Reverse Proxy to Make SPARQL Endpoint Available on Nice URI

When you set up an RDF repository such as Sesame2, you are likely to get a SPARQL endpoint at a URI such as http://my.server.org:8080/openrdf-sesame/repositories/repname. That's ok, but it doesn't look very nice. In order to serve the same endpoint at a nicer URI, say http://my.server.org/sparql, you can set up something called a reverse proxy on your Apache server. Here is how to do it on Debian:

Add the Reverse Proxy Rule

In /etc/apache2/conf.d, add a new configuration file sparql.conf. Make it look like this:

# Make the SPARQL endpoint of the Sesame server running on port 8080
# available as http://my.server.org/sparql.
#
# Configuration is done as per the Basic Example for Reverse Proxies
# in http://httpd.apache.org/docs/2.2/mod/mod_proxy.html .

<Proxy *>
 Order deny,allow
 Allow from all
</Proxy>
ProxyPass /sparql http://my.server.org:8080/openrdf-sesame/repositories/repname
ProxyPassReverse /sparql http://my.server.org:8080/openrdf-sesame/repositories/repname

Enable the Proxy Modules

In order for this to work, Apache has to load a number of modules having to do with proxies. The easiest and safest way to load them is by using the a2enmod command and select all proxy-related modules:

knumoe@exp3:/etc/apache2/conf.d$ sudo a2enmod
Your choices are: actions alias asis auth_basic auth_digest authn_alias authn_anon authn_dbd authn_dbm authn_default authn_file authnz_ldap authz_dbm authz_default authz_groupfile authz_host authz_owner authz_user autoindex cache cern_meta cgi cgid charset_lite dav dav_fs dav_lock dbd deflate dir disk_cache dump_io env expires ext_filter file_cache filter headers ident imagemap include info ldap log_forensic mem_cache mime mime_magic negotiation php5 proxy proxy_ajp proxy_balancer proxy_connect proxy_ftp proxy_http rewrite setenvif speling ssl status substitute suexec unique_id userdir usertrack version vhost_alias
Which module(s) do you want to enable (wildcards ok)?
proxy*

Restart Apache

sudo /etc/init.d/apache2 restart

Linux/Unix: Redirecting Output Using ">" with sudo

Problem: In Linux/Unix, you want to redirect output of something to a file with > you don't have write permissions for:

local$ ls -la > test.txt
-bash: test.txt: Permission denied

You think "I can solve this by using sudo"! But, alas:

local$ sudo ls -la > test.txt
-bash: test.txt: Permission denied

The shell won't even ask for your password. This indicates to me that somehow sudo only applies to the ls command here. What seems to happen is that first, the shell tries to open the file and only then runs the ls command to feed it with input. But since you don't have write permissions for this file, the execution of the whole expression is stopped. The way to solve this, as I found out in this blog post, is to run bash (or any other shell) as sudo and then give it the entire expression as a parameter:

local$ sudo bash -c "ls -la > test.txt"
Password:
local$ more test.txt
total 16
drwxr-xr-x  18 root  wheel   612 Oct  8 16:23 .
drwxr-xr-x@ 13 root  wheel   442 Dec  6  2007 ..
drwxr-xr-x@ 13 knud  staff   442 Sep  8 11:45 arc
drwxr-xr-x  61 root  wheel  2074 Apr 17 12:30 bin
drwxr-xr-x  14 root  wheel   476 Dec  4  2007 gwTeX
drwxr-xr-x  33 root  wheel  1122 Feb 17  2009 include
drwxr-xr-x  56 root  wheel  1904 Feb 17  2009 lib
-rw-r--r--   1 root  wheel     0 Oct  8 16:23 test.txt

Note that test.txt is already in the this list, proving that the file was created before the ls command was run!