1. Camtool - monitoring realtime information
webcam images for currently observing stations
taken from remote URL, cached in Erlang DB
webcam definitions stored in MySQL database
URL, associated databasekey, refreshrate
TSYS plots
taken from logfile monitoring utility’s gnuplot frontend
also cached locally in Erlang DB
generate webpage(s) with dynamic content
serve the images from Erlang i.s.o. disk
fault tolerant: if process dies, restart it
design, gen_server, supervisor (dynamic), mnesia, webtool

2. Startup of the utility start() ->
% gen_servers make interfacing to a
% piece of code very simple!
% They are trivial to create and GREAT
% our mnesia database interface, a gen_server
dumpstore:start(),
% virtual time server, also a gen_server
nowserver:start_link(),
% these are supervisors
webcammonitor:start_link(),
tsysmonitor:start_link(),
% webserver, mapping URLs to Erlang calls
webtool:start( standard_path,
[{port, 8000}, {bind_address, {0,0,0,0}}] ).
design, gen_server, supervisor (dynamic), mnesia, webtool

3. a supervisor: webcammonitor
init(_) ->
{one_for_one, 10, 1}, [
% This is the list of (static) ChildSpecifications
% {Module, Function, [Arguments]}
% the database connection first
{dbconn, {mysql, start_link,[pool,”Host”, “User”, “Pass”, “DB”]},
permanent, 1000, worker, []},
% Poll(*) JSON from “.py” URL for current status
% implemented as a gen_server
{jsonneer, {jsonserver, start_link, []},
% Poll(*) the current status from the jsonserver
% also implemented as a gen_server
{monitorloop, {webcamloop, start_link, []},
permanent, 1000, worker, []} ] }.
(*) In future version maybe publish-subscribe architecture
design, gen_server, supervisor (dynamic), mnesia, webtool

4. a gen_server: jsonserver
% Handle a clock-tick, fetch new “current” data.
handle_cast(tick, State) ->
% Get the current time
{{Y,M,D}, {HH,MM,SS}} = nowserver:datetime(),
% Call a pythonscript over HTTP (queries SQLite DB and returns result as JSON)
B = “ %d:%d:%d”,
Url = io_lib:format(B, [Y,M,D,HH,MM,SS]),
% Actually GET the json
JSON = case of
% only accept http-status “200” (succesfully retrieved url)
{ok, {{_200,_}, _, Body}} -> Body;
_ > []
end,
% And parse it
Obj = case rfc4627:decode(JSON) of
{ok, {obj, KVList}, _} -> KVList; % Parsed into a [{Key, Value}[,{Key,Value}]]
{ok, List, _} > [{stations, List}];
_ > [{stations, []}]
{noreply, dict:store(currentjson, Obj, State)}.
design, gen_server, supervisor (dynamic), mnesia, webtool

5. Dynamic supervision: add/remove childs otf
% Experiment stayed the same, now add/remove watchers
update(NewExp, NewExp, CurrentStations, State) ->
OldSet = sets:from_list(dict:fetch(stations, State)),
NewSet = sets:from_list(CurrentStations),
Removed = sets:subtract(OldSet, NewSet),
Added = sets:subtract(NewSet, sets:intersection(OldSet,NewSet)),
S = unwatch_tsys(sets:to_list(Removed), State),
S = watch_tsys(sets:to_list(Added), S0)
% dynamically add a child to the supervisor
watch_tsys([Atom|Rest], State) ->
% call helper function which creates a “ChildSpecification”
Child = tsysgetter:mk(Atom, Expt, Station, DBKey),
supervisor:start_child(?TSYSMONITOR, Child)
% And remove it from supervision
unwatch_tsys([Atom|Rest], State) ->
supervisor:terminate_child(?TSYSMONITOR, Atom),
supervisor:delete_child(?TSYSMONITOR, Atom),
design, gen_server, supervisor (dynamic), mnesia, webtool

6. mnesia - the database It is transactional - enforced and working.
get_data(Id) ->
% query the datenbank for a record where the lumpdata is still alive
% (i.e. its “age” is <= Now + records’ “time-to-live” (ttl)
Fun = fun() ->
Now = nowfunc(),
qlc:eval( qlc:q( [X || X <- mnesia:table(Table),
X#lump.id =:= Id,
Now<X#lump.last_refresh+X#lump.ttl] ) )
end,
case mnesia:transaction( Fun ) of
% data in record “too old”
{atomic, []} > {reply, stale_data, Table};
% Still up-to-date
{atomic, [R]} > {reply, R#lump.data, Table};
% something went wrong
{aborted, Reason} -> {reply, {error, Reason}, Table}
end;
design, gen_server, supervisor (dynamic), mnesia, webtool

7. mnesia - the database It stores binary data, actually any data.
% From the webcam-image-getter-process
Url = “
case of
% Only accept statuscode 200
{ok, {{_,200,_}, _Headers, Jpeg}} -> dumpstore:update(DBKey, Jpeg, Now);
_ > error
end,
% From the tsys-image-getter-process
case gnuplot, make_plot, [....]) of
{ok, Png} > dumpstore:update(DBKey, Png, Now);
{error, Why} -> Why;
_ > fail_but_buggered_if_we_know_why
design, gen_server, supervisor (dynamic), mnesia, webtool

8. webtool - mapping URLs to Erlang fn calls
-compile(export_all).
% return configdata for the “webtool” application
webtool(_Args) ->
{camtool, [{web_data,{"CamTool","/webcams/camtool/camtool"}},
{alias, {erl_alias, "/webcams", [camtool]}}]}.
% The {alias, {erl_alias ...}} statement maps URLs of the form
% “/webcams/camtool/function?arguments”
% into the Erlang function call [Module:Function(Arguments)]
% camtool:function(WebserverEnvironment, Arguments) %
camtool:image(Env, Args) ->
case get_input_data(Args, "id") of
undefined -> [head(),”Please give an Id”,closing()];
ImgId > case dumpstore:get_lump(list_to_atom(ImgId)) of
undefined -> [head(), “Invalid ImageId given”, closing()];
R > MimeType=R#lump.mimetype,
Data=R#lump.data,
"Content-Type: "++MimeType++"\r\n\r\n"++Data
end
end.
design, gen_server, supervisor (dynamic), mnesia, webtool