Having been working in the telco industry for over 10 years, I can't help feeling a bit ashamed for not having learned Erlang. Erlang was developed by Ericsson - the leading Swedish-based network equipment provider, and has been used in many of the network switching equipment produced by Ericsson. Erlang has gained more traction recently, especially in the last year or two.
Here, I write an Erlang program to consume a demo RESTful service that I developed a couple of months ago. The Erlang code is based on example by Dave Thomas - the author of Programming Ruby. The detail of the RESTful service is available in my previous post.
There are two services that I want to consume.
The first one accepts a social web site URL and scrapes the page for the person's interests in movies and music to return a list of comma-delimited keywords. For example,
http://localhost:8080/SvdemoRestful/resources/webPageKeywords?url=http://localhost/someWebPage.html
if any keywords are found, then they are returned in the body of the HTTP response as a string; otherwise, the HTTP body is empty. For example,
folk songs, pop music, chinese music ,battle, action, comedy
Notice the spaces in the above string. The second service I want to consume accepts the above string to search for any matching promotion in a mock up XML database. If any is found then the XML string will be returned in the HTTP body; otherwise, the string <Empty/> is returned in the HTTP body. For example, the following URL will return the promotion information in XML below:
http://localhost:8080/SvdemoRestful/resources/promo/jazz
the returned XML:
<Promotion>
<Code>802</Code>
<Name>Jazz Night</Name>
<Description>Jazz lovers' do not miss this once in a lifetime opportunity.</Description>
<Venue>The Jazz Club</Venue>
<DateTime>2008-10-30 21:00:00</DateTime>
<Tags>Jazz</Tags>
</Promotion>
Now, let's do this in Erlang.
Create a file called svdemoClient.erl. The following code will consume the first RESTful service:
-module(svdemoClient).
-export([get_keywords/1]).
-define(BASE_URL, "http://localhost:8080/SvdemoRestful/resources").
-define(PROMO_URL, ?BASE_URL ++ "/promo/").
-define(KEYWORDS_URL, ?BASE_URL "/webPageKeywords"). % also works without ++
keywords_url_for(Url) -> ?KEYWORDS_URL ++ "?url=" ++ Url.
get_keywords(Url) ->
URL = keywords_url_for(Url),
{ ok, {_Status, _Header, Body} } = http:request(URL),
Body.
In Erlang, function names and Atoms must start with lower-case letters; variable names must start with upper-case letters or underscore (meaning the variable is not used/read).
The -define() macro in Erlang is similar to #define in C/C++. In the above example, after defining BASE_URL, any occurance of ?BASE_URL will be replaced with the string "http://localhost:8080/SvdemoRestful/resources".
The get_keywords() function returns the body of the HTTP response from requesting the given Url. The Body is either a comma-delimited string, or an empty collection. Executing the above code in Erlang:
127> c("d:/projects/svdemoErl/svdemoClient.erl").
{ok,svdemoClient}
128> svdemoClient:get_keywords("http://localhost/myWebPage.htm").
"folk songs, pop music, chinese music ,battle, action, comedy "
129> svdemoClient:get_keywords("http://localhost/someOtherPage.htm").
[]
130>
To consume the second RESTful service, the search_promo() function is added.
promo_url_for(Keywords) -> ?PROMO_URL ++ utils:url_encode(Keywords).
search_promo(Keywords) ->
URL = promo_url_for(Keywords),
{ ok, {_Status, _Header, Body} } = http:request(URL),
%%% Now that the XML is in the Body variable, let's parse it.
if
Body == "<Empty/>" ->
not_found;
true ->
{ParseResult, _Misc} = xmerl_scan:string(Body),
[ #xmlText{value=Code} ] = xmerl_xpath:string("//Code/text()", ParseResult),
[ #xmlText{value=Name} ] = xmerl_xpath:string("//Name/text()", ParseResult),
[ #xmlText{value=Description} ] = xmerl_xpath:string("//Description/text()", ParseResult),
[ #xmlText{value=Venue} ] = xmerl_xpath:string("//Venue/text()", ParseResult),
[ #xmlText{value=DateTime} ] = xmerl_xpath:string("//DateTime/text()", ParseResult),
{ Code, Name, Description, Venue, DateTime }
end.
