Études for Erlang

Étude 11-1: Get the Weather

In this étude, you will create a weather server using the gen_server OTP behavior.This server will handle requests using a four-letter weather station identifier and will return a brief summary of the weather. You may also ask the server for a list of most recently accessed weather stations.

Here is some sample output:

1> c(weather).
{ok,weather}
2> weather:start_link().
{ok,<0.42.0>}
3> gen_server:call(weather, "KSJC").
{ok,[{location,"San Jose International Airport, CA"},
     {observation_time_rfc822,"Mon, 18 Feb 2013 13:53:00 -0800"},
     {weather,"Overcast"},
     {temperature_string,"51.0 F (10.6 C)"}]}
4> gen_server:call(weather, "KITH").
{ok,[{location,"Ithaca / Tompkins County, NY"},
     {observation_time_rfc822,"Mon, 18 Feb 2013 16:56:00 -0500"},
     {weather,"A Few Clouds"},
     {temperature_string,"29.0 F (-1.6 C)"}]}
5> gen_server:call(weather,"NONE").
{error,404}
6> gen_server:cast(weather, "").
Most recent requests: ["KITH","KSJC"]

Obtaining Weather Data

To retrieve a web page, you must first call inets:start/0; you will want to do this in your init/1 code. Then, simply call httpc:request(url), where url is a string containing the URL you want. In this case, you will use the server provided by National Oceanic and Atmospheric Administration. This server accepts four-letter weather station codes and returns an XML file summarizing the current weather at that station. You request this data with a URL in the form

http://w1.weather.gov/xml/current_obs/NNNN.xml

where NNNN ...