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.

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.