Erlang/OTP download comes with XML parser and XPath support in the xmerl application, which is not part of the Erlang standard library (stdlib). To use the XML functions, the header file must be included:
-include_lib("xmerl/include/xmerl.hrl").
Note that the keywords contain spaces, which must be URL-encoded before passing to Erlang's http:request() function. I stole the url_encode() function from YAWS and put it in utils.erl file.
To string the two service consumptions together:
search_promo_from_url(Url) ->
Keywords=get_keywords(Url),
if
Keywords == [] ->
not_found;
true ->
search_promo(Keywords)
end.
Calling the function in Erlang shell:
126> svdemoClient:search_promo_from_url("http://localhost/MyWebPage.htm")
.
{"801",
"Batman The Dark Knight",
"\n\t\t\tMeet stars in Batman in person - Chritian Bale, Michael Caine.\n\t\t",
"Star City",
"2008-7-30 10:00:00"}
The final svdemoClient.erl file:
-module(svdemoClient).
-export([get_keywords/1, search_promo/1, search_promo_from_url/1]).
-include_lib("xmerl/include/xmerl.hrl").
-define(BASE_URL, "http://localhost:8080/SvdemoRestful/resources").
-define(PROMO_URL, ?BASE_URL ++ "/promo/").
-define(KEYWORDS_URL, ?BASE_URL "/webPageKeywords"). % also works without ++
keywords_url_for(Url) -> ?KEYWORDS_URL ++ "?url=" ++ Url.
get_keywords(Url) ->
URL = keywords_url_for(Url),
{ ok, {_Status, _Header, Body} } = http:request(URL),
Body.
promo_url_for(Keywords) -> ?PROMO_URL ++ utils:url_encode(Keywords).
search_promo(Keywords) ->
URL = promo_url_for(Keywords),
{ ok, {_Status, _Header, Body} } = http:request(URL),
%%% Now that the XML is in the Body variable, let's parse it.
if
Body == "<Empty/>" ->
not_found;
true ->
{ParseResult, _Misc} = xmerl_scan:string(Body),
[ #xmlText{value=Code} ] = xmerl_xpath:string("//Code/text()", ParseResult),
[ #xmlText{value=Name} ] = xmerl_xpath:string("//Name/text()", ParseResult),
[ #xmlText{value=Description} ] = xmerl_xpath:string("//Description/text()", ParseResult),
[ #xmlText{value=Venue} ] = xmerl_xpath:string("//Venue/text()", ParseResult),
[ #xmlText{value=DateTime} ] = xmerl_xpath:string("//DateTime/text()", ParseResult),
{ Code, Name, Description, Venue, DateTime }
end.
search_promo_from_url(Url) ->
Keywords=get_keywords(Url),
if
Keywords == [] ->
not_found;
true ->
search_promo(Keywords)
end.
The utils.erl file (copied from YAWS):
-module(utils).
-export([integer_to_hex/1, url_encode/1]).
integer_to_hex(I) ->
case catch erlang:integer_to_list(I, 16) of
{'EXIT', _} ->
old_integer_to_hex(I);
Int ->
Int
end.
old_integer_to_hex(I) when I<10 ->
integer_to_list(I);
old_integer_to_hex(I) when I<16 ->
[I-10+$A];
old_integer_to_hex(I) when I>=16 ->
N = trunc(I/16),
old_integer_to_hex(N) ++ old_integer_to_hex(I rem 16).
url_encode([H|T]) ->
if
H >= $a, $z >= H ->
[H|url_encode(T)];
H >= $A, $Z >= H ->
[H|url_encode(T)];
H >= $0, $9 >= H ->
[H|url_encode(T)];
H == $_; H == $.; H == $-; H == $/; H == $: -> % FIXME: more..
[H|url_encode(T)];
true ->
case integer_to_hex(H) of
[X, Y] ->
[$%, X, Y | url_encode(T)];
[X] ->
[$%, $0, X | url_encode(T)]
end
end;
url_encode([]) ->
[].
using 
