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+
+Volta
+=====
+
+What is volta?
+--------------
+
+Volta is a high performance, low resource URI rewriter for use with the
+Squid caching proxy server (http://www.squid-cache.org/.)  With it, you
+can dynamically alter URI requests that pass through Squid based on
+various criteria.
+
+It uses a state machine to parse URIs and rules, and a constant database
+to store and access those rules.  It can then either perform conditional
+rewrites internally, or by evaluating Lua scripts.
+
+
+Why is it called "volta"?
+-------------------------
+
+It's a type of old Italian music written in triple-time.  Quick!
+
+
+How fast is it?
+---------------
+
+On a 2Ghz Xeon 5130, it can process a million squid requests against
+10000 rules in less than 8 seconds, using about 800k of ram.  On an
+1.8Ghz Intel E4300, it can do it in 3 seconds.
+
+Your mileage may vary, but for most all intents and purposes the answer
+is "definitely fast enough."
+
+
+Configuring squid
+-----------------
+
+You must enable url rewriting from within the squid.conf file.
+
+	url_rewrite_program /usr/local/bin/volta
+
+... and that's it.  You may need some additional customization, like where
+the volta database is stored on disk:
+
+	url_rewrite_program /usr/local/bin/volta -f /var/db/squid/volta.db
+
+Busy servers:
+
+While Volta is lightweight enough to simply increase the amount of
+rewriter children, it also supports Squid's rewrite_concurrency format
+if you find that to be more efficient for your environment.  Adjust to
+taste.
+
+	url_rewrite_children 5 startup=1 idle=2 concurrency=50
+
+
+Using volta
+-----------
+
+See the INSTALL file for instructions on how to compile volta.
+
+Volta reads its rewrite rules from a local database.  You can create the
+rules in a text editor, then convert it to the database like so:
+
+	% volta -c rules.txt
+
+You'll be left with a "volta.db" file in the current directory.  Put it
+wherever you please, and use the -f flag to point to it.
+
+
+Rule file syntax
+----------------
+
+Volta's rule syntax is designed to be easy to parse by humans and
+machines.  Blank lines are skipped, as is any line that starts with the
+'#' character, so you can keep the ascii version of your rules well
+documented and in version control.  There is no practical limit on the
+number of rules in this database.
+
+When compiling the ruleset into the database format, volta detects
+malformed rules and stops if there are any problems, leaving your
+original database intact.  You can change the ruleset at any time while
+volta is running, and the new rules will take affect within about 10
+seconds.  No need to restart squid!
+
+There are two types of rules -- positive matches, and negative matches.
+Positive matches cause the rewrite, negative matches intentionally allow
+the original request to pass.  Rule order is consistent, top-down, first
+match wins.  Fields are separated by any amount of whitespace (spaces or
+tabs.)
+
+
+### Positive matches:
+
+**First field**: *the hostname to match*
+
+	You can use an exact hostname (www.example.com), or the top level
+	domain (tld) if you want to match everything under a specific host
+	(example.com.)  You can also use a single '*' to match every request,
+	though this essentially bypasses a lot of what makes volta quick, it
+	is included for completeness.  You may have an unlimited amount of
+	rules per hostname.  Hostnames are compared without case sensitivity.
+
+**Second field**: *the path to match*
+
+	This can be an exact match ('/path/to/something.html'), a regular
+	expression ('\.(jpg|gif|png)$'), or a single '*' to match for any
+	path. Regular expressions are matched without case sensitivity.  There
+	is currently no internal support for captures, though you can use
+	a Lua rule (see below) for more complex processing.
+
+**Third field**: *the redirect code and url to rewrite to*
+
+	Any pieces of a url that are omitted are automatically replaced
+	with the original request's element -- the exception is a hostname,
+	which is required.  If you omit a redirect code, the URL rewrite is
+	transparent to the client.  You can attach a 301: or 302: prefix to
+	cause a permanent or temporary code to be respectively sent, instead.
+
+	If you require more complex processing than what volta provides
+	internally, you can also specify a path to a Lua script (prefixed
+	with 'lua:'.)  See the 'Lua rules' section of this README for more
+	information.
+
+
+### Negative matches:
+
+**First field**: *the hostname to match*
+
+See above -- all the same rules apply.
+
+
+**Second field**: *the path to match*
+
+See above -- all the same rules apply.
+
+
+**Third field**: *the 'negative' marker*
+
+This is simply the '-' character, that signals to volta that this is
+a negative matching rule.
+
+
+You can easily test your rules by running volta on the command line, and
+pasting URLs into it.   Boost the debug level (-d4) if you're having any issues.
+
+
+Examples
+--------
+
+Rewrite all requests to Google to the SSL version:
+
+	google.com * 302:https://www.google.com
+
+	This will redirect the request "http://www.google.com/search?q=test" to
+	"https://www.google.com/search?q=test".
+
+
+Transparently alter all uploaded images on imgur to be my face:  :)
+
+	i.imgur.com \.(gif|png|jpg)$ http://www.martini.nu/images/mahlon.jpg
+
+
+Expand a local, non qualified hostname to a FQDN (useful alongside the
+'dns_defnames' squid setting to enforce browser proxy behaviors):
+
+	local-example * local-example.company.com
+
+
+Cause all blog content except for 2011 posts to permanently redirect to
+an archival page:
+
+	martini.nu /blog/2011 -
+	martini.nu /blog 301:martini.nu/content-archived.html
+
+
+Send all requests to reddit/r/WTF/* through a lua script for further processing.
+
+	reddit.com /r/wtf lua:/path/to/a/lua-script.lua
+
+
+Turn off rewriting for specific network segment or IP address:
+
+	Squid has this ability built in -- see the 'url_rewrite_access' setting.
+	Alternatively, do the checks in lua.
+
+
+
+Lua Rules
+---------
+
+Volta has an embedded Lua interpreter that you can use to perform all
+kinds of conditional rewrites.  Read more about the syntax of the Lua
+language here: http://www.lua.org/manual/5.1/
+
+### Loading a script
+
+To use a Lua script, prefix the rewrite target of a volta rule with
+'lua:'.  The rest of the target is then treated as a path to the script.
+(You can find an example in the Examples section of this README.)
+
+You can specify a path to either an ascii file, or Lua bytecode. (If
+speed is an absolute premium, I'm seeing around a 25% performance
+increase by using Lua bytecode files.)
+
+You can use different scripts for different rules, or use the same
+script across any number of separate rules.
+
+There is no need to restart squid when modifying Lua rules.  Changes are
+seen immediately.
+
+
+### Environment
+
+* Global variable declarations are disabled, so scripts can't accidently stomp on each other.  All variables must be declared with the 'local' keyword.
+* There is a global table called 'shared' you may use if you want to share data between separate scripts, or remember things in-between rule evaluations.
+* The details of the request can be found in a table, appropriately named 'request'.  HTTP scheme, host, path, port, method, client_ip, and domain are all available by default from the request table.
+* Calling Lua's print() function emits debug information to stderr.  Use a debug level of 2 or higher to see it.
+
+
+### Return value
+
+The return value of the script is sent unmodified to squid, which should
+be a URL the request is rewritten to, with an optional redirect code
+prefix (301 or 302.)
+
+Omitting a return value, or returning 'nil' has the same effect as a negative
+rule match -- the original request is allowed through without any rewrite.
+
+
+An extremely simple Lua rule script can be found in the 'examples'
+directory, distributed with volta.
+