Flask extension implementing HTTP forwarding

This flask makes it possible to implement an HTTP forwarding system using Flask.

HTTP forwarding is how Fleischwolf handles mass queries of documents.

To use it, you'll need to start by setting forwarding headers in the following format:

X-Forward-Id: 12345678-1234-1234-1234-1234567890AB
X-Forward-Errors-To: http://errors.com
X-Forward-Referer: http://previous-server.com
X-Forward-To: http://nextserver.com
X-Forward-Query-Params: foo=bar
X-Forward-Method: POST

The first 3 headers are mandatory for all connections:

• X-Forward-Id is a conventional 128-bit UUID. It uniquely identifies the forwarding request.
• X-Forward-Errors-To is the server that error messages should be dispatched to.
• X-Forward-Referer is the name of the preceding server.
  If you're dispatching the reuqest manually, you can set the last of these to ORIGIN.

The remaining 3 are required to initiate forwarding:

• X-Forward-To is the HTTP hostname of the next server in the forwarding chain.
• X-Forward-Query-Params should hold any query params you want to be set on the URL pulled by X-Forward-To.
• X-Forward-Method is the HTTP verb used in the forwarded request. Only PUT and POST are supported.

You can set as many different forwarding destinations as you like in X-Forward-To. So for instance, if you were to send the following headers:

...
X-Forward-To: http://one.com, http://two.com, http://three.com
X-Forward-Query-Params: foo=bar, flavor=lemon&color=yellow, loneliest_number=1
X-Forward-Method: POST, POST, PUT
...

The request will be forwarded as a POST to one.com, then as a PUT from one.com to two.com, then as a POST from two.com to three.com. The IRI will have the query parameters appended, so if you were monitoring the server logs for all three hosts, you'd see something like this:

• POST http://one.com/us/judgment/us/1986-06-30/05-274/eng@published#test/2014-01-01/main.xml?foo=bar
• POST http://three.com/us/judgment/us/1986-06-30/05-274/eng@published#test/2014-01-01/main.xml?loneliest_number=1
• PUT http://two.com/us/judgment/us/1986-06-30/05-274/eng@published#test/2014-01-01/main.xml?flavor=lemon&color=yellow

In keeping with HTTP semantic convention, a PUT is a store, and a POST is an action. Therefore, a PUT should generally be a terminal action, specifying that the output should be stored as the final step in the chain.