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+# Building a remote-control for real cars
+
+[[!meta date="2011-05-01"]]
+[[!tag debian msg blog editorial]]
+
+## SMS is used to lock car doors
+
+[Copenhagen Carpool][] soon receive a new batch of electric cars.
+The new cars will contain a cellphone tied to the door locks,
+to only let into the car the member who booked its current timeslot.
+
+I am setting up the system to connect the cars with their booking calendar.
+
+The general idea is to...
+
+ 1. Connect a standard cellphone to a computer.
+ 1. Forward sms messages received on the phone to the calendar.
+ 1. Send messages from the calendar through the phone.
+ 1. Leave the system in a corner, turned on 24/7.
+
+[[!graph  src="""
+rankdir=LR
+phone -> computer -> "calendar" [dir="both"];
+"""]]
+
+Most of my work is in trying to avoid ways things might go wrong
+-- which so often happens when computers are involved.
+
+What if...
+  * messages are delayed or lost?
+  * a chain of messages arrive out of order?
+  * messages are sent faster than they can be delivered?
+
+[Copenhagen Carpool]: http://www.kobenhavnsdelebiler.dk/
+  "Københavns Delebiler"
+
+
+## Reliable conversation
+
+Human conversation is most reliable when using plain speak.
+Slang, sarcasm or jokes raise the risk of misunderstandings.
+This is quite similar in computer conversation.
+
+When surfing the Web,
+you normally "GET" a web page or "POST" new data like a search query.
+The words "GET" and "POST" are "plain speak" of the World Wide Web
+-- defined as a principle called "[REST][]" or "RESTful design".
+
+[[!graph  src="""
+rankdir=LR
+"fetch document" -> website [label="GET", dir="back"];
+"provide info" -> website [label="POST"];
+"""]]
+
+I use the [Kannel][] tool to talk directly to the phones.
+When an sms arrives on the phone,
+Kannel "GET"s it to the calendar.
+That's weakly expressed
+-- better if it had "POST"ed the message instead.
+
+[[!graph  src="""
+rankdir=LR
+"provide sms info" -> "calendar" [label="GET" style="dotted"];
+"""]]
+
+Kannel is a quite reliable tool, but not RESTful in conversations.
+That requires extra attention at other parts of the setup
+to avoid breakage in special situations.
+
+That worries me.
+
+[Kannel]: http://www.kannel.org/
+  "Kannel: Open Source WAP and SMS Gateway"
+
+[REST]: http://en.wikipedia.org/wiki/REST
+  "Representational State Transfer"
+
+
+## Each car has a blog
+
+So I setup a gateway from Kannel to RESTful conversation.
+And choose classic blogging as style of conversation:
+
+ * The carpool fleet is a blogroll
+ * each car has a blog
+ * each sms emitted from a car is a blog entry
+
+Blogging comes in two flavors,
+-- [RSS][] and [Atom][].
+Both cover distribution of messages.
+I use Atom because it is newest and coolest,
+and because it also covers creating and editing meesages
+with the [AtomPub][] extension.
+
+AtomPub allows the calendar to create an empty message
+-- which then triggers an sms from the car which "fills it in."
+
+AtomPub also let the calendar "PUT" tags onto existing messages,
+to mark them as processed, read etc.,
+and let an admin or a cleanup script "DELETE" outdated entries or blogs.
+The words "PUT" and "DELETE" are RESTful conversation as well.
+
+[Atom]: http://en.wikipedia.org/wiki/Atom
+  "Atom"
+
+[RSS]: http://en.wikipedia.org/wiki/RSS
+  "RSS"
+
+[AtomPub]: http://en.wikipedia.org/wiki/AtomPub
+  "AtomPub"
+
+
+## The system is dancing!
+
+My sms-to-blog gateway is written in [Perl][],
+using the [Dancer][] toolkit.
+
+Dancer makes it simple to setup a RESTful web conversation,
+and when I contacted the authors on [IRC][],
+they swiftly added support for the non-RESTful quirk that I needed.
+
+[[!graph  src="""
+rankdir=LR
+phone -> "sms gateway" [dir="both"];
+"sms gateway" -> "blog gateway" [dir="both", style="dotted"];
+"blog gateway" -> calendar [dir="both"];
+"""]]
+
+My sms-to-blog gateway is not yet ready, but basic routines work.
+The [project is public][], and code is [Free Software][].
+
+[Perl]: http://en.wikipedia.org/wiki/Perl
+  "Perl"
+
+[Dancer]: http://www.perldancer.org/
+  "Dancer: Perl web development framework"
+
+[IRC]: http://www.perldancer.org/irc
+  "IRC Chat system (Internet Relay Chat)"
+
+[project is public]: git://source.delebilfonden.dk/msg/
+  "Kannel-to-AtomPub Gateway"
+
+[Free Software]: http://www.debian.org/social_contract#guidelines
+  "The Debian Free Software Guidelines (DFSG)"