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-# 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.
 
 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.
 
-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.
+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
+four daemons that talk to each other via zeromq:
+
+- **collector** that keeps open TCP connections with the terminals
+  and publishes received messages,
+- **storage** that subscribes to the messages 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), and
+- **lookaside** that subscribes to "rough" location messages, quieries
+  an external source (in our implementation, opencellid database),
+  and responds with approximated location.
+
+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.