-# A server to collect data from zx303 GPS trackers
+# A server to collect data from zx303 ZhongXun Topin Locator
zx303 GPS+GPRS module is a cheap and featureful GPS tracker for pets,
children, elderly family members, and, of course, illegal tracking
of people and objects, though the latter absolutely must not be done.
+## Introduction
+
This work is inspired by [this project](https://github.com/tobadia/petGPS),
-but is more of a complete reimplementation than a derived work.
+but it is more of a complete reimplementation than a derived work.
+
+When powered up, the module makes TCP connection to the configured
+(via SMS) server, identifies itself (via IMEI) in the first message,
+and continue to send periodic messages with location and other status
+updates. Some of these messages require a response from the server.
+In particular, when the module has no GPS coverage, it sends information
+about neaby GSM+ cell towers and WiFi access points, to which the server
+is expected to respond with a message contaning approximate location
+derived from this data. That may require querying some external service.
+
+Because we would usually want location information reach consumer
+instantly upon arrival, and _also_ to be stored, it makes sense to
+design the system in "microservices" way, using a message bus in
+"publish-subscribe" model. And then, as we already have a message-
+passing infrastructure anyway, it makes sense to decouple processes
+that prepare responses to the module's messages from the server that
+keeps TCP connections with the modules.
+
+This leads us to the current implementation that has consists of
+five daemons that talk to each other via zeromq:
+
+- **collector** that keeps open TCP connections with the terminals
+ and publishes received messages _and_ sent responses,
+- **storage** that subscribes to the messages and responses from the
+ collector and stores them in a database,
+- **termconfig** that subscribes to messages that need non-trivial
+ response (most of them are about configuring various settings in
+ the terminal, hence the name),
+- **lookaside** that subscribes to "rough" location messages, quieries
+ an external source (in our implementation, opencellid database),
+ and responds with approximated location, and
+- **wsgateway** that is a websockets server that translaes messages
+ between our internal zeromq messaging and websocket clients, i.e.
+ web pages. This daemon is also capable of responding to http with
+ a single html file. This functionality is mainly for debugging.
+ Users of this package are expected to implement their own single
+ page web application that communicates with this server.
+
+There is also a command-line tool to send messages to the terminal.
+There is a number of useful things that can be requested in this way.
+
+## Websocket messages
+
+Websockets server communicates with the web page with json encoded
+text messages. The only supported message from the web page to the
+server is subscription message. Recognized elements are:
+
+- **type** - a string that must be "subscribe"
+- **backlog** - an integer specifying how many previous locations to
+ to send for the start. Limit is per-imei.
+- **imei** - a list of 16-character strings with IMEIs of the
+ tracker terminals to watch.
+
+Each subscription request nullifies preexisting list of IMEIs
+associated with the client, and replaces it with the list supplied
+in the message.
+
+Example of a subscription request:
+
+```
+{"imei":["8354369077195199"],
+ "type":"subscribe",
+ "timestamp":1652134234657,
+ "backlog":5}
+```
+
+Server sends to the client a backlog of last locations of the
+terminals, that it fetches from the database maintained by the
+storage service, one location per websocket message. It then
+continues to send further messages when they are received from
+the module, in real time.
+
+Example of a location message:
+
+```
+{"imei": "8354369077195199",
+ "timestamp": "2022-05-09 21:52:34.643277+00:00",
+ "longitude": 17.465816,
+ "latitude": 47.52013}
+```
+
+## Homepage and source
-As of the time of creation of this README, the program can successfully
-collect data and store it in sqlite database, but does not use/provide
-location service based on mobile cell and wifi data.
+Home page is [http://www.average.org/gps303/](http://www.average.org/gps303/)
+Get the source from the [origin](git://git.average.org/gps303.git)
+or [Github mirror](https://github.com/crosser/gps303).
projects / loctrkd.git / blobdiff