

  http://motherlode.ucar.edu:8080/thredds/fileServer/station/metar/Surface_METAR_20090209_0000.nc

(this file is probably timed off, but you can go to http://motherlode.ucar.edu:8080/thredds/catalog/station/metar/catalog.html to find a current one)

You should get something like the following in the Live Headers window:

GET /thredds/fileServer/station/metar/Surface_METAR_20090209_0000.nc HTTP/1.1
   Host: motherlode.ucar.edu:8080
   User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.2; en-US; rv:1.9.0.6) Gecko/2009011913 Firefox/3.0.6 (.NET CLR 3.5.30729)
   Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
   Accept-Language: en-us,en;q=0.5
   Accept-Encoding: gzip,deflate
   Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7
   Keep-Alive: 300
   Connection: keep-alive
 

HTTP/1.x 200 OK
   Server: Apache-Coyote/1.1
   Last-Modified: Mon, 09 Feb 2009 20:56:13 GMT
   Accept-Ranges: bytes
   Content-Type: application/x-netcdf
   Content-Length: 44414600
   Date: Mon, 09 Feb 2009 20:56:14 GMT

In particular you should see these two headers:

 Accept-Ranges: bytes

 Content-Length: 12345

3. Try the same thing with the server you want to test.

This document is maintained by Unidata. Send comments to THREDDS support. Last updated: June 29, 2009